An Annotated Checklist of the Main Representatives of Meiobenthos from Inland Water Bodies of Central and Southern Vietnam—II—Annelid Worms (Oligochaeta and Aeolosomatidae)

Abstract

This work continues a series of publications on the diversity of the main groups of invertebrates found through the investigation of meiobenthic communities in various inland waterbodies in central and southern Vietnam. Here, a checklist of oligochaetes and aeolosomatids (the annelid worms) discovered for the period 2010–2021 is presented, as well as data on the structure and abundance of their populations found in the country, ecology, local and global distribution, and, where necessary, taxonomic, morphological and other comments. In total, 71 representatives of the species and supra–species ranks from 3 families and 17 genera are listed. In total, 39 representatives were identified to the species level. During the present research, 2 species new to science were discovered and described in previously published works. This paper reports 9 more forms that need further study and may belong to still undescribed species. In addition to the new and still-unidentified species, the checklist reports about 30 more members of species and supra–species ranks that were recorded in Vietnam for the first time. Based on new and previously published data, the currently known diversity of aquatic oligochaetes and aeolosomatids in Vietnam is estimated at approximately 100–113 and 8–9 representatives, respectively. It is shown that members of the family Enchytraeidae remain practically unexplored in the waterbodies of the region. The subfamily Tubificinae and the family Aeolosomatidae also continue to be relatively poorly studied here at the species level.

1. Introduction

The present paper continues a cycle of publications on the diversity of the main groups of small bottom and near-bottom invertebrates—traditionally studied within the community of meiobenthos—in inland waters of Vietnam. Comprehensive studies on the benthic meiofauna in Southeast Asia, and in Vietnam in particular, have not been carried out until recently. The first results of our research, which began in 2008, showed that round (Nematoda) and annelid (Annelida) worms, entomostracan crustaceans (Cladocera, Copepoda, Ostracoda) and non-biting midge larvae (Chironomidae) play the leading roles in terms of abundance and diversity of benthic meiofauna in the country’s inland waterbodies [1,2]. A review of the fauna of nematodes discovered during our research was published previously [3]. This paper presents an annotated list of the members of annelid worms that have been found and identified to date.

The current subclass Oligochaeta (the class Clitellata) includes about 30 families whose representatives inhabit a variety of marine, brackish, fresh and terrestrial habitats, while the freshwater family Aeolosomatidae—microscopic worms with a still not completely clear systematic position within the phylum Annelida—consists of 3 genera, the richest of which (28 species) is the genus Aeolosoma [4,5,6,7]. Along with the aeolosomatids, the diversity and abundance of annelid worms in the meiobenthic communities of various freshwater bodies worldwide are primarily based on the microdrile oligochaetes from the current families Naididae and Enchytraeidae [4]. In some areas of the Oriental biogeographic region, the composition of these groups has been studied quite well. Most of the research on the fauna of aquatic annelids has been conducted in India, China, Indonesia and some neighbouring countries [8,9,10,11,12,13,14,15,16,17,18,19]. According to the generalized data, approximately 7–9 species from the genus Aeolosoma, along with about 120–130 representatives of the aquatic oligochaetes from the family Naididae and 1–3 each from the Enchytraeidae, Lumbriculidae and Phreodrilidae have been registered in the region so far [5,9,13,20,21,22,23]. In turn, there are still very few works devoted to the analysis of the composition and diversity of the Vietnamese aquatic annelids from the discussed groups. In rare sources where there is data on these worms, the names of only a few species found during the study of benthos and/or other aquatic communities are usually given [24,25,26]. Apparently, the most complete list presented before our research was given in the monograph “Hydrobiology of inland freshwater bodies of Vietnam”, published in Vietnamese (“Thủy sinh học các thủy vực nước ngọt nội địa Việt Nam”) [27]. It includes 3 species from the genus Aeolosoma and 43 members of the family Naididae from 4 subfamilies (according to the current classification [7]): 1 Rhyacodrilinae, 27 Naidinae, 9 Tubificinae, and 6 Pristininae. The first lists based on the results of our studies of the meiobenthic communities in waterbodies of Vietnam, which also included members of the annelids identified at that time, were reported in the works [1,2]. However, since then, a lot of new materials have been analysed, the identification of some taxa has been refined and new representatives have been found. This work aims to summarize and update the global information on the diversity, distribution and ecology of the oligochaetes and aeolosomatids found in Vietnamese waterbodies in view of the latest data.

2. Materials and Methods

The materials included in the checklist were collected in 2010, 2012, 2014, 2015, 2018, 2019, and 2021 in three provinces belonging to Central Vietnam—Dak Lak (Đắk Lắk), Khanh Hoa (Khánh Hòa) and Lam Dong (Lâm Đồng), as well as in the southern regions of the country—the province of Dong Nai (Đồng Nai) and the Co Chien (Cổ Chiên) branch (located within the provinces of Ben Tre (Bến Tre), Vinh Long (Vĩnh Long) and Tra Vinh (Trà Vinh)) of the Mekong (Mê Kông) river delta (Figure 1). In the first two provinces, most of the investigated waterbodies are of artificial origin and, as a rule, are situated in densely populated areas subject to intensive agricultural and other human activities. One of the most populated regions of the country is the Mekong Delta. The stations studied in the Co Chien branch cover the lower section of the river from freshwater to the saline seaside zone. In Dong Nai province, the work was carried out mainly in forest and floodplain natural waterbodies located within the protected area of Cat Tien (Cát Tiên) National Park. In Lam Dong, a mountain river in a relatively sparsely populated area was explored. The list and main characteristics of the studied waterbodies and habitats are given in Appendix A. In total, 98 quantitative and 14 qualitative samples of bottom sediments and the rhizospheres of aquatic macrophytes from 51 diverse bodies of water were collected and analyzed, and more than 5100 annelid specimens were found and examined. Although we studied the meiofauna, larger (macrobenthic) representatives of oligochaetes were also found in the samples. Since one of the focuses of the present paper is the diversity and composition of aquatic annelids in the region, and it would be unreasonable to omit additional information about the finds of the group’s members in the collected material, we have also included these finds in the checklist. The checklist does not contain the first materials we obtained in Vietnam in 2008 [1] since they were not kept and the identification of the worms found in these samples cannot be rechecked. However, where necessary, these data are used in comments on some listed taxa and a general discussion of the study results.

The quantitative samples were collected using an S–1 microbenthometer with a corer diameter of 34 mm (capture area ~9 cm²) or simply using the corer in shallow sites. Each sampling event included three cores of the bottom sediments and near-bottom water (about 5 cm each), combined into a single integrated sample. The qualitative samples of the sediments were taken by hand and placed in 0.1–1.0 L containers until full. Fixation was made with 4% formaldehyde. For analysis of the qualitative composition of the fauna inhabiting the rhizosphere of floating macrophytes, medium-sized plants were selected, placed in 10 L containers with prefiltered water and rinsed carefully. Then the plants were removed, and the remaining material was filtered, preserved and processed following the same technique as used for the bottom sediment samples. The main environmental parameters of the studied habitats (temperature, pH, mineralization, oxygen concentration) were measured using a Hanna HI 9828 device (Hanna Instruments Inc., Woonsocket, RI, USA).

In the laboratory, the samples were filtered through a sieve with a mesh size of 82 × 82 µm. The sieved residues were stained with Rose Bengal by the method of [28] and viewed in a Bogorov counting chamber under a stereoscopic microscope. Selected worms were mounted in glycerol on slides. Their examination, measurement and photography were conducted using a Nikon Eclipse 80i light microscope equipped with Nomarski DIC accessories, a Nikon DS–Fi1 digital camera and a PC with NIS–Elements D 3.2 software for imaging and analysis of research objects. Most of the slides with studied worms are stored in the Laboratory of Ecology of Aquatic Invertebrates of the Papanin Institute for Biology of Inland Waters, Russian Academy of Sciences (Borok, Yaroslavl Province), in the collection of representatives of meiobenthos from waterbodies of Vietnam. The holotypes of new species already described during the research are deposited in the collection of the Zoological Institute of the Russian Academy of Sciences (St. Petersburg). The respective storage locations and slide inventory numbers for each of the representatives listed in the checklist are presented in Supplementary Table S1.

3. Results

3.1. Taxonomy, Structure and Conventions of the Checklist

In the current paper, we use the classification and accepted taxa names (taking into account all synonymized names) of the aquatic oligochaetes and aeolosomatids given in the WoRMS database at the time of the checklist’s origination [7]. The discovered species and supra-species taxa are presented within families/subfamilies in alphabetical order and numbered through the entire list. In braces after the name of the taxon, the serial numbers of the samples in which a particular taxon was found are indicated. These numbers correspond to those given in the third column of Appendix A. Further, the occurrence frequency (%) of the representative in different types of the studied waterbodies is presented in parentheses according to the following scheme: (reservoirs and big quarries, natural lakes, small permanent and temporary waterbodies, rivers and brooks; total). Below, information on the structure and abundance of the worms’ populations found in Vietnam is given, as well as data on their ecology and distribution, and, if necessary, taxonomic, morphological and other remarks and comments. A single asterisk before the taxon name denotes worms registered in Vietnam for the first time since the beginning of the present study on the meiobenthos; two asterisks denote new species found and already described during the research; three asterisks denote incomprehensible specimens that require further study and which are also possibly undescribed species, as they differ from the currently known putative congeners.

3.2. Checklist

In total, 65 representatives of the oligochaetes from the families Naididae and Enchytraeidae and 6 members of the family Aeolosomatidae were recognized in the studied material:

PHYLUM ANNELIDA Lamarck, 1809

CLASS CLITELLATA Michaelsen, 1919

Subclass OLIGOCHAETA Grube, 1850

Order TUBIFICIDA Jamieson, 1978

Family Naididae Ehrenberg, 1831

Subfamily Naidinae Ehrenberg, 1831

1. Allonais inaequalis (Stephenson, 1911)—{50, 51, 59}, (0, 9, 0, 0; 3).

Population structure and abundance: This is a relatively rare species and few in number in the studied material. The maximum amount (14 immature solitary individuals) was found in a floodplain lake in the washout from roots of Salvinia sp. (qualitative sample {51}; here and hereinafter, see Appendix A for more details about the named samples). Sexually mature worms were not found.

Ecology and distribution: The worm lives in various freshwater bodies, predominantly in non- or slightly flowing habitats, among aquatic vegetation, in colonies of sponges and detritus of sediments. It can actively swim by a wriggling movement. The species is recorded on all continents (except Antarctica), as well as in some island states of the Atlantic, Indian and Pacific Oceans. The range covers mainly tropical and subtropical regions [9,22,23,29,30,31]. Allonais inaequalis was repeatedly observed in South and Southeast Asia, including Vietnam [9,23,27].

2. Allonais pectinata (Stephenson, 1910)—{30, 40, 41, 50, 51}, (0, 9, 13, 0; 4).

Population structure and abundance: Only immature, complete and regenerating after fragmentation individuals were detected—from 1–2 to 15 (in qualitative sample {51}) and 23 (~9 thous. ind./m²; in {30}) specimens per sample.

Ecology and distribution: The ecology and lifestyle are similar to the previous species. Most of the finds are concentrated in the tropical and subtropical zones of South America, Africa, Australia and Asia, including Vietnam [9,23,27]. There is data on the discovery of the species in one waterbody of Slovakia (Europe) [32]. Indications of the presence of A. pectinata in North America need, apparently, to be rechecked [29,31].

3. *Amphichaeta sp.—{4, 12–14, 70, 71}, (9, 6, 0, 5; 5).

Population structure and abundance: Only immature solitary individuals and numerous chains of 2 zooids were registered. The total abundance varied from 1–2 to 15–20 thous. ind./m² (from 1–5 to 40–55 specimens in a sample). The most numerous populations were recorded in samples {4, 13, 14}.

Ecology and distribution: In the studied region, these oligochaetes were found in open areas with shallow depths and silty, silty–sandy or silty–clayey sediments in man–made reservoirs, a natural lake and a river. This is the first record of Amphichaeta in Vietnam.

Remarks: Initially, we attributed the first Amphichaeta individuals discovered in Vietnam to the common Holarctic species A. leydigi Tauber, 1879 based on the chaetae pattern, particularly due to the presence of 5 chaetae in the dorsal bundles of segment III (see text and Figure 3.2.1 on pages 177–178 in [2]); it should be noted that in this source on page 170 in Table 3.2.2 erroneously typed another species—A. sannio Kallstenius, 1892). However, subsequent samples showed that the chaetae number and pattern in the anterior part of the body in the Vietnamese specimens vary markedly even within the same population (sample). For example, as it turned out, in the mentioned bundles of segment III, 2 to 5 chaetae may be present (most often 4, but other variants are not rare). In addition to A. leydigi, the Vietnamese worms are also similar to the rare and still poorly studied Asian member of the genus A. asiatica Liang, 1958 and the more known and widespread A. sannio [32,33,34]. However, as far as can be judged from the studied fixed material, the combination of external morphological characters in the Vietnamese individuals (foremost, general chaetae pattern, their shape, length and thickness, as well as the length ratio of the chaetae teeth) does not fully correspond to that of these species. Thus, the Amphichaeta worms discovered in Vietnam at this stage cannot be unambiguously assigned to any already-known species of the genus. They surely require additional study, including examination of live material, which is necessary to observe the important diagnostic details of internal morphology (e.g., the number and location of ventral vessel loops, nephridia, etc.). It can be assumed that the specimens found by us may be a mix of different species or a possibly undescribed Amphichaeta species.

4. *Aulophorus flabelliger Stephenson, 1931—{16, 25, 26, 51, 82, 100}, (9, 6, 0, 5; 5).

Population structure and abundance: This is a species which is few in number. No more than 1–3 single specimens in a sample were registered. Sexually mature individuals were not found.

Ecology and distribution: The species dwells among aquatic vegetation and its remains, as well as at the bottom of freshwater bodies, mainly in shallow, stagnant or weakly flowing habitats. Worms build hyaline tubes with inclusions from organic and inorganic materials. Being out of the tube, they can swim. Aulophorus flabelliger occurs generally in tropical and subtropical regions. It is registered in Africa, Australia, Asia and North America [23,35,36]. There are also data on finds of the species in Europe (Italy) and South America [9]. Before the beginning of present studies of the meiobenthos [2], this species was not documented in Vietnam.

5. Aulophorus furcatus (Müller, 1773)—{15, 16, 19, 39, 40, 80, 84, 100}, (0, 6, 20, 7; 7).

Population structure and abundance: This species is relatively few in number. The maximum noted abundance was up to 7 individuals per sample (~3 thous. ind./m²; sample {40}). Most of the specimens found were single immature worms. Less often, chains of 2 zooids were registered. In sample {16}, there was 1 mature individual with a well–developed clitellum.

