*Article* **The Coume Ouarnède System, a Hotspot of Subterranean Biodiversity in Pyrenees (France)**

**Arnaud Faille 1,\* and Louis Deharveng <sup>2</sup>**


**\*** Correspondence: arnaud.faille@smns-bw.de

**Abstract:** Located in Northern Pyrenees, in the Arbas massif, France, the system of the Coume Ouarnède, also known as Réseau Félix Trombe—Henne Morte, is the longest and the most complex cave system of France. The system, developed in massive Mesozoic limestone, has two distinct resurgences. Despite relatively limited sampling, its subterranean fauna is rich, composed of a number of local endemics, terrestrial as well as aquatic, including two remarkable relictual species, *Arbasus caecus* (Simon, 1911) and *Tritomurus falcifer* Cassagnau, 1958. With 38 stygobiotic and troglobiotic species recorded so far, the Coume Ouarnède system is the second richest subterranean hotspot in France and the first one in Pyrenees. This species richness is, however, expected to increase because several taxonomic groups, like Ostracoda, as well as important subterranean habitats, like MSS ("Milieu Souterrain Superficiel"), have not been considered so far in inventories. Similar levels of subterranean biodiversity are expected to occur in less-sampled karsts of central and western Pyrenees.

**Keywords:** troglobionts; stygobionts; cave fauna

Stretching at the border between France and Spain, the Pyrenees are known as one of the subterranean hotspots of the world [1]. This remarkable diversity is unevenly distributed along the Pyrenean range, reaching its highest value on the northern slope of central and western Pyrenees.

The Arbas massif is located on the northern slope of central Pyrenees, about 70 km south of Toulouse, at the limit of the departments of Haute-Garonne in the west and Ariège in the east (Figure 1). Extending the Lestelas massif to the west, it develops north of the Bouigane valley, east of the Ger valley, and south of rolling hills of Comminges. It ranges from 500 m to 1608 m in altitude.

The massif is formed by relatively complex series of Mesozoic limestones, generally overlaid by massive Urgonian limestones [2]. Under high rainfall exceeding 2000 mm a year, the massif is mostly covered by beech forest, with small stands of fir locally. Water transfers are very rapid due to a well-organized drainage and high surface karstification [2]. Not under threat and not under formal protection measures, the biological richness of the massif is nevertheless remarkable, and has been labeled as Zone Naturelle d'Intérêt Écologique, Faunistique et Floristique (ZNIEFF) *Massifs d'Arbas*, *Paloumère et Cornudère* (national ID: 730011048).

The main cave system of the massif is the Réseau Félix Trombe—Henne Morte, from the name of the French engineer and caver Félix Trombe, also known for his pioneer works on solar energy. This large system is commonly called Coume Ouarnède or Coumo d'Hyouernedo by cavers. It was explored for years by the famous speleologist Norbert Casteret who often referred to these explorations in his writings (see [3] for a list of references) and remains a well-known cave system for speleologists worldwide.

**Citation:** Faille, A.; Deharveng, L. The Coume Ouarnède System, a Hotspot of Subterranean Biodiversity in Pyrenees (France). *Diversity* **2021**, *13*, 419. https://doi.org/10.3390/ d13090419

Academic Editors: Michael Wink and Spyros Sfenthourakis

Received: 7 August 2021 Accepted: 27 August 2021 Published: 31 August 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

**Figure 1.** Location of the Arbas massif, in central French Pyrenees.

Various taxon-centered studies since more than one century ago progressively brought to light that the Coume Ouarnède system hosted a high diversity of cave animals. In the only global synthesis of its fauna, published in 1982, 29 cave-restricted species were listed from the system, including 18 stygobionts and 11 troglobionts [4]. Today, that is, 40 years later, there are 38 cave-restricted species recorded, including 21 stygobionts and 17 troglobionts, making of the Coume Ouarnède the richest cave fauna of the Pyrenees, which is itself a major European hotspot. The richness of the Pyrenean range can be explained by its biogeographical history [5]. The richness of the Coume Ouarnède can be explained by the great extent of the cave system and the diversity of its subterranean habitats. It may also result from the relatively good knowledge we have of its fauna, as the system is close to the renowned Subterranean Laboratory of Moulis where a number of biospeologists from all over the world have worked for decades.

In the framework of the special issue of the journal "Diversity" that deals with world hotspots of subterranean biodiversity, the Coume Ouarnède clearly deserves some attention. We shall provide in this paper an updated and comprehensive checklist of its subterranean fauna, put in its ecological and biogeographical context.

#### **2. The Coume Ouarnède System**

The Coume Ouarnède system is the longest subterranean system of France, and one of the most famous sites regarding French speleology. Today, it has a development of more than 112 km for a depth of 1020 m, with 57 inter-connected caves, while the massif of Arbas has about 500 caves in total (Figures 2 and 3) ([6]; S. Clément pers. comm. 05.2021). The network offers a great variety of geomorphological features, with countless fossil and active galleries and shafts, some of large dimensions (Pont de Gerbaut, Pène Blanque), and subterranean rivers [7].

**Figure 2.** Coume Ouarnède. Synthesis of the networks (modified from [7] and S. Clément pers. comm.). 1–9: Localities with three or more listed species, by decreasing species richness. 1: Goueil di Her; 2: Grotte de Pène Blanque; 3: Poudac Gran; 4: Hount deras Hechos; 5: Henne Morte; 6: Puits du Mistral; 7: Gouffre du Pont de Gerbaut; 8: Trou Mile; 9: Gouffre Raymonde.

**Figure 3.** A transverse view of the system, with the location of the nine caves from which more than three taxa are listed (modified from [7]). Cave numbers as in Figure 2.

The Coume Ouarnède system has two distinct resurgences: the Goueil di Her at Arbas, and the Hount deras Hechos at Herran, 2.5 km to the west ("*Hount-des-Heretchos*" sensu [8,9], that is, «the source of the ash trees» after [10]). The Goueil di Her is the resurgence of the main hydrological system, the Réseau Félix Trombe. The Hount deras Hechos is the resurgence of the smaller Henne Morte system, regarded as a secondary derivation of the main system [2,7]. The two systems are interconnected.

#### **3. Methods**

Data synthetized here are drawn from the literature, that is, species lists [4,9,11–14] and taxonomic literature. Most specimens were collected in nine caves or cave complexes of the system: Gouffre de la Henne Morte (and spring nearby), Gouffre du Pont de Gerbaut, Poudac Gran, Gouffre Raymonde, Grotte de Pène Blanque, Goueil di Her, Hount deras Hechos (cave and spring), Puits du Mistral, and Trou Mile (Figures 2 and 3). These caves are now interconnected, except Poudac Gran. Most of the collections in the Goueil di Her, the richest of these caves, were made between the entrance and the first sump.

Species names and species validity have been checked from [15] and public databases [16,17]. Species ecological status has been inferred from the taxonomic literature, from [18], and from [19] for spiders.

Abbreviations and terms defining species ecology that are used in the text are defined below: Endogean or euedaphic species: living or assumed to live only in deep soil, often common at cave entrances.

Eutroglophile: a species with permanent populations inside and outside caves.

Hyporheic species: living or assumed to live only in interstitia of sediment beneath and alongside streams.

MSS: Milieu Souterrain Superficiel (often translated as "Superficial Underground Compartment" in the literature).

Stygobiont: a species living or assumed to live only in groundwater.

Subtroglophile: a species spending a part of its life cycle in caves.

Troglobiont or troglobiotic species: living or assumed to live only in caves or in the MSS.

Troglophile or troglophilic species: living inside as well as outside caves.

Only troglobionts, stygobionts and the most important troglophiles are considered in this paper. Guano and shallow subterranean habitats have not been sampled, though these habitats are present and promising in the massif d'Arbas [20].

#### **4. The History of Biological Explorations**

The Goueil di Her cave, the eye of Hell ("*uèlh d'in hèrn*") in the Gascon dialect, is the main resurgence of the Coume Ouarnède system. The first explorations of the cave are related in [21]. In 1908, 30 years later, Jeannel visited the cave, and did the first biological collections, during a speleological expedition conducted by E.A. Martel which aimed at studying the hydrological characteristics of Pyrenees [22,23]. He introduced the cave in a dramatic way [23]: *«Près d*´*Arbas se trouve une étrange caverne, le Goueil di Her, redouté des habitants du pays, parce qu*´*après les pluies une puissante rivière souterraine jaillit sous pression hors de la grotte, produisant une détonation qui s*´*entend à plusieurs kilomètres»*. "Near Arbas is a strange cave, the Goueil di Her, feared by the inhabitants of the country, because after the rains a powerful subterranean river gushes under pressure out of the cave, producing a detonation which can be heard several kilometers away". During subsequent trips, Jeannel, Racovitza and several biologists continued to regularly sample this cave (Figure 4).