Ecology and distribution: This is one of the most known representatives of the genus. Species live among emergent, submerged or floating macrophytes, algae, their remains and in bottom sediments of a wide variety of freshwater bodies. Worms build attached or portable mucous tubes covered with foreign material. They can also swim freely. The species is cosmopolitan [9,22,23,32,37,38]. It was repeatedly noted in various areas and waterbodies of Vietnam [24,27].

6. *Aulophorus lodeni (Brinkhurst, 1986)—{9, 15, 16, 22, 27, 59, 61, 62, 64, 72, 100, 102}, (14, 15, 7, 7; 11).

Population structure and abundance: Aulophorus lodeni is the most common, but not numerous members of the genus in the studied waterbodies. In most samples, only 1–2 complete and/or incomplete (regenerating anterior or posterior end of the body after fragmentation) immature individuals were found. The maximum abundance (9 exemplars; ~3 thous. ind./m²) was noted in sample {61}.

Ecology and distribution: The species inhabits different types of freshwater bodies, from rivers and lakes to acid swamps and alkaline pools [39]. Literary data on lifestyle and ecology could not be found. They are probably similar to those of other congeners. In our material, the species most often occurred in shallow, plant-filled or open areas of reservoirs and lakes on silty sediments or substrates containing plant remains and detritus. Under original and synonymized names (see Remarks), this worm is known from South and North America (Florida), Martinique (Caribbean, Atlantic), Africa and Asia (Middle and South China, Bangladesh) [10,13,23,39,40]. The identification of specimens from Africa needs to be rechecked [39]. In Vietnam, prior to our studies [2], this species (including known synonyms) was not recorded.

Remarks: According to the database [7], A. lodeni combines several synonymized names of oligochaetes—Aulophorus pectinatus Stephenson, 1931, Dero (Aulophorus) intermedia Loden & Harman, 1982, D. (A.) lodeni Brinkhurst, 1986 and D. (A.) pectinatus (Stephenson, 1931) (for more detailed taxonomic history of the species, see, for example, [39] (p. 84)). Aulophorus pectinatus is the most known of these synonyms, and it is still listed as a separate species in some modern sources [22,23,40]. For Vietnam, A. lodeni was also initially identified by us as A. cf. pectinatus [2].

7. Aulophorus sp.—{59}, (0, 3, 0, 0; 1).

Population structure and abundance: We found 2 immature individuals.

Remarks: These specimens apparently belong to the same species (but not A. lodeni which is also present in this sample), which could not be correctly identified, since most of the dorsal chaetae were broken off. In the work [2], we preliminarily designated these specimens as Aulophorus cf. gravelyi Stephenson, 1925, but most likely this is a different representative of the genus.

8. Aulophorus tonkinensis (Vejdovský, 1894)—{50, 51}, (0, 6, 0, 0; 2).

Population structure and abundance: The species was found only in the qualitative samples of washouts from the roots of water hyacinth (Eichhornia crassipes) and salvinia (Salvinia sp.)—about 20 immature individuals in both cases. Several worms were in the process of budding, representing chains of 2 zooids.

Ecology and distribution: This is a freshwater species ecologically close to A. flabelliger. The worm builds tubes and can swim [9,34]. It is known from the tropical and subtropical regions of Central and South America, Africa, Australia and Southern Europe (Italy). Aulophorus tonkinensis was repeatedly noted in several countries of South, East and Southeast Asia, including Vietnam [9,10,22,23,25,27,41].

Remarks: Aulophorus tonkinensis belongs to a group of close members of the genus with moderately (unlike A. flabelliger) palmate needle chaetae, namely A. carteri Stephenson, 1931, A. ghanensis Hrabĕ, 1966, A. huaronensis Piguet, 1928 and A. schmardai (Michaelsen, 1905), which are sometimes difficult to distinguish morphologically [42,43]. According to [43], the group needs additional revision based on analysis of the reproductive apparatus, alimentary tract and branchial fossa in living specimens and electron microscopic studies of the needle chaetae. Individuals of A. tonkinensis found in Vietnam are most similar to the descriptions given for the African and Indian populations of this species [9,44].

9. *Branchiodrilus hortensis (Stephenson, 1910)—{15, 16, 58, 65, 67, 72, 75, 78, 79, 81, 82, 90, 91}, (0, 9, 13, 20; 12).

Population structure and abundance: There were only immature specimens. In the majority of samples, 1–3 single individuals or chains of 2–3 zooids were detected. The maximum abundance (8–10 exemplars; ~3–4 thous. ind./m²) was recorded in samples {16, 72, 78}.

Ecology and distribution: The worm dwells among aquatic vegetation and its remains at the bottom of various freshwater bodies. According to some sources, the species does not swim [9,34], according to others, it is a good swimmer [38]. The species is mostly distributed in tropical and subtropical regions of Africa, Australia, East and South Asia [9,22,23,32,34,38]. Recently, it has been also noted in some European countries (England, Netherlands, Russia), where it was probably introduced via aquaria [23,32,45]. There are reports of the appearance of the species in North America (Florida) too [31,45]. We failed to find cases of the discovery of B. hortensis in Vietnam before the start of the present studies of the meiobenthos [2].

Remarks: Branchiodrilus hortensis is one of the most common oligochaetes in the waterbodies of Vietnam that we studied. This and other species of the genus are characterized by the largest size (up to 50 mm) among the members of the subfamily Naidinae and, as was believed until recently, Branchiodrilus species are relatively easily distinguished from each other by external morphological characters—foremost, by the arrangement of gills and chaetae of various shapes on the body. However, the latest molecular investigations have shown that B. hortensis is apparently a complex of cryptic species, requiring additional detailed study [46,47]. Obviously, this applies equally to the populations living in Vietnam.

10. Branchiodrilus semperi (Bourne, 1890)—{5, 6, 25, 26, 69, 100}, (18, 0, 7, 2; 5).

Population structure and abundance: The species is rarer than the previous congener. There were 1–8 specimens per sample (up to 3 thous. ind./m²). The maximum number was registered in {5}. In the same sample, 4 mature worms were observed.

Ecology and distribution: The worm inhabits sediments of freshwater bodies, mainly silty. It makes burrows in mud and resides in them. This oligochaete does not swim. Branchiodrilus semperi has a narrower distribution than the previous member of the genus. Until now, it has been observed only in South and Southeast Asia, including Vietnam [9,22,23,27,38].

Remarks: As with B. hortensis, based on molecular data, the status of B. semperi as a separate species is questionable and requires additional research [47].

11. Branchiodrilus spp.—{59, 68, 73, 84, 87, 102}, (0, 3, 7, 10; 5).

Population structure and abundance: Single immature specimens were noted in a sample

Remarks: This species group unites damaged (incomplete) individuals that cannot be unambiguously assigned to whatever member of the genus.

12. **Bratislavia gusevi Gusakov, 2021—{72, 77, 79, 93}, (0, 0, 0, 10; 4).

Population structure and abundance: This is a rare, non-numerous species. The maximum number (6 individuals) was noted in the qualitative sample {79}. Only immature full worms or incomplete fragments with missing or regenerating anterior and/or posterior ends of the body were found.

Ecology and distribution: So far, the species has been detected only in 2 small forest streams in Vietnam. Possibly, it is rheophilic.

Remarks: Bratislavia gusevi is a recently described species discovered in Vietnam [48]. In a previously published list, it was given as Pristina sp. No. 2 [2].

13. Chaetogaster diastrophus (Gruithuisen, 1828) (+(?) Ch. langi Bretscher, 1896)—{2, 6, 8, 12, 14, 19, 25, 26, 30–32, 33–36, 38–40, 47, 50, 59, 60, 69, 71, 72, 77, 79, 80, 82, 87, 90, 91, 96, 98}, (27, 35, 33, 27; 30).

Population structure and abundance: Only asexual specimens were found—solitary individuals and chains consisting of 2, rarely 3, zooids. In most samples, there were no more than 1–5 specimens (up to 2 thous. ind./m²). The maximum abundance (20–45 individuals or ~8–17 thous. ind./m²) was recorded in samples {2, 31–33, 38, 39}.

Ecology and distribution: These worms live on the bottom, as well as among aquatic plants and sponge colonies, in a wide variety of freshwater bodies, mainly in the shallow zone [9,45,49]. There is also evidence that the species can penetrate into brackish water up to 8‰ [50]. It is cosmopolitan and well-known in Vietnam [22,23,27,32,40,51].

Remarks: This is one of the most widespread representatives of oligochaetes in all types of water bodies studied by us. The majority of the found individuals correspond to the descriptions of the “typical” Ch. diastrophus, as far as can be judged from the general size of the body, the type and size of the chaetae and a distinct prostomium with long sensory hairs, well expressed even in fixed individuals (Figure 2A) [9,32,34,45]. At the same time, the material also contains specimens without a clearly conspicuous prostomium (with a “blunt” anterior body end) (Figure 2B), which is typical for Ch. langi Bretscher, 1896—a close congener of Ch. diastrophus. Chaetogaster langi is also relatively widespread and known in Vietnam [27]. Unfortunately, the fixed material does not allow us to confidently judge whether individuals with vestigial prostomium are indeed Ch. langi or recently separated juveniles (zooids) of Ch. diastrophus. The difficulty of identifying Ch. diastrophus and Ch. langi in a fixed state, and even the possibility of them belonging to the same species, has been repeatedly discussed in the literature [9,45,52,53]. In addition, recent molecular studies have shown that Ch. diastrophus is probably not a monophyletic taxon but includes some cryptic species [46]. Thus, further research is needed, both on living individuals and at the molecular level, to resolve questions about the Vietnamese populations of the “diastrophus–langi” group.

14. *Chaetogaster cf. krasnopolskiae Lastočkin, 1937—{97}, (0, 0, 0, 2; 1).

Population structure and abundance: A single individual was found, probably a recently detached zooid (Figure 2C).

Ecology and distribution: This is a relatively rare, little-studied species. It is known from Eastern Europe (Russia, Ukraine, Belarus, Poland) [23,32,40,54]. There is also evidence of the discovery of similar specimens in the USA [55]. Obviously, it is a rheophilic worm, preferring flowing gravel–sand biotopes, including the hyporheic zone. Most of the finds known to date (and in Vietnam as well) were made in large and small rivers, streams or in recently built reservoirs [1,23,54,55,56].

Remarks: Chaetogaster krasnopolskiae is one of the known representatives of the genus with bifid chaetae that have partially reduced (shortened) upper teeth. The second species with similar chaetae is Ch. crocodilus Semernoy, 1985, endemic to Lake Baikal [57]. The known descriptions of Ch. krasnopolskiae, including the original one, are not complete enough, and they are based on few specimens, which is why the morphological variability of the species is still poorly understood [54,55,56]. Therefore, it is difficult to judge whether the individuals found in Europe, North America and Vietnam belong to the same or different, morphologically similar species. The first 3 specimens of a worm similar in chaetae to Ch. krasnopolskiae were discovered in Vietnam as early as 2008 in a lowland part of a small river (early June, water temperature 28 °C) [1]. Unlike European specimens, which have only 1–2 chaetae in bundles of segment II [32,54], these Vietnamese exemplars had 4 chaetae [2] (Figure 3.2.1, p. 178). Unfortunately, the material from 2008 was not kept, which does not allow a comparison of the Ch. cf. krasnopolskiae specimens that were first discovered in Vietnam with the current find. The present find was made in a small mountain river at an altitude of ~1500 m above sea level in December at a water temperature of 16.5 °C, which is consistent in terms of ecology with the occurrence of Ch. krasnopolskiae in the more northern latitudes of Europe and North America. The new specimen has 2–3 chaetae with a characteristic shortened upper tooth in segment II and 5–6 similar chaetae in the rest of the chaetiferous body segments (Figure 2D,F).

15. ***Chaetogaster sp.—{48, 50, 51}, (0, 9, 0, 0; 3).

Population structure and abundance: Qualitative samples {50, 51} contained 9–25 individuals, and 1 specimen was in {48}. All worms were solitary and immature (Figure 2E).

Ecology and distribution: So far, this species have been recorded only in 1 shallow, floodplain, forest lake. The main amount was noted in washes from the roots of floating plants (hyacinth and salvinia).

Remarks: These Chaetogaster individuals require additional study, including on living material. Perhaps this is an undescribed species. The main feature that distinguishes these Vietnamese specimens from other known congeners with bifid tips of chaetae [9,32,34,40,57] is relatively long and thin (especially in the posterior segments), slightly divergent and slightly curved teeth of the chaetae (Figure 2G–I). The upper tooth is 6.0–6.8 µm long, and the lower one is 4.6–5.1 µm long. All chaetiferous body bundles have 4 chaetae with a proximal nodulus. The chaetae length in segment II is 72–77 µm, and from segment VI onwards it is 49–62 µm. The prostomium in a fixed state is not expressed (Figure 2E). The general body size corresponds to the worms from the “diastrophus–langi” group (0.4–0.6 mm). Another species of the genus, Ch. limnaei von Baer, 1827, has similar long and thin teeth of the chaetae. However, it is larger than these Chaetogaster specimens from Vietnam (up to 5.0 mm), and the general number of chaetae per bundle in Ch. limnaei is usually 8–20 pieces [32,34].

16. *Dero cooperi Stephenson, 1932—{16, 40, 50, 59}, (0, 9, 7, 0; 4).

Population structure and abundance: There were mostly 1–3 individuals per sample and up to 11 (~4 thous. ind./m²) in {40}. Generally solitary, immature individuals were found. In {50}, there was also a chain of 2 zooids.

Ecology and distribution: Dero cooperi lives among aquatic vegetation and its remains at the bottom of various freshwater bodies, mainly stagnant. It builds mucous tubes with organic and inorganic inclusions and can swim [9,38,58]. The species is registered in several countries in South America, Africa, Europe, the Middle East and South Asia [9,13,23,38]. According to [22], it is also known from Australia. Somewhat earlier, Pinder [51] noted that the discovery of D. cooperi in Australia requires re-verification. This worm has not previously been recorded in Vietnam.

17. Dero digitata (Müller, 1773)—{6, 8, 26, 61, 62, 63, 65, 102}, (14, 9, 7, 2; 7).

Population structure and abundance: In most samples, there were only 1 to 3 specimens. In shallow forest water bodies {61, 62, 65}, up to 13–19 individuals per sample (~5–7 thous. ind./m²) were noted. Sexually mature individuals were not found. There were mostly solitary immature individuals, sometimes chains of 2 zooids.

Ecology and distribution: Dero digitata dwells among aquatic vegetation and at the bottom of different waterbodies, usually on silty and silty–sandy sediments. It builds mucus tubes with inclusions of sand and can swim. The worm tolerates a wide range of environmental conditions, including severe organic pollution and deficiency of oxygen [9,34,39,45,49]. It is cosmopolitan—one of the most widely known members of the genus, found in a wide variety of fresh and slightly brackish waters around the world [9,22,23,32,34,45]. This oligochaete was repeatedly recorded in waterbodies of Vietnam [27].