**Figure 4.** René Jeannel at the entrance of the Goueil di Her cave, modified from [11].

A summary of their collections was published in the "Enumérations des Grottes Visitées" [22,24,25]. In 1908, Jeannel visited several caves of the Arbas massif, and sampled their fauna (Gouffre du Pont de Gerbaut, Grotte de Pène Blanque, Poudac Gran, Hount deras Hechos, Grotte de Gourgue) [22]. In particular, he returned repeatedly to the Goueil

di Her (1908, 1910, 1912). An important integrative work was done by F. Trombe and his collaborators (1945–1947), based on a compilation of data in the fields of speleology, karstology, climatology, hydrology, ecology, and biospeology of the Comminges area, with a strong focus on the Arbas massif karst [12,26] Some years later, the Pène Blanque cave fauna was the subject of another contribution [13]. The last and most important investigations for aquatic subterranean fauna were done by Lescher-Moutoué, Gourbault and Rouch, who studied the composition, distribution and ecology of the stygobiotic fauna and characterized abiotic parameters of the Goueil di Her habitats in great detail [9,14]. A synthesis of our knowledge on the hydrology and biospeology of the massif was published by Bou [4,27]. Aside from these fundamental works, data available in the literature are few, based on punctual samples in a small number of caves.

#### **5. The Fauna**

The Goueil di Her was ranked among the world hotspots of subterranean biodiversity, with 26 cave-restricted species taxa (14 aquatics, 12 terrestrial species) in [28,29], then with 29 species [30]. This last number is actually an underestimation of the biological richness of the cave, and of the whole system. Our knowledge of the Coume Ouarnède terrestrial biodiversity relies on disparate sampling surveys in a few caves and pits, that often focused on peculiar groups only, mostly beetles; conversely, aquatic fauna has been the object of an intensive taxonomic and ecological study in karst and hyporheic habitats, but limited to the first part of the Goueil di Her cave itself and to the hyporheic zone of the stream that emerges from this cave [9].

Although the system counts more than 50 entrances, faunistic records come from a few of them: nine caves have three cave species or more, but among them, only two were investigated thoroughly, and provided more than seven cave-restricted species: the Pène Blanque cave (eight troglobionts, no stygobionts) and the Goueil di Her, the main resurgence of the system and by far the richest (17 troglobionts and 13 stygobionts). Fourteen stygobionts were collected in the hyporheic of streamlet under Goueil di Her. Faunistic data are detailed in Tables 1 and 2.

#### *5.1. Stygobiotic Taxa*

The works of Lescher-Moutoué, Gourbault, and Rouch are the main sources of information about stygobionts of the system [9,14]. They mainly focused on crustacea of Goueil di Her cave and of the hyporheic of the stream down to 2 km from the resurgence, that is, a limited number of habitats compared to those of the whole system. In most cases, Copepoda were the dominant group in abundance and diversity, rarely surpassed by Ostracoda for abundance. The taxonomic coverage of the collected fauna is globally good, but Ostracoda remain unidentified and only two species of Gastropoda were mentioned. Below, we browse the most interesting stygobiotic species of the Coume Ouarnède system.

#### 5.1.1. Tricladida

*Plagnolia vandeli*, the only representative of the genus *Plagnolia*, is a blind white flatworm endemic of a few karsts in central northern Pyrenees. Metabolism of the species has been shown to be strongly reduced compared to epigean species and is associated with a considerable lengthening of all biological processes, such as regeneration and life expectancy [31].

#### 5.1.2. Gastropoda

Both listed species, *Moitessieria simoniana* and *Islamia moquiniana*, are wide-range endemics of subterranean aquifers, the former in eastern Pyrenees and Montagne Noire, the later in eastern Pyrenees [32].

**Table 1.** List of stygobionts and troglobionts of the Coume Ouarnède system (CO), Grotte de Lestelas (L), Grotte de Gourgue (G). \*, ectoparasite; hab, habitats; c, cave; h, hyporheic; s, spring; x, species present in CO, L or G.



**Table 2.** List of troglophiles of the Coume Ouarnède system (CO), Grotte de Lestelas (L), Grotte de Gourgue (G). ecol, ecology; endo, endogean; TPeu, eutroglophiles; TPsub, subtroglophiles; x, species present in CO, L or G.

#### 5.1.3. Amphipoda

Five species of Amphipoda in three different genera are present in the system. *Parasalentinella rouchi*, from the hyporheic zone of the Arbas and Escalette streams, is a small species known from the hyporheic of a few stations in Ariège and eastern Haute-Garonne [33]; it is present in this habitat downstream of Goueil di Her resurgence, where it sometimes occurs together with *Salentinella petiti*.

*Niphargus* are present but less common in Pyrenees than in most other French regions [34]. Three species of *Niphargus* have been collected in the system, in three different habitats: *N. pachypus* in the hyporheic downstream of the resurgence [4], a species of the group *longicaudatus* Costa, 1851, probably *N. robustus*, inside the Goueil di Her [4,34] and *N. foreli* at a spring in the upper part of the system [35]. Further sampling as well as taxonomic work would be necessary to confirm these findings.

#### 5.1.4. Isopoda

*Proasellus racovitzai* was described from the Goueil di Her where it coexists with *Stenasellus virei hussoni*. It is considered endemic of this cave [36]. *Stenasellus virei hussoni* (Figure 5), a "carnivore facultative omnivore" after [37], is widespread in caves of the northern part of central Pyrenees. This typical stygobiont has a life span of 15–20 years [38].

**Figure 5.** *Stenasellus virei hussoni*, a stygobiotic species common in hypogean waters of the Coume Ouarnède system (specimen from the Tute de Jovis cave, in the Sourroque massif, east of the Arbas massif; body 10 mm long; reproduced with permission from S. Huang).

#### 5.1.5. Copepoda

The Coume Ouarnède is especially rich in Copepoda, with nine stygobiotic species recorded so far, of which six occur at the Goueil di Her. A large number of epigean species are associated to these strictly hypogean species, for example 11 troglophilic or trogloxenic species for four troglobiotic ones among harpacticoids are cited from Goueil di Her [14]. Copepoda are by far dominant in number and diversity in all hypogean compartments.

#### *5.2. Terrestrial Taxa*

#### 5.2.1. Acari

Rhagidiidae, a family of tiny predatory mites, comprises the largest number of troglobionts among Acari of temperate caves. *Troglocheles vandeli* is a troglomorphic mite only known so far from its type locality, the Trou Mile in Arbas [39]. The remarkable blind, slender, and transparent mite mentioned in [40] in the Goueil di Her might be this species. Given the rarity of cave Rhagidiidae, the presence of a second troglobiotic species of this family, *Rhagidia (Deharvengiella) troglomorphica* Zacharda, 1987, in the Grotte de la Buhadère, a cave of the Source Bleue system adjacent to the southwestern part of the Arbas massif, may indicate that these rare mites may be more diversified than expected.

#### 5.2.2. Araneae

The Coume Ouarnède system, as well as the Arbas and surrounding massifs, are riche in troglophilic spiders, but very poor in troglobiotic ones. A single troglobiotic spider, *Leptoneta microphthalma*, is recorded from system, and present in several caves of the massif d'Arbas and surrounding karsts. It has six reduced eyes. *L. infuscata*, a troglophilic but mostly cave dwelling species, is also frequent in the caves of the region [41].

#### 5.2.3. Opiliones

*Arbasus caecus* is a relictual species of harvestmen, the only species in the genus *Arbasus* (Figure 6). The superfamily Travunioidea, to which *Arbasus* belongs, is a north temperate lineage known from North America, Europe and East Asia (Japan, Korea) and counts

four families and 24 genera, some of them troglobiotic [42]. The peculiar morphology of *Arbasus* makes difficult its attribution to a family, and it is regarded as a representative of either Travuniidae [43] or Cladonychiidae [42]. Genetic data are still lacking to confirm this attribution.

**Figure 6.** *Arbasus caecus*, a monospecific genus of harvestmen Travuniidae described from Pène Blanque cave (specimen from Artigouli cave, Estadens; body 2 mm long; reproduced with permission from C. Vanderbergh).