Remarks: As noted by [59], D. digitata probably includes a number of cryptic species.

18. Dero dorsalis Ferronière, 1899—{15, 26, 59, 61, 62, 65, 67}, (5, 12, 13, 0; 6).

Population structure and abundance: The maximum abundance (10–30 individuals in a sample; 4–11 thous. ind./m²) was recorded in small shallow forest waterbodies {61, 62, 65, 67}. In other samples, there were no more than 1–6 specimens (up to 2 thous. ind./m²). The main number of the worms found was represented by immature single individuals, sometimes by chains of 2 zooids. In {65}, 2 premature specimens with inchoate clitellum were present.

Ecology and distribution: This is a well-known member of the genus. It occurs in almost all kinds of freshwater habitats and lives on submerged vegetation and on the bottom on a wide variety of sediments, from muddy to sandy. The worm can build tubes from detritus and various plant materials. It does not swim [9,34,38,45]. According to most sources, D. dorsalis is distributed in Europe, Asia (South, East, Southeast) and Australia [9,22,23,32,40], although the discovery of the species in Australia seems to need confirmation [51]. The book [9] also mentions findings of this species in Uruguay and Argentina (South America). The species is common in Vietnam [27].

19. Dero indica Naidu, 1962—{5, 22, 58, 100}, (9, 3, 0, 2; 4).

Population structure and abundance: This is a rare and not numerous species. The maximum abundance (4–6 individuals in a sample; up to 2 thous. ind./m²) was noted in {5, 22}. In the same samples, 2–4 sexually mature individuals were also found. The remaining exemplars recorded were solitary young worms.

Ecology and distribution: The worm dwells in soft sediments and among the roots of floating macrophytes (hyacinth) in fresh waters, usually stagnant. It does not build tubes, lives freely and can swim [9,23,27]. So far, the species has been registered in several countries of South, Southeast and East Asia (India, Bangladesh, Vietnam and Japan) and on the Sakhalin and Kuril Islands [9,10,13,27]. According to [22], it is also known in the Australian biogeographic region, but where exactly is not indicated in this source.

Remarks: In external morphology, D. indica is very similar to D. digitata. Differences exist, in fact, only in the presence of 2 hair and needle chaetae in the dorsal bundles of the anterior and middle parts of the body in the first species, while D. digitata has only 1 chaeta of each type [9,34]. In this regard, it has been suggested that both species are possibly synonyms [34]. However, recent studies based on molecular methods have shown that D. indica is well separated from D. digitata [59].

20. Dero obtusa d’Udekem, 1855—{7, 10, 15, 16, 22, 25, 26, 29, 33, 35, 39–42, 45, 48, 50, 58, 59, 61, 64, 65, 69, 82, 100, 102}, (27, 29, 47, 7; 23).

Population structure and abundance: This is one of the most common species of oligochaetes in the studied material, but not numerous. As a rule, there were only 1–2 specimens in a sample, less often up to 4–6 (up to 2 thous. ind./m²). The most specimens found were single immature individuals and chains of 2–3 zooids. We observed 1 sexually mature individual in sample {22}.

Ecology and distribution: The species lives in a variety of stagnant and slow-flowing freshwater bodies, generally in shallow biotopes on sandy and silty–sandy substrates, as well as among the thickets of aquatic vegetation and roots of floating (hyacinth, etc.) plants. It builds mucus tubes and can also swim freely [9,37,39,45,49,58]. Dero obtusa is a widespread species, almost cosmopolitan. It is registered on every continent except Australia and Antarctica [9,22,23,32] and known in Vietnam from previous studies [27].

21. *Dero palmata Aiyer, 1929—{27, 61}, (5, 3, 0, 0; 2).

Population structure and abundance: In the first sample, there was only a solitary individual, but in {61}, the species was very numerous—74 exemplars (~27 thous. ind./m²), a significant part of which were chains of 2 zooids. Sexually mature worms were not found.

Ecology and distribution: This is one of the smallest members of the genus. Single immature individuals are usually no longer than 3 mm [9,60]. The ecology is little studied. We recorded it in small stagnant waterbodies at a depth of ~0.4 m on sandy and clayey substrates rich in detritus and various plant remains. In Guyana (South America), the species was found in a river at a depth of 0.3–3.5 m on muddy biotopes, including those with aquatic vegetation [61]. Dero palmata is a relatively rare species. To date, it has been noted only in India and 2 countries of South America (Guyana and Argentina) [9,22,23,61]. The present checklist is the first mention of the species in Vietnam and, apparently, Southeast Asia in general.

22. Dero pectinata Aiyer, 1929—{2, 8, 45, 50, 60, 71, 87}, (9, 9, 0, 5; 6).

Population structure and abundance: This species is not numerous. As a rule, there were 1–4 individuals in a sample. The maximum number (17 specimens; ~6 thous. ind./m²) was detected in {8}. Almost exclusively solitary immature individuals were presented, very rarely chains of 2 zooids. Sexually mature exemplars were not found.

Ecology and distribution: This is one of the smallest representatives of the genus, like the previous species (a single individual is up to 2.5 mm) [9,38,60]. The ecology is also still poorly known. In India, it was recorded in detritus and among green algae covering the muddy and sandy bottom in shallow, slow-flowing river habitats [38]. In our material, it was registered in various waterbodies in shallow stagnant or slow-flowing biotopes on clayey (and, as a rule, moderately silted) substrates, as well as in water hyacinth roots. The species occurs in countries of the tropical and subtropical belt in North, Central and South America, on the Caribbean islands and the Azores (Atlantic), in Africa and Australia [23,34,51]. In Asia, it is known from India and Vietnam [9,25,27].

23. Naidinae gen. spp.—{26, 27, 60, 73, 89, 100, 102}, (9, 3, 7, 7; 6).

Population structure and abundance: There were single immature individuals in each sample.

Remarks: This group of species unites specimens that could not be identified more precisely than the subfamily due to damage to the body and/or chaetae.

24. Nais pardalis Piguet, 1906—{8, 25, 26}, (14, 0, 0, 0; 3).

Population structure and abundance: This is a rare species and few in number. The maximum abundance (6 exemplars; ~2 thous. ind./m²) was noted in sample {8}. Only solitary immature individuals were found.

Ecology and distribution: The species dwells mainly in the littoral of moderately flowing and stagnant freshwater bodies on rocky, sandy and silty–sandy substrates, including among higher aquatic vegetation and in periphyton on stones. This oligochaete can swim [34,37,39,45,49,62]. It is a well-known and relatively widespread representative of the genus. According to various sources, N. pardalis has been registered on all continents except Antarctica [9,22,23,32,39]. Previously, it was repeatedly noted in Vietnam [27].

25. Nais spp.—{18, 68, 102}, (0, 3, 7, 2; 3).

Population structure and abundance: There were single immature specimens in a sample.

Remarks: This taxon includes Nais individuals whose exact species could not be determined for various reasons.

26. *Nais variabilis Piguet, 1906—{99}, (0, 0, 0, 2; 1).

Population structure and abundance: We found 1 immature specimen.

Ecology and distribution: This is a well-known and common member of the genus. The worm lives in a wide diversity of freshwater bodies, but it is also found in brackish habitats. As a rule, the species prefers shallow-water biotopes rich in aquatic vegetation and/or algae, although it also occurs in open littoral and moderately flowing habitats. This species swims well [9,37,39,45,49]. It is cosmopolitan, not yet registered only in Antarctica [9,22,23,32,34,39,49]. In Vietnam, N. variabilis has not previously been recorded.

Remarks: As shown by recent molecular studies, the individuals usually identified as N. variabilis obviously represent a complex of various morphologically similar species [46].

27. Slavina appendiculata (d’Udekem, 1855)—{45, 48}, (0, 6, 0, 0; 2).

Population structure and abundance: There were 1–2 individuals per sample. Only immature specimens were found, including 1 chain of 2 zooids.

Ecology and distribution: This is a widely known species. It occurs in a large variety of freshwater bodies and habitats on different bottom sediments, among aquatic plants and their roots and in colonies of sponges and bryozoans. However, it seems to prefer shallow, muddy areas rich in thickets. Slavina appendiculata is one of the typical species of oligochaetes in swamp systems. The species does not swim [9,37,39,45,49]. It is cosmopolitan, not found only in Antarctica [9,22,23,32,34,39]. This worm is known in Vietnam from previous research [27].

28. *Slavina evelinae (Marcus, 1942)—{6, 17, 18, 25, 26, 50, 51}, (14, 12, 0, 0; 6).

Population structure and abundance: The maximum number (10–18 individuals) was noted in the qualitative sample {50} (a washout from the roots of water hyacinth) and in {17, 18} (up to ~7 thous. ind./m²). Other samples contained 1–3 individuals. There were only solitary complete and regenerating after fragmentation immature specimens.

Ecology and distribution: This is a less known and common representative of the genus than the previous one. The ecology and biology are probably similar to S. appendiculata. Judging by our finds and the literature data [61,63,64,65,66], S. evelinae prefers habitats of shallow, weakly flowing and stagnant freshwater bodies, rich in vegetation and its remains. It also occurs among roots of floating aquatic macrophytes (Eichhornia, Pistia). So far, the species has only been recorded in Central and South America and the Caribbean [22,23,34,64]. Apparently, this is the first mention of the species outside the named regions.

Remarks: Considering the findings of S. evelinae in the roots of water hyacinth, which is of neotropical origin, and Pistia, which is used as a decorative plant in aquaria, the introduction of the worm together with these plants into the waterbodies of Vietnam is quite probable.

29. Stephensoniana trivandrana (Aiyer, 1926)—{2, 4–6, 8, 10, 12–14, 25, 26, 43–45, 47, 49, 52–54, 100, 102, 105}, (41, 29, 0, 7; 20).

Population structure and abundance: This is one of the most common species of oligochaetes in the studied waterbodies, especially in the littoral and shallow water zones of reservoirs and lakes. In most samples, from 1 to 9 individuals were found. The worm was more numerous in {4, 5, 14, 52–54}—12–18 exemplars or ~4–7 thous. ind./m². Mature individuals were not noted. Only young, single specimens and chains of 2–3 zooids were presented.

Ecology and distribution: The species lives mainly on silted bottom sediments of freshwater bodies, including rivers. It can swim [9,34,39]. Stephensoniana trivandrana is distributed generally in tropical and subtropical regions of North and South America, Africa and Asia [9,22,23,39], but on the east coast of the Asian continent, the species range reaches Japan, Sakhalin Island and the Kuril Islands [10,23]. There is also evidence of the finding of species individuals in the hyporheic zone of one of the New Zealand rivers [67]. This oligochaete was repeatedly noted in waterbodies of Vietnam [27].

30. Stylaria fossularis Leidy, 1852—{50, 53, 95}, (0, 6, 0, 2; 3).

Population structure and abundance: This is a rare, not numerous species. There were no more than 1–3 individuals per sample. Only immature solitary specimens were recorded.

Ecology and distribution: The worm dwells in stagnant and moderately flowing freshwater bodies, primarily among algae and macrophytes, but it can also be found in bottom sediments in areas devoid of aquatic vegetation. This species swims well [9,41,49,62]. According to the generalized data, it is cosmopolitan. So far, S. fossularis has not been registered only in Antarctica [9,22,23,40]. For Vietnam, the species is known from previous studies [27].

Subfamily Opistocystinae Černosvitov, 1936

31. ***Trieminentia sp.—{49, 52, 59, 61, 82}, (0, 12, 0, 2; 4).

Population structure and abundance: This is a relatively rare worm and few in number. The maximum abundance (6–8 individuals or ~2–3 thous. ind./m²) was recorded in {49, 59}. In other samples, no more than 1–2 specimens were present. Only immature solitary individuals and 1 with an inchoate budding zone after segment XIV were found.

Ecology and distribution: The ecology and biology of the genus Trieminentia (with its only species, T. corderoi (Harman, 1970)), to which our individuals presumably belong (see Remarks), as well as of the entire subfamily Opistocystinae, are still poorly studied. Usually, opistocystins are found in shallow freshwater habitats rich in living and dead vegetation [39,68,69]. In similar biotopes, the worm was also detected in Vietnam (see Appendix A). The center of origin of the subfamily Opistocystinae, apparently, is the tropical and subtropical regions of North, Central and South America, where the main finding points of all its few species (including T. corderoi) are concentrated thus far [22,23,69,70]. Information about the findings of Opistocystinae members outside the indicated area is only available in single works. Thus, Brinkhurst [71] notes the discovery of 2 immature specimens, presumably of Opistocysta funiculus Cordero, 1948, in the White Nile (Africa), while, according to [68], representatives of the subfamily—the authors do not specify which genera and species—also occur in Asia, namely in Nepal, Indonesia (Kalimantan Island) and possibly Malaysia. In Vietnam and adjacent countries, no members of the Opistocystinae have been observed to date.

Remarks: Due to the presence of three distinct posterior appendages (Figure 3A–C), the immature worms found in Vietnam clearly belong to the Opistocystinae and, judging by the similarity of morphology, to the same species. The main distinguishing feature of the subfamily’s genera is the structure of the reproductive system. In the absence of sexually mature individuals, the complex of all other characters that we were able to consider in our specimens does not correspond to any of the currently known species of the subfamily. Thus, all exemplars found in Vietnam have smooth hair chaetae, which distinguishes the genus Trieminentia with the only species T. corderoi from other Opistocystinae genera [39,70,72,73]. Milligan [39] points out that the lack of serrations on the hair chaetae, along with the reproductive system, seems to be a fairly characteristic and persistent feature between the opistocystins. Taking into account the smoothness of the hair chaetae, we have for the time being assigned the specimens from Vietnam to the genus Trieminentia. However, the Vietnamese specimens have several notable differences from the available descriptions of T. corderoi [39,70,72,73], such as the longer hair (150–430 μm vs. 72–360 μm in T. corderoi) and needle (50–64 μm vs. about 30 μm in T. corderoi) dorsal chaetae (Figure 3D,G), the greater number of hair and needle chaetae per bundle (up to 3 of both types vs. 1–2 in T. corderoi) and the distinctly proximal or near–median location of the nodulus on the ventral chaetae (vs. distal in T. corderoi) (Figure 3E,F). Thus, it is obvious that the Vietnamese worm requires further study for the accurate identification, including the discovery and analysis of sexually mature individuals. Perhaps it is a not-yet-described species from Trieminentia. However, it also cannot be ruled out that the specimens from Vietnam represent a morphological form of T. corderoi, the description of which is still scarce, and the variability of its characters has not been studied at all [39,70,72,73].

Subfamily Pristininae Lastočkin, 1921

32. Pristina (Pristina) aequiseta Bourne, 1891—{8, 15, 18, 19, 25, 26, 30, 39, 45, 47, 48, 50, 51, 59, 79, 87, 91, 92}, (14, 26, 13, 10; 16).