> First described in the genus *Phalangodes* by Simon [44], *Arbasus caecus* was subsequently included in a new genus, *Arbasus*, by Roewer in his revision of European Opiliones [45]. Contrary to what was mentioned by Bou [4], the species was not described from Gourgue cave, where it is present, but on a single specimen collected in the Pène Blanque cave, probably by R. Jeannel [22,44]. Since then, the species was found in Goueil di Her, as well as a few other caves in karsts surrounding the Arbas massif: Riusec cave of the Source Bleue system [12], Cap de Payssas cave at Juzet-d'Izaut [46], the Lestelas cave [47] of the Caussanous system [2], and more recently, the Artigouli cave at Estadens (C. Vanderbergh pers. comm.), a cave of the small Peyrein system northwest of Arbas [48].

#### 5.2.4. Chilopoda

One of the three troglophilic *Lithobius*, *L. troglodytes*, has a strong affinity for cave habitats as more than 70% of its 73 French records listed by Iorio in 2014 were collected in caves [49]. The two other species are more abundant outside, rather than inside caves.

#### 5.2.5. Diplopoda

All three species listed in Table 1 are detritivores. Four Blaniulidae (two species and four subspecies) have been reported in various caves of the Coume Ouarnède system in the literature. A taxonomic re-examination of this material would be necessary to validate these forms. We provisionally listed only the two species, *Blaniulus lorifer* and *B. troglobius*, regardless of their subspecies. The third species, *Spelaeoglomeris jeanneli*, is a small pillmillipede only known from the Arbas massif (Goueil di Her, Grotte de Gourgue, Grotte de Paloumère and Poudac Gran). This species is the only troglobiont endemic of the Arbas massif [4,11,50].

#### 5.2.6. Isopoda Oniscida

The species *Scotoniscus macromelos*, endemic of central Pyrenees, is split into nine parapatric subspecies, all strictly troglobiotic. *S. macromelos macromelos* is the easternmost form, endemic of a cluster of karstic massifs including Arbas and Lestelas [51–53].

#### 5.2.7. Collembola

Collembola are usually the dominant arthropod species in caves. They are detritivore, but troglobiotic species mostly ingest clay, like many deep soil species, perhaps feeding on the micro-organisms it contains, while many litter species ingest fungi mycelium.

Three cave-restricted species are known from the system. At least two additional ones from two genera are expected to occur, as they are known in caves of other systems of the region: a *Micronychiurus* and a blind *Pseudosinella*. No species from the guano has been cited from the system, but they probably exist, and a guanobiotic endemic Hypogastruridae, like those known from the Grotte de Mont de Chac and Grotte de Payssa nearby can also be expected.

*Pseudosinella theodoridesi*. It is a common species in the caves of Ariège and Haute-Garonne. Its eyes and pigmentation are reduced, but present. Its appendages are, however, clearly elongated. Several forms differing by eye reduction have been recognized in the caves of the region [54], but their taxonomic status is unclear.

*Oncopodura tricuspidata.* Described from the grotte de Pène Blanque, this troglomorphic species has populations in several caves of central Pyrenees, which are likely to be undescribed species [55].

*Tritomurus falcifer.* This highly troglomorphic species is blind, depigmented, and has long antennae and elongate claw, in contrast to all other western European Tomoceridae. It is geographically isolated from the two other species of its genus which are located in caves of the Dinarides [56] and can be qualified as relictual. The species is the only troglobiont of the Coume Ouarnède system to live in hygropetric habitats [57], where it moves relatively slowly, often associated to *Aphaenops ehlersi*; though equipped with a long furca, "the large Collembola, special to" the Goueil di Her "jump with difficulty and prefer to run away: they walk by sweeping the ground with their very long antennae which strike the roughness of the ground while folding back." [40]. It is known from the Goueil di Her and Trou Mile in the system, but also from a few caves in the surroundings of the Arbas massif, that is, Grotte du Béguet, Grotte de la Buhadère and Grotte de Riusec.

#### 5.2.8. Coleoptera

The system is rich in 11 species of Coleoptera, among which six are troglobitic (genera *Aphaenops* and *Speonomus*, both endemic of the Pyrenees), three are endogean and two are troglophilic. Though narrow endemics, all Coume Ouarnède beetle species in their current acceptation have distribution areas exceeding the Coume Ouarnède and even the Arbas massif. None of the species are endemic of the Arbas massif itself, but all the troglobiotic ones have a narrow distribution in Arbas and surrounding massifs, like the Leiodidae *Speonomus (Machaeroscelis) infernus arbasanus* (Figure 7) [58].