Population structure and abundance: This is one of the most common representatives of the genus and oligochaetes in general in the studied region. It was relatively numerous in some samples. There were 65 and 37 specimens (~24 and 14 thous. ind./m²) in {30} and {39}, respectively, 15–18 specimens (~6–7 thous. ind./m²) in {8, 47, 92}, and 1–8 specimens (up to 3 thous. ind./m²) in the rest of samples. Sexually mature individuals were not found. In most samples, both solitary young worms and chains of 2–3 zooids were present.

Ecology and distribution: This is a well-known and widespread species. It lives in a wide variety of freshwater bodies, mainly in the shallow zone, in different bottom sediments, among aquatic vegetation and its roots, algae, sponges and bryozoans. It was also noted in brackish waters, up to 6.5‰. The worm does not swim. It is cosmopolitan, not recorded only in Antarctica [9,23,32,39,45]. The species was repeatedly recorded in Vietnam [27].

Remarks: Pristina aequiseta has a morphologically similar form—”foreli”. External differences between them only consist of the presence of giant chaetae in the ventral bundles of segments IV–V in the form “aequiseta” and the absence of such chaetae in “foreli”. The question of whether both forms belong to the same or different species has often been raised in the literature. Some authors consider P. foreli (Piguet, 1906) to be an independent species [9,34,45,49,54], while others treat it as a synonym of P. aequiseta, i.e., as an ecomorph devoid of the giant chaetae [22,39,40,42,74]. In our material, both forms were found together in most samples, and no morphological differences except the presence/absence of the giant ventral chaetae were observed between them. However, a recent integrated study by [75], which also included a molecular analysis, showed that P. aequiseta and P. foreli are most likely distinct, independent species. Given that study, in the present checklist, we also consider them separately (see the paragraph about P. foreli below).

33. *Pristina (Pristina) biserrata Chen, 1940—{19, 41, 50, 51, 59, 60, 100}, (0, 12, 13, 2; 6).

Population structure and abundance: This is a relatively rare species and not numerous. In quantitative samples, there were no more than 1–4 individuals (up to ~1 thous. ind./m²). In washes from the roots of aquatic macrophytes {50, 51}, there were 8–16 specimens. Sexually mature exemplars were absent. Only solitary immature individuals and 1 chain of 2 zooids were noted.

Ecology and distribution: Judging by the literature and our data, the species prefers abundantly overgrown areas of stagnant freshwater bodies, including swampy ones, with aquatic vegetation, but it also dwells in moderately flowing waters. It can swim [34,76]. The worm occurs in bottom sediments rich in detritus and/or plant residues, in periphyton covered aquatic vegetation, in the rhizosphere of floating macrophytes and in sponge colonies [12,18,41,61,62,77]. According to the generalized information, so far, the species has been registered in East, South and Southeast Asia, the Indonesian Islands, South America and the Caribbean [10,12,18,23,38,40,62,78]. In the checklist of [22], it is also indicated for the Australasian biogeographical region, but the specific find sites are not given in this source. In Vietnam, the species was first discovered during the present studies of the meiobenthos [2].

34. *Pristina (Pristina) foreli (Piguet, 1906)—{8, 15, 25, 26, 30, 38, 39, 42, 45, 47, 77–80, 82, 87, 91, 92, 100}, (14, 12, 20, 22; 17).

Population structure and abundance: This is a quite common species and in places relatively numerous. The maximum abundance (24–34 individuals; ~9–13 thous. ind./m²) was noted in samples {8, 30, 91}. Most of the remaining samples contained 5–12 individuals (up to ~4 thous. ind./m²); less often, only 1–3. In each habitat, as a rule, both solitary immature specimens and chains of 2–3 zooids occurred. Sexually mature worms were not found.

Ecology and distribution: The species inhabits a wide spectrum of freshwater bodies and biotopes. It lives in bottom sediments, among aquatic vegetation and its roots, algae and moss. The worm is able to withstand low pH values (4.1–4.2). It does not swim. This species is cosmopolitan, not recorded only in Antarctica [9,23,32,39,45]. In Vietnam, P. foreli was not previously directly noted, but, possibly, it was recorded together with P. aequiseta [27] as its morphological form (see Remarks to P. aequiseta above).

35. Pristina (Pristina) longiseta Ehrenberg, 1828—{8, 15, 19, 26, 30, 32, 33, 41, 45, 48, 50, 51, 59, 60, 86, 92, 98}, (9, 29, 13, 7; 15).

Population structure and abundance: This is one of the most common members of the genus in the studied region, especially in the samples from natural lakes. As a rule, the species was not numerous (1–6 individuals per sample; up to ~2 thous. ind./m²), but in some samples, its abundance reached considerable values—119 individuals (~44 thous. ind./m²) in {32}, 36 individuals (~13 thous ind./m²) in {92}, 17 individuals (~6 thous. ind./m²) in {33, 59}. In qualitative samples of washes from the roots of hyacinth and salvinia {50, 51}, 10–11 specimens were noted. Almost everywhere, along with solitary worms, there were also budding individuals, usually consisting of 2, less often 3 zooids. Sexually mature specimens were not found.

Ecology and distribution: This is one of the most known and widespread representatives of the genus. It occurs mainly in the shallow zone of various stagnant, or less often flowing, freshwater bodies among higher vegetation, algae, colonies of sponges and bryozoans. The species has also been noted among the roots of floating macrophytes and in bottom sediments, mainly rich in detritus and plant remains. It does not swim [9,45,49,58,62]. This oligochaete is cosmopolitan, not registered yet only in Antarctica. It is common in Vietnam [9,22,23,27,32,40].

36. Pristina (Pristina) proboscidea Beddard, 1869—{96}, (0, 0, 0, 2; 1).

Population structure and abundance: We found 3 immature solitary individuals.

Ecology and distribution: The species inhabits rivers and streams, various lentic fresh waters and swamps. It lives in bottom sediments, among aquatic macrophytes, in colonies of sponges and bryozoans. This worm does not swim [9,39,45,62,79]. Pristina (P.) proboscidea is nearly cosmopolitan. It is distributed mainly in tropical and subtropical regions of South America and on the islands of the Caribbean, in Africa, South, East and Southeast Asia, Australia and New Zealand. The species was also registered in some countries of North America, Europe and Western Asia [9,22,23,34,39,45,51,79]. According to [27], P. (P.) proboscidea is well-known in Vietnam.

37. *Pristina (Pristina) synclites Stephenson, 1925—{16, 25, 26, 39, 79, 81, 91, 92}, (9, 3, 7, 10; 7).

Population structure and abundance: In most samples, 1–5 individuals were present (up to 2 thous. ind./m²), and only in some was the species more numerous—24–26 individuals (~9–10 thous. ind./m²) in {39, 92} and 73 individuals in qualitative sample {81}. The worms were mainly represented by solitary immature individuals, less often by chains of 2 zooids. No sexually mature individuals were found.

Ecology and distribution: This species is common in rivers and streams, but also found in other freshwater bodies. It lives in soft bottom substrates, as well as among filamentous algae and floating macrophytes. It has also been noted in colonies of bryozoans. This worm does not swim [9,37,38,39,45,80]. The species is registered in North and South America, Africa and in one European country (Netherlands). It is relatively widespread in the countries of South, Southeast and East Asia [9,23,32,39,45,80]. According to [22], it is also known from the Australasian biogeographic region. In Vietnam, prior to the present studies of the benthic meiofauna [1], the species was not recorded.

38. ***Pristina (Pristina) sp.—{92}, (0, 0, 0, 2; 1).

Population structure and abundance: The worm was found in a single sample, but in large number—82 individuals (~30 thous. ind./m²). The composition included immature single individuals and chains of 2–3 zooids, as well as 12 mature individuals.

Ecology and distribution: This Pristina member was found in a small forest stream on clayey ground with pebbles. Judging from the remains covering some specimens, the worms apparently build mucus tubes with inclusions of sand and other foreign material.

Remarks: Despite the large number of individuals found, including sexually mature ones, it has not yet been possible to accurately identify the species. Currently, the study of these worms is ongoing. Perhaps this is an undescribed representative of the genus. By the presence of a proboscis, the absence of particularly elongated hair chaetae and single-pointed needle chaetae in the dorsal bundles, this worm is most similar to P. proboscidea [9,79]. However, the Vietnamese specimens differ from the latter in the shape of the needle chaetae (curved vs. straight in P. proboscidea) and their noticeably longer length (75–85 μm vs. 42–54 μm in P. proboscidea), as well as in several other features. The whole complex of morphological signs of Pristina sp. from Vietnam, which we considered, also does not fit other similar congeners, such as P. biserrata and P. longiseta.

Regarding the genus Pristina in Vietnam as a whole, it should also be added that the authors of [27] mention 2 representatives, P. vietnamica Thai and P. dangi Thai, which apparently were described directly in Vietnam. Unfortunately, the authors provide neither the year of description of these species nor a reference to the corresponding literary source. Because the subgenus is not indicated in the book [27] too, it also remains unknown whether P. vietnamica and P. dangi have a proboscis (subgenus Pristina) or not (subgenus Pristinella). So far, the search for the source describing P. vietnamica and P. dangi, as well as any other works where they are mentioned and illustrated, has not been successful. There are no data on these species (including lists of synonyms, doubtful species, etc.) and in the main world databases on oligochaetes [7,23]. Thus, at the moment, we cannot exclude the possibility that the Pristina (Pristina) sp. we are considering here, as well as other unidentified representatives from the subgenus Pristinella listed below in the present checklist, may actually be one of the above-named “lost” species, which are already known from Vietnam.

39. *Pristina (Pristinella) acuminata Liang, 1958—{101}, (0, 0, 0, 2; 1).

Population structure and abundance: We found 2 immature individuals.

Ecology and distribution: This is a relatively rare species. The ecology and biology are poorly studied. According to the literature data, it occurs mainly in shallow, warm, stagnant or slow-flowing waterbodies with rich vegetation on silty, gravel, sandy and mixed soils [37,38,81,82]. Harman et al. [83] noted the species in the composition of the fauna of bromeliad pools. Until recently, the species has been recorded in South, East, Southeast and West Asia, as well as in North and South America [9,22,23,31,40,81,82]. In Vietnam, the species was first noted by us in one of the rivers in the material of 2008, which was not kept [1].

40. *Pristina (Pristinella) aequidentata (Liang & Xie, 1997)—{6, 12–14}, (9, 6, 0, 0; 4).

Population structure and abundance: The maximum abundance was recorded in {6, 12}—47 (~17 thous. ind./m²) and 18 (~7 thous. ind./m²) individuals, respectively. The remaining samples contained 1–3 worms. Most of the exemplars were solitary, non-budding individuals. Sporadically, chains of 2 zooids were also encountered. Sexually mature specimens were absent.

Ecology and distribution: This is a rare, little-studied species. Judging by the literature and our data, it occurs in shallow waters of lakes and reservoirs on silty–sand and silty sediments, as well as in thickets of macrophytes (reed) [84]. Before the present study of meiobenthos in Vietnam [2], the worm was recorded only in South China and in the south of the Russian Far East (Lake Hanka) [23,84].

41. *Pristina (Pristinella) cf. bilobata (Bretscher, 1903)—{75, 81, 86, 94, 105}, (0, 0, 0, 12; 4).

Population structure and abundance: This is a relatively rare species and few in number. In most samples, there were 1–3 specimens. The maximum abundance (11 exemplars; ~4 thous. ind./m²) was recorded in {94}. Only single immature worms and 1 chain of 2 zooids were noted.

Ecology and distribution: Pristina bilobata inhabits various freshwater bodies but apparently prefers rivers and streams. It is recorded on silty, sandy, gravel, stony and mixed substrates, in interstitial waters, as well as in macrophytes thickets and peat bogs [45,49,54,85,86]. It is mostly known from Europe and Western Asia. There are also single finds in South America (Argentina), Africa (Cameroon) and South Asia (Sri Lanka) [22,23,32]. The mention of its find in Australia requires confirmation [51]. In Vietnam, the species was first noted during the current studies of meiobenthos [1,2].

Remarks: The specimens found in Vietnam generally fit the known descriptions of P. bilobata, and they are clearly not juveniles of some Tubificinae, with which they may be confused when immature [32,34,40,49]. However, in some specimens, variations in the length of teeth of the needle chaetae were observed, which are not typical for the species—either both teeth could be shortened relative to the “normal” state for the species, or a partial and even complete reduction of the upper tooth was observed, due to which the needle looked single-pointed. Furthermore, the “normal” bifid needles and modified ones (in different variants) were usually found on different segments in the same individual. Given the small number of specimens found, it is clear that populations of P. cf. bilobata from Vietnam need further study.

42. Pristina (Pristinella) jenkinae (Stephenson, 1931)—{25, 26, 72, 77, 99}, (9, 0, 0, 7; 4).

Population structure and abundance: This is a relatively rare species and not numerous in the studied material. There were up to 10 individuals in qualitative samples {25, 26} and not more than 4 individuals in quantitative ones. All specimens found were non-sexual and non-budding.

Ecology and distribution: Pristina (P.) jenkinae is a widely-known species. It occurs in various stagnant freshwater bodies, but, apparently, prefers shallow, running habitats, where it lives on a diversity of hard substrates (sandy, sandy–silt, gravel, rocky, etc.), as well as in the hyporheic zone. Occasionally, the worm is found among various aquatic vegetation, algae and mosses. It is also known from temporary forest pools such as bromeliad tanks and stump holes. Possibly, Pristina (P.) jenkinae is amphibious, as it has also been recorded in moist forest soils. It does not swim [9,42,45,55,67,87]. The species is cosmopolitan and known from all continents except Antarctica [9,23,32,40]. It was previously observed in Vietnam [27].

Remarks: In some sources, including recent ones, P. jenkinae appears as a synonym for P. (Pristinella) rosea (Piguet, 1906). The taxonomic history and comparison of these two and some other closely-related representatives of the genus (the “rosea” group) are considered in more detail in the works [12,45,88,89]. In this paper, as declared above, we adhere to the taxonomy given in the WoRMS database [7], which considers P. jenkinae to be a distinct species.

43. *Pristina (Pristinella) osborni (Walton, 1906)—{32, 33, 35, 41, 42}, (0, 9, 13, 0; 4).

Population structure and abundance: This is a rare but locally numerous species. The maximum abundance (88 individuals; ~33 thous. ind./m²) was recorded in sample {32}. In other quantitative samples, it ranged from 2 to 6–10 specimens (up to 2–4 thous. ind./m² in {33, 41}). In the qualitative sample {35}, it was 19 individuals. Sexually mature worms were not found. Most specimens collected were solitary young worms. Everywhere there were also chains of 2 zooids (up to 10 exemplars in {32}).

Ecology and distribution: This is a widespread member of the genus. It inhabits a wide range of freshwater bodies and biotopes, from lakes and rivers to swamps, streams and caves. The species occurs in bottom sediments, including in the hyporheic zone, among various aquatic vegetation, and in periphyton. It has also been registered in wet soils and mosses, thus being an amphibiotic species of the oligochaetes. This worm does not swim [9,45]. The species is cosmopolitan. It has been found on every continent except Antarctica [9,22,23,32,40]. In some sources, for instance [9], this species appears under the name P. (Pristinella) minuta (Stephenson, 1914), which is currently recognized as a synonym for P. (Pristinella) osborni [7]. Prior to the present studies of meiobenthos, the species was not recorded in Vietnam. It was first mentioned here as P. cf. osborni in the works [1,2].

44. *Pristina (Pristinella) sima (Marcus, 1944)—{8, 42, 72, 77, 79, 82, 86, 87, 90, 91, 105}, (5, 0, 7, 22; 10).

Population structure and abundance: The species was relatively numerous in some samples. There were 61 individuals in the qualitative sample {79}. In quantitative samples {77, 86}, 28–33 individuals (~10–12 thous. ind./m²) were recorded, and in {8, 72} 8–10 specimens (~3–4 thous. ind./m²) were noted. In the rest of the samples, there were 1–5 exemplars (up to 2 thous. ind./m²). Mostly immature solitary individuals, rarely chains of 2–3 zooids, occurred. In {77},1 sexually mature specimen was found.

Ecology and distribution: Despite the rather wide distribution, the ecology of this species is still not well understood. To date, P. sima has been reported in both fresh and brackish estuarine waters on various sediments, sometimes among sparse thickets of various vegetation and algae. However, most likely, it prefers flowing biotopes in rivers and streams with hard substrates (sandy, silty–sand, pebbly, mixed). It is also common in hyporheic zones [37,45,55,61,90]. This is a widespread, but not often occurring species. According to the cumulative data, it is currently registered on all continents except Antarctica [10,22,23,32,40,51,90]. The species was not previously referred to in Vietnam.

45. ***Pristina (Pristinella) sp. No. 1—{71}, (0, 0, 0, 2; 1).

Population structure and abundance: In total, 3 single immature individuals and a chain of 2 zooids were found.

Ecology and distribution: These specimens were discovered in the riparian zone of a river on silty clay among stones.

Remarks: Due to the small number of found individuals, the absence of sexually mature specimens among them, and the general high morphological variability of some Pristina species, these worms require additional study. It is possible that this is an undescribed member of the genus. The body lengths of the single specimens are 0.9–1.6 mm (Figure 4A), and the chain of zooids is 2.2 mm. The budding zone is located after segment XI. Dorsal bundles consist of alternating hair and needle chaetae, from 3–5 of each kind in the anterior to 1–2 in the posterior parts of the body. The former are visually smooth (at 1500× magnification with using immersion and DIC contrast), relatively short (maximum somewhat larger than the corresponding body diameter), slightly curved, and with a relatively blunt tip (Figure 4B). The needle chaetae in all individuals on all segments are unidentate, nearly 35–40 μm long, slightly curved in the dorsal third, with a weak nodulus below the curve and a relatively blunt, rounded tip (Figure 4D). Ventral chaetae are all of the same type, slightly longer than others on segment II, up to 3–4 in anterior bundles and 1–2 in posterior ones. Their upper tooth is always thinner than the lower one and somewhat longer in several anterior segments. Further, the teeth are approximately equal in length.

The absence of a proboscis, numerous chaetae of each type in the dorsal bundles in the anterior body part (usually no more than 1–2 of hair and needle chaetae throughout the body) and simple-pointed, sigmoid needle chaetae are a unique combination of features of these individuals, not known in other representatives of the genus. Pristina acuminata and P. aequidentata, considered above lack proboscises and have alternating hair and needle chaetae in dorsal bundles in a quantity similar to P. (Pristinella) sp. No. 1. However, the needles in both these species are bifid. Pristina (Pristinella) menoni (Aiyer, 1929) has a very fine, reduced or sometimes even completely absent upper tooth of the needle chaetae, but there are only 1–2 hairs and needles per bundle in this species. Among the representatives of the subgenus Pristina (the species with a proboscis, which is occasionally absent in young individuals), P. (Pristina) proboscidea and P. (Pristina) foreli are closest to P. (Pristinella) sp. No. 1. Both have up to 4 hair and needle chaetae in the dorsal bundles. However, the needles of the former, although unidentate, are straight and without a nodulus, while P. (Pristina) foreli has bifid needles [9,32,34].

We also recall that the issue of P. (Pristinella) sp. No. 1 and the other still unidentified Pristina members possibly belonging to undescribed species remains in a “suspended condition” until the descriptions of 2 “puzzling” species from Vietnam, P. vietnamica and P. dangi [27] (see Remarks to P. (Pristina) sp. above), are found, and they will either be redescribed or declared as nomina dubia.

46. ***Pristina (Pristinella) sp. No. 2—{81, 91, 92}, (0, 0, 0, 7; 3).

Population structure and abundance: A total of 2 individuals were present in the qualitative sample {81}, and 6–9 individuals (~2–3 thous. ind./m²) were recorded in the rest samples. All worms were immature, 1 represented a chain of 2 zooids.

Ecology and distribution: These specimens were found in small forest streams, in weakly flowing sections on clayey–pebble substrates with plant litter. Judging by the remains of the body, the worm lives in mucus tubes with various inorganic inclusions.

Remarks: This is another taxon that possesses all signs of belonging to the genus Pristina, subgenus Pristinella, and which differs from all other known species in several morphological characters and requires additional study. These are quite large worms for Pristina. The chain of 2 zooids in a fixed state is 6.8 mm long, while solitary immature individuals are 4.0–5.0 mm long. The budding zone in the dividing individual is after segment XXX. The dorsal bundles bear 1–2 hair and needle chaetae each (Figure 4C,E–G). The former are clearly hispid (at 900–1500× magnification with using DIC contrast and/or immersion), approximately equal to the body diameter at the anterior and posterior ends and nearly 1.8–1.9 times the body diameter in its middle (up to 500–600 μm long). The needles are 70–80 μm long, almost straight and relatively stout. Some of the needles are distinctly single-pointed with a clearly rounded, blunt tip (Figure 4E). At the same time, the needles with a peculiar bifid end also often occur, especially in the second half of the body. Their tips split into 2 comparatively small (but usually visible, even at a magnification of 600× due to the general big thickness of the chaetae), approximately equal, apical teeth (Figure 4F,G). Ventral chaetae are alike throughout the body, 4–6 per bundle anteriorly, and 1–2 posteriorly. In several first segments, both teeth of the ventral chaetae are nearly equal in length. Further, the upper tooth becomes slightly shorter than the lower. The upper tooth is always thinner.

In general, it is still not completely clear whether P. (Pristinella) sp. No. 2 is a morphological variation of some close congeners, or whether it is an undescribed species. In shape and thickness of the needle chaetae in the single-pointed variant, P. (Pristinella) sp. No. 2 is most similar to P. (Pristinella) menoni. However, in the bifid variant of the needles, the upper tooth in P. (Pristinella) menoni is situated not apically next to the second one as in P. (Pristinella) sp. No. 2, but strongly subapically (displaced proximally). In addition, the hair chaetae of P. (Pristinella) menoni are smooth rather than hispid as in P. (Pristinella) sp. No. 2 [9,32,34].

47. ***Pristina (Pristinella) sp. No. 3—{13, 71, 81}, (0, 3, 0, 5; 3).

Population structure and abundance: This is a not numerous worm. There were no more than 1–2 individuals in a sample. Only immature solitary specimens and a chain of 2 zooids were found.

Ecology and distribution: This oligochaete was recorded at shallow depths in a lake, river and stream on silty and clay substrates.

Remarks: This representative of the genus requires further study. It is possibly an undescribed species. The maximum body length is 3.8 mm. The budding zone is after segment XXII. The dorsal bundles contain 1, sometimes 2, hair and needle chaetae. The former are weakly hispid, not long, maximally slightly longer than the corresponding body diameter (Figure 4H). Their tips are common, thin and sharp. The needle chaetae are mostly bifid, with comparatively long, parallel, equal (rarely upper slightly shorter) teeth of the “P. bilobata” type (Figure 4K,L). However, there are also trifid needles with a thin intermediate tooth, minute but already clearly visible at 600× magnification (Figure 4M,N). In addition, in 2 individuals, single-pointed needles with blunt, widely rounded tips were also noted at the end of the body (Figure 4I,J). All types of needles are markedly curved in their dorsal third. The ventral chaetae are all similar in length and shape, up to 6–7 per bundle in the anterior and 1–3 in the posterior body parts. Their upper tooth is nearly 1.5 times as long as the lower one on the anterior and 1.2–1.3 times on the posterior segments.

In terms of the shape and length of the main teeth of the needle chaetae, P. (Pristinella) sp. No. 3 is most similar to P. bilobata, but the latter has no trifid or single-pointed needles. The trifid needles are known in P. (Pristinella) trifida Collado & Schmelz, 2002, and they are also sometimes noted (primarily in young specimens) in P. (Pristinella) silvicola Collado & Schmelz, 2000, P. (Pristinella) osborni and P. (Pristinella) sima. However, both the length and shape of the teeth of the needles in all these species are clearly different from those in P. (Pristinella) sp. No. 3, even without taking into account the entire set of the needles and discrepancies between some other morphological features [32,34,45,90,91,92].

Subfamily Rhyacodrilinae Hrabě, 1963

48. Branchiura sowerbyi Beddard, 1892—{6, 14, 15, 25, 26, 81}, (14, 6, 0, 2; 5).

Population structure and abundance: In the studied material, the species was relatively rare and few in number. In quantitative samples, there were no more than 1–2 individuals. In qualitative samples {25, 26, 81}, up to 5–9 specimens were recorded. Sexually mature worms were not found.

Ecology and distribution: This is a widespread and well-known species. It lives in a variety of fresh and, occasionally, in slightly brackish waterbodies, mainly on silty enriched with organic material sediments, in open and overgrown areas, including heavily polluted and anaerobic [9,38,45,93]. The worm is cosmopolitan, not noted only in Antarctica. Its center of origin seems to be Asia. In the majority of other regions, the species has most likely been introduced [9,22,23,32,34,38,39,40,45]. Branchiura sowerbyi is common in Vietnam [2,25,27].

49. ***Monopylephorus sp.—{102}, (0, 0, 0, 2; 1).

Population structure and abundance: There were 2 incomplete individuals without a tail end of the body, 1 of which is sexually mature.

Ecology and distribution: These specimens were found in the freshwater part of the Mekong Delta (Co Chien Branch) in the riparian zone on gray clayey silt with sand and coarse plant residues.

Remarks: This is presumably an undescribed species. The clitellum is located on segments XI–1/2XII. Penial chaetae are not observed. Internal sexual organs are not visible. Dorsal and ventral bundles of chaetae start from segment II (Figure 5A). In the anteriormost dorsal bundles there are only bifid chaetae. Beginning from segments VII–VIII, the dorsal bundles contain 1–2 hair chaetae of a characteristic type—very thin and long (from 376 to 1031 µm) with a spirally twisted plumose tip. The tip’s plumage is distinct, but it is well visible only at high magnification (1500×) using immersion and DIC contrast (Figure 5H–J). Non-hair chaetae in the ventral and dorsal bundles are alike, 3–4 in the anterior bundles and 2 after segments XII–XIII. The upper teeth of these chaetae are twice as long as the lower ones in several anterior segments. Onwards, the lower teeth become gradually shorter until they disappear completely. As a result, starting from segments XII–XIII, all chaetae become unidentate both in the dorsal and ventral bundles (Figure 5B–G).

In the peculiar hair chaetae, these specimens from Vietnam are similar to members of the genus Monopylephorus such as M. cuticulatus Baker & Brinkhurst, 1981, M. aucklandicus (Benham, 1909) and M. irroratus (Verrill, 1873). In other genera of the subfamily and family, similar hair chaetae are not known. However, none of the named species have single-pointed chaetae among non-hair ones (only bifid), as well as plumage at the tips of the hair chaetae [34,94,95]. In turn, Pinder [51] (p. 12) illustrates the hair chaetae with twisted and plumose tips similar to the Vietnamese specimens for groundwater species from Australia named Monopylephorus n. sp. WA29. Unfortunately, there are no other details about the species in this paper, and we could not find other sources with a more detailed description of this Australian Monopylephorus. Therefore, it remains unclear whether the individuals from Australia and Vietnam are similar in other signs as well.

Subfamily Tubificinae Claus, 1876

50. *Aulodrilus acutus Ohtaka & Usman, 1997—{6, 18}, (5, 3, 0, 0; 2).

Population structure and abundance: This is a rare species and few in number. The maximum abundance (5 individuals; ~2 thous. ind./m²) was noted in {18}. Only solitary immature exemplars were found.

Ecology and distribution: This is a little-known member of the genus. The ecology and biology are poorly studied. Apparently, the worm prefers shallow-water biotopes with a muddy bottom. It builds tight mud tubes and lives in them [96]. The species can survive the temporary drying of waterbodies in slimy capsules [18]. It was recorded on rice fields, in ponds, open and overgrown shallow areas of lakes and reservoirs ([11,18,96] and our data). Perhaps A. acutus is endemic to Southeast Asia. Until now, the species has been noted only in the waterbodies of Indonesia (on the islands of Sumatra and Kalimantan) and Cambodia [11,18,68,96]. The checklist [22] indicates that the species is also known from the Australasian biogeographical region, but specific data on the finds are not given here. In Vietnam, the worm is registered for the first time.

51. Aulodrilus limnobius Bretscher, 1899—{74, 76, 79, 80}, (0, 0, 0, 10; 4).

Population structure and abundance: In the studied material, the species is rare and not numerous. Only solitary immature specimens were found in the amount of 1–3 per sample.

Ecology and distribution: This is a relatively well-known species. It dwells in a variety of fresh waters. Evidently, the worm prefers silty and silty–sand biotopes of stagnant water bodies and slow-flowing sections of rivers, including areas with sparse to moderate macrophyte cover, although it has also been recorded in streams on substrates dominated by sand, gravel and stones. The species lives in tubes [37,39,45,49]. It is almost cosmopolitan with a predominant distribution in the Northern Hemisphere [22,23,34,40]. This species was previously recorded in Vietnam [27].

52. *Aulodrilus pectinatus Aiyer, 1928—{5, 6, 12, 13, 102}, (14, 3, 0, 2; 4).

Population structure and abundance: This is a rare species and not numerous. There were 1 to 4 individuals per sample. Only solitary juvenile individuals were recorded.

Ecology and distribution: This is a relatively rare representative of the genus. The ecology is poorly understood. The species is noted in lakes, reservoirs, rivers and various small waterbodies mainly on silty substrates in non-flowing habitats, including thickets of aquatic vegetation. The worm builds tubes ([9,49,93] and our data). Aulodrilus pectinatus is chiefly known from the countries of South, Southeast and East Asia (India, Bangladesh, China, the Far East of Russia and Japan) [9,13,23,40,49]. According to [9], there are also finds of the species in South America (Brazil), but there are no such indications in other cited sources. It was not previously noted in Vietnam.

Remarks: Naidu [9] indicates that A. pectinatus was found in Europe, namely in Russia. Apparently, this mention refers to the only discovery place of the species known so far in Russia—Lake Khanka, which is located in the Asian part of the country (Far East) [49]. If so, then the species remains unknown in the European part of the continent. At the same time, Timm and Všivkova [84] note that since the report by [49] A. pectinatus has never been found again in Lake Khanka, and they suggest that the species has probably been confused with A. japonicus Yamaguchi, 1953.

53. Aulodrilus pigueti Kowalewski, 1914—{5, 6, 10, 12, 14–17, 25, 26, 59, 61–63, 71, 90, 100, 102}, (27, 24, 0, 10; 16).

Population structure and abundance: This is one of the most common oligochaetes in the studied areas of Vietnam, especially in shallow zones of reservoirs and lakes. The species was quite numerous in some samples, namely {102} (57 specimens; ~21 thous. ind./m²) and {6, 100} (21–22 specimens; ~8 thous. ind./m²). In other quantitative samples, the abundance varied from 1–3 to 7–11 (up to 4 thous. ind./m²) exemplars. Qualitative samples {25, 26} included 12–15 individuals. Most of the found worms were young solitary specimens. Less often, chains of 2–3 zooids were occurred. Sexually mature individuals were not found.

Ecology and distribution: Aulodrilus pigueti is one of the most known members of the genus worldwide. It lives in bottom sediments—mostly soft, but also on sands and other types—in a wide variety of habitats of diverse freshwater bodies, but occasionally it also occurs in slightly brackish waters. The worm builds tubes from mucous, sand and clay particles [9,38,39,45]. It is cosmopolitan [9,22,23,32,34,39,40]. In Vietnam, this species has been earlier noted under the synonymous name A. prothecatus Chen, 1940 [25,27].

54. Aulodrilus pluriseta (Piguet, 1906)—{2, 5–7, 9, 10, 13–15, 22, 24, 43–49, 54–57, 59, 62–65, 67, 94, 102}, (36, 50, 20, 5; 27).

Population structure and abundance: This is one of the most common and relatively numerous worms in the studied waterbodies and biotopes. The largest abundance (42–44 individuals; ~15–16 thous. ind./m²) was recorded in {15, 67}. In {13, 14, 59, 62}, the abundance reached 24–31 specimens (~9–11 thous. ind./m²), and in {6, 65}, it was 10–16 specimens (~4–6 thous. ind./m²). In the rest of samples, there were from 1 to 5–6 exemplars (up to 2 thous. ind./m²). Despite the large quantity, no sexually mature specimens were found. Populations included generally young solitary individuals, rarely chains of 2–3 zooids.

Ecology and distribution: The lifestyle and ecology are similar to the previous species. The worm lives in tubes of mucous and inorganic inclusions. It was found in a wide variety of freshwater bodies and biotopes, but it evidently prefers stagnant or weakly flowing habitats with silty bottoms enriched with organic matter, including those with a low oxygen concentration [9,38,39,45,49]. The species is most widely distributed in the Holarctic and Oriental regions. It is also registered in North America, Australia, New Zealand and Africa [22,23,34,39,40]. Aulodrilus pluriseta is well-known in Vietnam [27].

Remarks: According to recent molecular studies, A. pluriseta probably represents a complex of morphologically similar species [97].

55. Tubificinae (?) gen. spp. No. 1—{6, 8, 10, 25, 26, 34, 35, 38, 39, 61, 62, 67, 68, 72, 73, 81, 86, 87, 90, 95, 101, 105, 106, 108, 110}, (23, 12, 27, 29; 22).

Population structure and abundance: In the majority of the listed samples, 1–5 individuals were assigned to this group of species, less often up to 8–9 individuals (in {6, 28, 29, 34}), and a maximum of 21–29 individuals (in {62, 90}). The group includes mostly young immature specimens. Sexually mature worms with a developed clitellum were presented only in {101} (one whole individual and one anterior body part) and in {106} (1 individual devoid of a tail end).

Remarks: This species group combines immature and/or damaged members of the subfamily that have not been identified to the genus and species and that have only “ordinary”—without any characteristic peculiarities, as, for example, in some species of the genus Aulodrilus—bidentate chaetae in all bundles. The genus/species membership of the few found sexually mature individuals also could not be established due to the poor visibility of the reproductive system. Obviously, in most samples, there is not one but several species, as can be understood from the general appearance of the body, the shape and number of chaetae, etc. The name of the subfamily Tubificinae given to this and the next species group does not exclude the presence among the included individuals (especially young) representatives of other modern subfamilies previously united in the family Tubificidae [31,40,98]. This is emphasized by us with a question mark in the names of these groups.

It should be noted that the aquatic fauna of the former Tubificidae in Vietnam remains practically unexplored. Today, the best-known representatives in the country that do not have hair chaetae in their dorsal bundles—apart from some Aulodrilus species considered above—are the cosmopolitan Limnodrilus hoffmeisteri Claparède, 1862 and the Asiatic L. grandisetosus Nomura, 1932 [27].

56. Tubificinae (?) gen. spp. No. 2—{25, 58, 94}, (5, 3, 0, 2; 3).

Population structure and abundance: There was 1 immature individual in each sample.

Remarks: Unlike the previous one, this group of species includes worms that have both bifid and hair chaetae in the dorsal bundles. The specimens included in this group lack gills, as in Branchiura sowerbyi, or the characteristic non-segmented section at the posterior end of the body, specific for species of the genus Aulodrilus. Obviously, they belong to other genera of tubificins (or former Tubificidae). For Vietnam, no concrete species with such signs have yet been documented.

57. ***Tubificinae gen. sp. No. 3—{101}, (0, 0, 0, 2; 1).

Population structure and abundance: A total of 4 individuals, including 3 sexually mature, 1 of which lacks a tail end, were found.

Ecology and distribution: These specimens were found in the Mekong Delta in the freshwater part of the Co Chien branch in the channel zone on gray clayey silt.

Remarks: These “unusual” worms are characterized by small papillae covering the body surface and the absence of any chaetae in both the dorsal and ventral bundles. The chaetae (and/or their fragments) were not found in any individual on any of the segments during careful examination. General habitus, as well as the presence of a covering with papillae similar to representatives in the genera Tubificoides, Embolocephalus, Spirosperma (i.e., Tubificoides benedii (d’Udekem, 1855), Embolocephalus velutinus (Grube, 1879), Spirosperma ferox Eisen, 1879, etc.) [32,45], gave reasons to preliminarily assign the Vietnamese specimens to the same subfamily as the indicated genera. However, neither in these nor in other genera and subfamilies of the aquatic oligochaetes, are there species with a similar combination of signs—a body covered with papillae and a complete absence of chaetae. The above-named species and their relatives all have bifid or simple-pointed chaetae and some also have hair ones. Thus, the Vietnamese specimens probably belong to an undescribed species that requires additional special study.

These are rather large worms. The length of the immature individual is 6.2 mm, and that of the mature exemplars is up to 20.2–20.3 mm. The prostomium is small, triangular–rounded and without a proboscis (Figure 6A,C). The prostomium, the first two segments following it, the region of the clitellum, and in the young individual the last three segments also, do not have papillae on the surface (Figure 6C,D). The papillae on most of the body are located densely, relatively uniformly and without any visible order. Only near the anterior part of the body and in front of and behind the clitellum are there quite clear ring rows of alternating groups of papillae and empty spaces between them (Figure 6A–F). The clitellum is situated on segments XI, as far as can be judged from the segmentation of the anterior part of the body. In the region of the clitellum, there are protruding paired structures—apparently, the external genital papillae bearing male pores (Figure 6G). In 2 mature individuals, paired tubular structures protruding out of the body with harpoon-like tips are also visible, which are presumably genital (penial) chaetae. In the third mature individual, the tip of this structure is simple, needle-like, without a lateral process, which is possibly due to the different angle from which this object is seen here (Figure 6H–J). It was not possible to fully consider the internal structure of the reproductive system of the fixed and stained worms because of the papillae and dense, weakly transparent integuments of the body.

Order ENCHYTRAEIDA Kasprzak, 1984

Family Enchytraeidae d’Udekem, 1855

58. Achaeta spp. (and/or achaetous Marionina spp.?)—{42, 86, 90}, (0, 0, 7, 5; 3).

Population structure and abundance: The maximum number of worms of this type was recorded in {86}—19 individuals (~7 thous. ind./m²). The remaining samples contained 1–2 specimens. Most of the individuals found were young worms. Only in {90} were there 2 adult exemplars (2.5–2.9 mm long) with fully developed clitellums.

Ecology and distribution: Members of the genera Achaeta and Marionina have a worldwide distribution. Like most other enchytraeids, they live primarily in terrestrial biotopes (wet soils), but many species are also found in semi-aquatic or directly in freshwater and marine habitats [32,99,100,101]. Our specimens were registered in small forest streams and an artificial pond.

Remarks: Reliable identification of most enchytraeid species requires analysis of their internal structure, including the reproductive system. Since not all details of the anatomy are usually visible in fixed individuals, in most cases an intravital examination of the worms is also necessary [32,99,101]. Because we worked with fixed material, in this paper the representatives of the family are presented here and below as groups of species, united mainly according to the external morphological features, firstly according to the shape and numerical pattern of the chaetae. In the future, we plan to expand studies of the aquatic and semi-aquatic enchytraeids of Vietnam, including the involvement of molecular methods.

Achaeta is the only genus of the family that combines such features as the complete absence of chaetae and the location of the head pore on the prostomium. The last sign distinguishes the Achaetae species from some members of the genus Marionina, which are also completely devoid of chaetae, but in which the head pore is located on the border of the prostomium and the first body segment (peristomium) [99,101,102]. Unfortunately, it was not possible to discern the location of the pore in the fixed achaetous specimens from Vietnam. It seems that in 2–3 specimens, the pore is situated on the prostomium, but there is no exact certainty. Thus, the group considered in this section may include both Achaeta and achaetous Marionina species. In total, 4 species of Marionina lacking all chaetae are currently known, namely M. achaeta (Hagen, 1954), M. nothachaeta Matamoros, Rota & Erséus, 2012, M. nevisensis Righi & Kanner, 1979 and M. arenaria Healy, 1979, but, judging by the genetic data, there are several still undescribed ones [102,103]. Of these species, only M. nevisensis was previously collected from freshwater sites in some waterbodies in Japan [102]. Currently, 1 species of Achaeta, A. unibulba Graefe, Dózsa-Farkas & Christensen, 2005, has been noted in Vietnam (in soil samples), while no species of Marionina have been recorded here yet. From Southeast China bordering North Vietnam, Achaeta brevivasa Graefe, 1980 and A. cf. indica Prabhoo, 1960 are also known from terrestrial biotopes [104], and among the achaetous Marionina, M. nevisensis is known from coastal marine habitats around Hong Kong [23]. Therefore, the presence of these species in Vietnam is also not excluded.

Achaeta is the only genus of the family that combines such features as the complete absence of chaetae and the location of the head pore on the prostomium. The last sign distinguishes the Achaetae species from some members of the genus Marionina, which are also completely devoid of chaetae, but in which the head pore is located on the border of the prostomium and the first body segment (peristomium) [99,101,102]. Unfortunately, it was not possible to discern the location of the pore in the fixed achaetous specimens from Vietnam. It seems that in 2–3 specimens, the pore is situated on the prostomium, but there is no exact certainty. Thus, the group considered in this section may include both Achaeta and achaetous Marionina species. In total, 4 species of Marionina lacking all chaetae are currently known, namely M. achaeta (Hagen, 1954), M. nothachaeta Matamoros, Rota & Erséus, 2012, M. nevisensis Righi & Kanner, 1979 and M. arenaria Healy, 1979, but, judging by the genetic data, there are several still undescribed ones [102,103]. Of these species, only M. nevisensis was previously collected from freshwater sites in some waterbodies in Japan [102]. Currently, 1 species of Achaeta, A. unibulba Graefe, Dózsa-Farkas & Christensen, 2005, has been noted in Vietnam (in soil samples), while no species of Marionina have been recorded here yet. From Southeast China bordering North Vietnam, Achaeta brevivasa Graefe, 1980 and A. cf. indica Prabhoo, 1960 are also known from terrestrial biotopes [104], and among the achaetous Marionina, M. nevisensis is known from coastal marine habitats around Hong Kong [23]. Therefore, the presence of these species in Vietnam is also not excluded.

59. Enchytraeidae gen. sp. No. 1—{35}, (0, 3, 0, 0; 1).

Population structure and abundance: In the single quality sample, 12 individuals were found, 3 of which were subadults with an inchoate clitellum.

Ecology and distribution: The worms were recorded in a natural lake on a sandy open littoral with flooded terrestrial grasses and plant residues.

Remarks: These are small worms. The number of segments in 3 premature specimens is 30–35. The body length is 2.5–2.8 mm. An indistinct, probably still underdeveloped clitellum is on segment XII. Here, paired male pores are also visible from the ventral side. The chaetal formula, traditional for enchytraeids, has the form 0–0: (0,1),2–0 (here and below, the description of chaetae and the chaetal formula are given according to the guide of [99] (p. 15; Figure 5)). Lateral chaetae are completely absent. Ventral chaetae are present only in the preclitellar zone, in segments III–VII, usually 2 per bundle. In some immature specimens, bundles of segment VII bear only 1 chaeta each, or chaetae are completely absent here. In segment II and after VII, ventral chaetae are absent in all individuals. All chaetae in a bundle are equal in size, 16–19 µm long, straight distally, without nodulus, weakly bent proximally. Judging by the chaetae formula and kind, the specimens included in this taxon may belong to the genus Marionina. For example, the European species M. preclitellochaeta has a close chaetae pattern [105]. However, it is difficult to reliably determine the species from fixed exemplars.

60. Enchytraeidae gen. sp. No. 2—{30, 37, 40}, (0, 3, 13, 0; 3).

Population structure and abundance: The first two samples include 3–4 young individuals (up to 1 thous. ind./m²), while in {40}, there are 8 specimens (~3 thous. ind./m²), 1 of which is sexually mature, and 3 are premature with yet inchoate clitellum.

Ecology and distribution: These specimens were recorded in small natural and man–made waterbodies on sandy substrates enriched with detritus and plant remains. All basins are located near a sea coast.

Remarks: These are small worms. The number of segments in adults and preadults is 27, the body length is 2.4–2.6 mm. The clitellum is in segments XII–XIII. Lateral bundles of chaetae are absent. Ventral chaetae are present in all segments (except XII in mature and premature individuals), starting from III,1 per bundle (chaetal formula 0–0: (0),1–1). They are stout, a little thinner in the middle of the body, straight distally, without a nodulus, strongly bent proximally. The anterior chaetae are slightly shorter (17–18 µm) than those in the posterior half of the body (up to 19–22 µm). These individuals perhaps also belong to the genus Marionina, several species of which are also characterized by the full absence of lateral chaetae [99].

61. Enchytraeidae gen. sp. No. 3—{42, 84, 87}, (0, 0, 7, 5; 3).

Population structure and abundance: The first 2 samples include 1 juvenile individual each. In {87}, there are 7 specimens (up to 3 thous. ind./m²), 1 of which is adult with a distinct clitellum, and 2 are at the stage of maturation.

Ecology and distribution: The worms were noted in a small man-made pond and in forest streams on sandy and clay substrates with various plant remains.

Remarks: These specimens may also presumably belong to the genus Marionina (see Remarks for the next taxon). They are slightly larger worms than the previous ones. The number of segments in mature and premature individuals is 34–42, the body length is 2.6–4.0 mm. The clitellum is in segment XII. The chaetal formula is 0–0: (0,1),2–1. Only ventral bundles of the chaetae are present, beginning with segment III. Up to segments VIII–IX, each bundle bears 2 chaetae, from segments IX–X to the end of the body, only 1 (except for segment XII in the mature individual, where all chaetae are absent). The chaetae are uniform throughout the body, 18–22 µm long. They are straight distally, without a nodulus, weakly bent proximally.

62. Enchytraeidae gen. spp. No. 4—{8, 30, 32, 35, 42, 72, 76, 77, 79, 80, 86, 87, 96}, (5, 9, 7, 20; 12).

Population structure and abundance: The maximum abundance was recorded in samples {30, 32, 86}—26 (~10 thous. ind./m²), 14 (~5 thous. ind./m²) and 45 (~17 thous. ind./m²) individuals, respectively. In other samples, from 1 to 8 specimens were found (up to 3 thous. ind./m²). In about half of the samples, along with immature individuals, 1–4 mature and/or 1–7 premature exemplars were noted.

Ecology and distribution: This is the most common form of enchytraeids in the studied material, especially in the samples from small forest streams. The worms were also found in the inshore zones of reservoirs, lakes and various small waterbodies primarily on the sandy and clay substrates.

Remarks: This species group combines worms that also have only ventral bundles of chaetae, but, unlike Enchytraeidae gen. sp. No. 3, with 2 chaetae throughout the body. In rare cases, in the last 1 or 2 segments, the bundles bear 1 chaeta each. The bundles start from segment III. In the mature exemplars, they are missing in segment XII. Thus, the chaetal formula is 0–0: (0),2–(1),2. The chaetae are straight distally, without a nodulus, weakly bent proximally and equal in size in a bundle. Since the individuals included in this group have obvious differences in the general appearance of the body, size and length of the chaetae, they most likely belong to different species. In general, in the mature and premature specimens, the body length, the number of segments and the length of chaetae varied within 1.7–3.0 mm, 22–26 and 19–41 µm, respectively. In many individuals, the length and thickness of the chaetae in the posterior part of the body were somewhat greater than those in the anterior.

It is likely that the worms included in Enchytraeidae gen. spp. No. 4 also belong to the genus Marionina, as mentioned above for Enchytraeidae gen. spp. No. 3. Both taxa differ only in the presence of 1 or 2 chaetae in the bundles of the second half of the body. Among the species of the genus Marionina, for example, M. seminuda Xie & Rota, 2001 described from soil biotopes in China [106], and M. biwaensis Torii, 2012 from Lake Biwa [102], have a number and pattern of the chaetae similar to the Vietnamese Enchytraeidae gen. spp. No. 3 and No. 4. Unfortunately, without studying living specimens from Vietnam, a complete comparison of the species is impossible.

63. Enchytraeidae gen. spp. No. 5—{3, 42, 47, 72, 77, 79–81, 85, 86, 93}, (5, 3, 7, 20; 10).

Population structure and abundance: In most samples, there were 1–6 individuals, among which 1–2 mature and/or premature worms sometimes occurred. The maximum abundance was recorded in {86}—27 specimens (~10 thous. ind./m²), of which about half each were premature and mature.

Ecology and distribution: This is the second form of the enchytraeids in terms of distribution in the studied samples. The maximum number of finds was also confined to small forest streams. In some cases, these worms were found in shallow areas of reservoirs, lakes and in small man-made ponds.

Remarks: This group of species unites individuals with both lateral and ventral bundles, bearing 2, less often 3 chaetae, each, and starting from segment II. Judging by the general appearance and size of sexually mature individuals, as well as the shape and length of the setae, these are apparently 3 or 4 different species, the juveniles of which are difficult to distinguish from each other. The clitellum in mature individuals is located mainly on segment XII, but in some individuals, as far as can be seen from the fixed material, it also partially covers segments XI and/or XIII. Most of the worms included in this group have the chaetal formula 2–2,(3): 2–2,(3), i.e., single bundles with 3 chaetae are present only in the posterior (postclitellar) part of the body. However, in some samples, there are sporadic individuals in which triple bundles on the ventral side begin immediately with segments II–III, while bundles with 2 chaetae predominate laterally, as well as individuals in which lateral bundles with 3 chaetae may also start in the preclitellar zone (from segment VIII). The only one of its kind, the smallest of all the mature individuals (1.5 mm long), additionally differed in that it had no lateral bundles on segment II. The chaetae in most specimens are stout (usually slightly thicker anteriorly and posteriorly and in ventral bundles compared to the middle of the body and lateral bundles), straight distally, without a nodulus, weakly bent proximally. There are also individuals in which the chaetae are more strongly bent proximally. The size of adult and subadult specimens varies within a very wide range, from 1.5–2.0 to 4.0–5.0 mm, often even within the same sample. The smallest mature individual (1.5 mm) consists of 21 segments and has chaetae 26–31 µm long. The number of segments of other mature and premature exemplars is 24–44, and the length of their chaetae in the preclitellar zone is 23–33 µm, while in the posterior part of the body, it reaches 36–44 µm.

64. Enchytraeidae gen. sp. No. 6—{90, 91, 95, 96}, (0, 0, 0, 10; 4).

Population structure and abundance: There was 1 individual in each sample—a premature in {90} and immatures in the rest.

Ecology and distribution: These specimens were found in streams and a river on clayey and sandy substrates.

Remarks: These individuals have ventral and lateral bundles starting from segment II and bearing 2 chaetae each. Ventral bundles are present in all segments, while lateral bundles are present only up to segments VI–VII. Onwards, lateral chaetae are absent (chaetal formula 0,2–0: 2–2). The chaetae are stout, straight distally, without a nodulus, weakly bent proximally, 20–30 µm long. They are slightly longer and thicker at the posterior body part. The premature specimen is 1.7 mm long and consists of 26 segments.

65. Enchytraeidae gen. sp. No. 7—{95}, (0, 0, 0, 2; 1).

Population structure and abundance: There was 1 damaged (with tail torn off) immature specimen.

Ecology and distribution: The worm was found in a slow-flowing section of river on a sandy–clay bottom with plant remains.

Remarks: This exemplar is similar in shape and set of chaetae to members of the genera Mesenchytraeus or Cernosvitoviella. It has 4 bundles of chaetae in all 10 extant anterior segments, beginning with II. All chaetae are alike, slender, sigmoid, with a distinct nodulus, 20–22 µm long. The lateral bundles contain 3–4 chaetae, while the ventral bundles include 3–6. Among representatives of the genus Mesenchytraeus, only 1 species, M. pelicensis Issel, 1905, was previously recorded (in soil samples) in Vietnam [23]. Species of Cernosvitoviella have not yet been noted in Vietnam.

SUPERCLASS ANNELIDA incertae sedis

Family Aeolosomatidae Levinsen, 1884

66. *Aeolosoma cf. corderoi Du Bois-Reymond Marcus, 1944—{22, 25, 26, 32, 33, 39, 40, 45, 61, 65, 69, 77, 79, 82, 100} (14, 12, 27, 10; 13).

Population structure and abundance: This is a relatively common member of the genus in the studied material. The maximum abundance (27 specimens; ~10 thous. ind./m²) was recorded in sample {32}. In {39, 65, 69}, there were 7–10 exemplars (up to 4 thous. ind./m²). In other samples, no more than 1–3 individuals were found. Mostly they were solitary complete individuals; less often chains of 2–3 zooids and separate detached zooids were recorded.

Ecology and distribution: Aeolosoma corderoi is a rare, little-studied species. According to a few literary data, it lives in rivers and lakes on the bottom and among aquatic vegetation. It builds tubes from mud particles. So far, it is known only from South America (Brazil) [5,107,108,109]. In Vietnam, the worm was found in the bottom sediments of waterbodies of all studied types. If the identification of the species is subsequently confirmed (see Remarks), this would appear to be the first indication of the presence of the species in this part of the world.

Remarks: The specimens found in Vietnam have sigmoid chaetae (both in the dorsal and ventral bundles) characteristic of A. corderoi, with a group of teeth on the inner concave margin of the dorsal end (Figure 7A–C). However, in the Vietnamese specimens, these chaetae are less curved than is shown in the drawings of A. corderoi [107,108,109]. In addition, the number of teeth in Vietnamese populations varies from 2–3 (on separate chaetae, the teeth are not visible at all) to 7–9 even within the same individual, while 6–7 teeth are indicated in the original description [107]. Also, in Vietnamese individuals, the sigmoid chaetae are somewhat shorter (about 35 µm vs. 40–46 µm in A. corderoi). The extent to which named differences can be typical of A. corderoi is unknown because the morphological variability of the species has not been studied completely. All known descriptions of the species (e.g., [108,109]) are based on the initial material collected in one of the Brazilian rivers [107].

Inasmuch as the exact diagnosis of most aeolosomatids in the fixed state is difficult or not possible at all [32,108], it is obvious that the final confirmation of the discovery of A. corderoi in Vietnam will be possible only after studying live worms. Among other representatives of the genus, A. evelinae Marcus, 1944 and A. sawayai Marcus, 1944 are also close to these Vietnamese specimens in terms of the type of sigmoid chaetae. However, in the first species, the sigmoid chaetae are present only in the ventral body bundles, while in A. sawayai, the number of teeth on the concave side of these chaetae does not exceed 2–3, or the teeth are completely absent [108,109,110].

67. Aeolosoma spp. No. 1—{8, 30–32, 35, 37, 72, 75, 77, 79, 80, 86, 95, 96, 98–100}, (5, 12, 7, 27; 15).

Population structure and abundance: Specimens attributed to this group of species were relatively widespread in the studied material, and they were also numerous in some samples. The largest abundance was recorded in {37} and {32}—112 (~41 thous. ind./m²) and 24 individuals (~9 thous. ind./m²), respectively. In other samples, there were from 1–4 to 10 specimens (up to 4 thous. ind./m²). Sexually mature worms were not found. Only solitary immature individuals and chains of 2–3 zooids were present.

Ecology and distribution: Representatives of this group were registered in all types of the studied waterbodies. Most often, they were noted in samples collected in flowing waterbodies (streams, rivers), but they were not numerous here.

Remarks: This group of species unites Aeolosoma members, who bear only comparatively long, thin, straight hair chaetae in all chaetiferous body segments. Accurate separation of such individuals—having hair chaetae similar in shape and length—in a fixed state is usually impossible due to the discoloration of the epidermal glands, the inability to view the internal anatomy and some other diagnostic features. Of the species belonging to this group in South and Southeast Asia, including Vietnam, the best known are A. hemprichii Ehrenberg, 1828, A. headleyi Beddard, 1888 and A. bengalense Stephenson,1911 [9,25,27,111,112]. Aeolosoma bengalense reported for Vietnam by [25,27] is a synonym of A. headleyi [5,7,9]. Aeolosoma hemprichii is considered cosmopolitan, while A. headleyi is distributed in the Palearctic, Nearctic, Neotropical and Oriental regions [5,9,23]. It is possible that the wide distribution of these species is due to the erroneous indication of other similar representatives of the genus under these names [5]. Our material seems to be dominated by A. hemprichii, but we could not confidently separate it from A. headleyi and other similar species also possibly present in the studied waterbodies. Obviously, the forms of aeolosomatids from Vietnam, which have only hair chaetae, require additional studies, including living specimens.

68. Aeolosoma spp. No. 2—{25, 26, 47, 71, 86, 91}, (9, 3, 0, 7; 4).

Population structure and abundance: These worms were the most numerous in qualitative samples {25, 26}—194 and 38 individuals, respectively. In the remaining samples, 2–16 individuals (up to 6 thous. ind./m²) were found with a maximum in {86}. The material was dominated by solitary immature individuals. Less commonly, chains of 2 or 3 zooids and solitary, apparently recently detached, zooids were noted.

Ecology and distribution: The worms were recorded in streams, a river, the inshore area of a reservoir and a shallow lake on silty, silty–sand and clayey sediments.

Remarks: This group of species includes individuals with markedly shorter, thicker and more s-curved hair chaetae than the previous one (the sigmoid chaetae are also absent). In addition, in most individuals, part of the hair chaetae has noticeable serrations in the middle part. The last feature varies both within a specimen and within a bundle. There are exemplars in which serration is almost not expressed. To some extent, the length of the hair chaetae is also variable. Thus, this taxon may contain several similar species. In terms of the shape and length of chaetae (45–75 µm), these specimens from Vietnam are closest to A. quaternarium Ehrenberg, 1831. However, in descriptions of the latter, the serrations on chaetae are not mentioned. In turn, in terms of the presence of serrations on the chaetae’s middle part, our worms are similar to A. olivaceum Bunke, 1967, but in this species, the chaetae are longer and the serrations are finer and present only on the shortest hairs [32,108,109]. Aeolosoma quaternarium is distributed mainly in the European part of the Holarctic. There are separate finds in North and South America. In South and Southeast Asia, the species has not yet been recorded [5,23]. Like other aeolosomatids that have only ordinary hair chaetae, A. quaternarium is likely to be confused in the literature with similar species [109]. Aeolosoma olivaceum is known only in Europe so far [23,32].

69. ***Aeolosoma sp. No. 3—{95, 98}, (0, 0, 0, 5; 2).

Population structure and abundance: This is a rare and scarce form of the aeolosomatids in the studied area. There were 1 to 3 individuals per sample. Only solitary juveniles were found.

Ecology and distribution: Possibly, this is a rheophilic species. All exemplars were recorded in one of the mountain rivers on sandy–pebble substrates.

Remarks: These worms differ from all representatives of the genus in their unusual, short (up to 40–50 µm) hair chaetae with a thick base and a sharply tapering, whip-like distal part. Most chaetae bear a coarse serration on the inner side at the place of transition from the chaeta base to the tip (Figure 7D–F). The ventral and dorsal chaetae are similar. Each bundle contains 3–4 chaetae, 1–2 of which are usually shorter than the others. In terms of the presence of serrations on the inner part of the chaetae, these Vietnamese individuals are similar to A. olivaceum, but the intensity of serration, as well as the number, length and shape of the chaetae in both species, clearly differ [32,108,109]. It is likely that Aeolosoma sp. No. 3 is an undescribed species requiring further study.

70. Aeolosoma travancorense Aiyer, 1926—{2, 8, 16, 25, 26, 28, 37–41, 45, 47, 50, 59–62, 65, 69, 71, 72, 77, 79, 80, 82, 90–92, 98, 100, 102, 108}, (23, 24, 47, 32; 29).

Population structure and abundance: This is the most common and numerous representative of the aeolosomatids and annelids in general in the studied waterbodies and biotopes. Qualitative samples {25, 26, 79} included 46–153 individuals. Among quantitative samples, the maximum abundance was recorded in {92} (144 specimens; ~53 thous. ind./m²), {77} (119 specimens; ~44 thous. ind./m²) and {39} (79 specimens; ~29 thous. ind./m²). In samples {8, 38, 59, 72}, 12–21 individuals (~4–8 thous. ind./m²) were found, and in the remaining samples, there were from 1–3 to 6–9 exemplars (up to 3 thous. ind./m²). Everywhere, there were numerous solitary individuals and chains of 2–3 zooids. Sexually mature worms were not found.

Ecology and distribution: This species is one of the most famous representatives of the genus. Owing to its specific sigmoid chaetae, it is well identified in a fixed state. The worm dwells on the bottom, among aquatic vegetation, in periphyton in a wide variety of freshwater bodies and biotopes. It builds mud tubes [49,54,109]. According to the generalized information, the species is cosmopolitan. Apparently, it has not yet been recorded only in Antarctica [5,9,23,32]. Aeolosoma travancorense is well-known in Vietnam [27].

71. **Aeolosoma vietnamicum Gusakov, Tran Duc Dien, Nguyen Thi Hai Thanh, 2021—{27, 58}, (5, 3, 0, 0; 2).

Population structure and abundance: There was 1 immature, non-budding individual in each sample.

Ecology and distribution: This is a rare species. It is recorded in an almost completely dry floodplain lake and the coastal area of a sandy quarry on clayey and sandy deposits with plant remains.

Remarks: This is a recently discovered species and described in Vietnam [113]. Due to the presence of unique sigmoid chaetae in the dorsal and ventral body bundles, it can be easily distinguished from other members of the genus even in a fixed state.

4. Discussion

The presented checklist includes 65 taxa and groups of taxa of various oligochaete ranks from 2 families, Naididae and Enchytraeidae, of the order Tubificida and 6 species and groups of species of aeolosomatids (superclass Annelida incertae sedis, family Aeolosomatidae) registered in 2010–2021 during a study of the meiobenthos communities in various inland waterbodies of central and southern Vietnam. The number of reported genera is 17. In total, 39 representatives have been identified to the species level. Additional studies are required to accurately diagnose the remaining worms. As noted in the remarks for specific taxa, this is primarily due to the small number of individuals found in the material and/or damage to their bodies, as well as the lack of sexually mature specimens necessary to correctly diagnose some species and/or the need to also study them in vivo. Among the specimens still not identified to the species level, 8 representatives from the genera Chaetogaster, Trieminentia, Pristina, Monopylephorus and Aeolosoma and 1 from the subfamily Tubificinae, which we designated as a “possibly undescribed species” (the worms with 3 asterisks before the names in the checklist), should be noted especially. As far as can be judged from the specimens found, they differ from all putative congeners known today. Currently, their study and the search for new individuals for a comprehensive analysis are continuing. In addition, 2 new annelid species, discovered for the first time during the study of the Vietnamese meiobenthos, were already described not long ago. These are the naidid Bratislavia gusevi and the aeolosomatid Aeolosoma vietnamicum [48,113].

Besides the abovementioned 2 recently described species and 9 “possibly undescribed species”, among the representatives listed in the checklist, the aeolosomatid worm Aeolosoma cf. corderoi and the oligochaetes Amphichaeta sp., Aulophorus flabelliger, A. lodeni, Branchiodrilus hortensis, Chaetogaster cf. krasnopolskiae, Dero cooperi, D. palmata, Nais variabilis, Slavina evelinae, Pristina (Pristina) biserrata, P. (P.) foreli, P. (P.) synclites, P. (Pristinella) acuminata, P. (P.) aequidentata, P. (P.) cf. bilobata, P. (P.) osborni, P. (P.) sima, Aulodrilus acutus and A. pectinatus had not been reported in the territory of Vietnam before the start of the present study [24,25,26,27]. This obviously applies also to all discovered representatives of the family Enchytraeidae, which have not yet been identified to the species level. At that, Aeolosoma cf. corderoi, Chaetogaster cf. krasnopolskiae and Slavina evelinae, along with the unidentified members of the genera Trieminentia and Monopylephorus that we found in Vietnam, were also not previously known from the Oriental biogeographical region as a whole [5,9,13,20,21,22,23].

Naidid oligochaetes form the core of the diversity of annelids in the studied waterbodies and habitats. More than half of all members reported in the checklist belong to the subfamilies Naidinae and Pristininae (

Table 1. The number of annelid members noted in the studied waterbodies (n—number of the samples; dash—not found).

Waterbody Type	Naididae					Enchytraeidae	Aeolosomatidae	Total
	Naidinae	Opistocystinae	Pristininae	Rhyacodrilinae	Tubificinae
Reservoirs, big quarries (n = 22)	15	–	7	1	6	2	5	36
Natural lakes (n = 34)	25	1	8	1	6	4	5	50
Small permanent and temporary waterbodies (n = 15)	13	–	7	–	2	5	3	30
Rivers, streams, brooks (n = 41)	19	1	14	2	7	6	5	54
All waterbodies (n = 112)	30	1	16	2	8	8	6	71

). The genus Pristina is the richest in terms of number of identified representatives (16 species and supra-species taxa). The naidids of the genera Aulophorus (5 members), Dero (7) and Aulodrilus (5) are also relatively diverse in the studied material. The high species diversity of the listed genera is typical for tropical and subtropical regions [9,22,23,34,39,68,78].

Enchytraeid worms are represented in the reported investigation by 8 individual representatives and groups of species (Table 1). As noted in the checklist, the examined specimens appear to include members of the genera Achaeta and Marionina, and possibly Cernosvitoviella or Mesenchytraeus. Unfortunately, correct identification of the enchytraeid species in a fixed state is difficult. Considering that half of the presented in the checklist enchytraeids are groups of at least 2 or 3 different species, the real diversity of the family in the studied waterbodies is approximately 12–16 species. An analysis of the literature data has shown that aquatic and semiaquatic Enchytraeidae still remain practically unexplored in Vietnam and the surrounding regions. Undoubtedly, the taxonomic composition and diversity of these groups of oligochaetes require special studies in Southeast Asia.

All 6 members from the family Aeolosomatidae named in the presented checklist belong to the genus Aeolosoma. Of these, Aeolosoma spp. No. 1 and Aeolosoma spp. No. 2 seem to include at least 2 different species each that cannot be clearly separated in a fixed state (see related remarks in the checklist). Thus, the total diversity of the genus in the studied material is a minimum of 8 species, which approximately corresponds to the total number of the aeolosomatid members currently known in the Oriental area [5,9,13]. At the same time, the recently described A. vietnamicum [113], as well as, apparently, another representative of the genus found by us in Vietnam, Aeolosoma sp. No. 3, are species previously unrecorded in the country. Therefore, in tropical regions, the diversity of these microscopic annelids is probably much higher than is currently known. This group of worms has been weakly studied worldwide in the last decades. Only 5 new species of Aeolosoma, including A. vietnamicum, have been described since the comprehensive systematic monograph of the family by Bunke published in 1967 [5,7,108,113].

Among the representatives of the considered groups of annelids listed for the fauna of Vietnam in the monograph [27], the following taxa were not found in our material (taking into account the synonymized names according to the WoRMS database [7]): Naidinae—Chaetogaster limnaei von Baer, 1827, Ch. cristallinus Vejdovský, 1884, Nais communis Piguet, 1906, N. bretscheri Michaelsen, 1899, Haemonais waldvogeli Bretscher, 1900, Dero nivea Aiyer, 1929, Aulophorus hymanae (Naidu, 1962) and Allonais gwaliorensis (Stephenson, 1920); Tubificinae—Limnodrilus hoffmeisteri Claparède, 1862, L. grandisetosus Nomura, 1932, Limnodrilus sp. and 3 unidentified members of the genus Tubifex. In addition, this list contains 4 more uncertain species—the naidins Dero denticulata Thai and Allonais hanoiensis Thai, and the pristinins Pristina vietnamica Thai and P. dangi Thai. Unfortunately, neither the years of description of these taxa nor the corresponding references are given in the book [27]. Judging by the given name of the author and the names of some species, these worms were most likely discovered and described directly in Vietnam, but we have not yet been able to find these descriptions. There are also no mentions of these species in the main world checklists and databases on oligochaetes [7,22,23] or in any other sources. Thus, these worms can be both valid but “lost” species, and, possibly, synonyms of already known ones. Until the original descriptions are found, this question remains open.

In addition to the worms listed in the present checklist and in the review [27], in the first publication based on the study of the meiobenthos of Vietnam in 2008 [1], we also indicated the aeolosomatid Rheomorpha neizvestnovae (Lastochkin, 1935) and the oligochaetes Dero raviensis (Stephenson, 1914), Pristina cf. amphibiotica Lastočkin, 1927 and P. cf. notopora Černosvitov, 1937. As noted in the Section 2, we did not include the 2008 data in the checklist because this material was not kept, and, therefore, the identification of the found representatives could not be rechecked. Rheomorpha neizvestnovae is a microscopic interstitial rheophilic worm known from the European part of the Holarctic [5,23]. If our mention of the habitation of this species in Vietnam [1] was correct and is confirmed in the future, then this will be its first record in the Asian part of the continent. Other named species are known from Asia and/or tropical regions of other continents [22,23], so their presence in Vietnam is also quite probable.

Most worms in our checklist identified to the species level (about 30) are cosmopolitan or distributed on most continents, at least in tropical, subtropical and adjacent regions. About 10 representatives can be classified as relatively rare, having a limited distribution. Thus, Branchiodrilus semperi, Dero indica, Pristina (Pristinella) aequidentata and Aulodrilus acutus have only been observed in South, Southeast and East Asia thus far. The range of A. pectinatus is also limited to the same region, except for single finds in Europe and South America [9,22,23]. Dero palmata was previously known only in a few countries of Central and South America and India. Slavina evelinae was recorded in Central and South America and the Caribbean, and Aeolosoma corderoi was known only from Brazil [9,22,23]. Chaetogaster krasnopolskiae is a European species. Outside this region, in addition to our find in Vietnam, individuals similar to this species were previously noted only once in the United States [23,55]. Obviously, the new species described by us [48,113], as well as still unclear, undescribed worms (marked in the checklist with three asterisks), can also be attributed to rare representatives. Perhaps their modern distribution is restricted to Vietnam and neighboring territories since these worms have not yet been recorded anywhere else, including in Asian countries (India, China, etc.) that are relatively well studied in terms of the diversity of aquatic annelids.

The most widespread species in the studied Vietnam waterbodies and habitats were Chaetogaster diastrophus (total occurrence frequency 30%), Aeolosoma travancorense (29%), Aulodrilus pluriseta (27%), Dero obtusa (23%) and Stephensoniana trivandrana (20%). The first two species were common in all types of waterbodies—stagnant and flowing, large and small. Aulodrilus pluriseta and Dero obtusa were almost never found in rivers and streams, as opposed to stagnant habitats. Stephensoniana trivandrana was mainly present in reservoirs (as a rule, in littoral biotopes) and natural lakes. In certain types of waterbodies, the following species were comparatively frequent: (1) reservoirs and big quarries—Aulodrilus pigueti (found in 27% of samples); (2) natural lakes—Pristina longiseta (29%), P. aequiseta (26%) and Aulodrilus pigueti (24%); (3) small permanent and temporary pools—Aeolosoma cf. corderoi (27%), Aulophorus furcatus (20%) and Pristina foreli (20%); (4) rivers and brooks—P. foreli (22%), P. sima (22%) and Branchiodrilus hortensis (20%).

The greatest number of representatives of the studied groups of annelids was detected in natural lakes and flowing waterbodies, from which the largest number of samples was analyzed (Table 1). In individual quantitative samples, the highest species (taxonomic) richness (up to 8–15 taxa per sample) was commonly noted in the open or moderately overgrown littoral zones of some reservoirs and lakes, and sometimes in the shallow central zones of their water areas, as well as in some small artificial waterbodies (irrigation canals, flooded sandpits), riparian areas of rivers and small forest streams (Appendix A). In turn, the poorest (usually, with only 1–3 taxa) were, as a rule, samples from the deep profundal points of reservoirs, some man-made waterbodies, some eutrophic floodplain lakes, small, periodically drying up stagnant and flowing forest waterbodies and the lower, most saline section of the Co Chien branch of the Mekong Delta. In several such habitats, oligochaetes and aeolosomatids were not found at all (Appendix A). The latter, of course, does not mean that these groups do not live in these habitats at all, but their low diversity and abundance here is obvious, at least at the time of our research.

5. Conclusions

According to data summarized from the literary sources [1,2,27] and the present checklist, the total currently known diversity of the considered groups of annelids in the inland waters of Vietnam can be estimated at 100–113 taxa, where the maximum is the separate representatives conditionally united under “spp.”. These are nearly 80–88 Naididae (43–47 Naidinae, 1 Opistocystinae, 18–20 Pristininae, 2 Rhyacodrilinae, and 14–18 Tubificinae), 12–16 Enchytraeidae and 8–9 Aeolosomatidae. For the naidids, this is approximately two-thirds of all representatives of the family registered in the Oriental biogeographical region so far [9,13,21,22,23]. At the same time, the presence of a significant number of species previously unknown in Vietnam in the material studied by us and collected only in a few provinces, along with several new and possibly new ones, suggests, in our opinion, a much greater diversity of aquatic annelids, both directly in the country and in the surrounding territories. In addition, one should also consider the still poorly studied groups in the region such as Tubificinae, Enchytraeidae and Aeolosomatidae [5,21,23,27], as well as the possible presence of complexes of cryptic species among some of the worms we noted, traditionally (without using molecular methods) defined as a single species [46,47,59,97]. Thus, the prospects for further studies of the composition and diversity of aquatic annelids in Vietnam are obvious. In addition to standard hydrobiological observations with a wider coverage of various waterbodies, climatic regions and landscape zones of the country, these works should also include taxonomic, morphological and molecular studies specialized for this group of worms.