**Figure 7.** *Speonomus (Machaeroscelis) infernus arbasanus*, a hypogean Leiodidae present in the Arbas massif (specimen from Lespugue cave, Saleich; body 2.5 mm long; reproduced with permission from S. Huang).

The genus *Aphaenops*, endemic of the Pyrenees, is composed of many troglobiotic and a few endogean species. Five species are quoted from the system, all strictly troglobiotic. These *Aphaenops* are easily recognizable morphologically and were, until recently, distributed in different subgenera. Three of them are common (*A. ehlersi, A. t. tiresias*, *A. cerberus bruneti*) in the Goueil di Her cave, while the two other ones are rare.

*Aphaenops* (formerly *Hydraphaenops*) *ehlersi* (Figure 8) has a particularly interesting ecology, very different from the other species of the genus present in the Goueil di Her. During a visit of the cave just after a storm and a flood in July 1914, Jeannel observed numerous exemplars of "*Hydraphaenops*" (actually *A. ehlersi*) walking on wet clay around the sump [59], that were usually not present or rare in this habitat. He was convinced that he found here the real way of life of the *Hydraphaenops*-like species, which are always extremely rare in most of caves of Pyrenees. He hypothesized the existence of a terrestrial phreatic habitat restricted to the deep parts of the karsts, to which some species are subservient, coming out to the riverbanks of subterranean rivers only after flooding [60,61].

*Aphaenops* (formerly *Arachnaphaenops*) *tiresias* is a wide-range endemic that is present in several karsts in the Pyrenees of Ariège and Haute-Garonne.

*Aphaenops* (formerly *Cerbaphaenops) cerberus* is widespread in Ariège and Haute-Garonne, where it is differentiated in many genetically divergent populations, some taxonomically recognized as subspecies, the splits between the populations reflecting largely the fragmentation of the karst itself [62]. The *Aphaenops cerberus bruneti* populations of Coume Ouarnède and of Lestelas, *locus typicus* of the subspecies are slightly different morphologically. Because of morphological similarities, it was suggested that the Coume Ouarnède population, the westernmost of *A. cerberus*, might actually belong to the species *A. jauzioni* Faille, Déliot, and Quéinnec, 2007 which was described from Grotte d'Artigouli, a cave in a small isolated limestone outcrop 5 km northwest from Goueil di Her [63,64].

**Figure 8.** *Aphaenops ehlersi*, specimen from Goueil di Her (body 4.2 mm long; reproduced with permission from C. Vanderbergh).

*Aphaenops* (formerly *Cephalaphaenops) bucephalus* is known from many caves of Ariège and Haute-Garonne, where it is always rare [65] (Figure 9).

**Figure 9.** *Aphaenops bucephalus*, a species endemic from the Ariège and Haute-Garonne departments, always rare in its distribution area, and known by only a few records in Goueil di Her cave (specimen from Mount cave, Juzet-d'Izaut; body 5.6 mm long; reproduced with permission from S. Huang).

*Aphaenops crypticola* is quoted from a single record under the name *A. parallelus* Coiffait, 1954, and its identification needs confirmation [66].

One species of the Coume Ouarnède system initially described as distinct subspecies (*A. tiresias* ssp. *proserpina* Jeannel, 1909) was recently synonymized with its nominal subspecies. Another one, *A. ehlersi* ssp. *longiceps* Jeannel, 1926, is no more recognized as a valid subspecies. As a result, none of the recognized *Aphaenops* of the Coume Ouarnède is currently restricted to the system, but all remain relatively narrow regional endemics.

Among the three endogean species found in the system, *Geotrechus orpheus consorranus* belongs to a genus very close to *Aphaenops*, but with as many endogean forms as troglobiotic ones. All *Geotrechus* are blind and depigmented, but none exhibit strong appendage elongation. The species found in the Coume Ouarnède, often endogean, is also present in Grotte de Lestelas. However, this last cave also has another species of the same genus, *G. trophonius trophonius*, which is rare.

#### 5.2.9. Laboulbeniomycetes

Pyrenean Trechinae frequently carry on their integument a species of fungi of the Laboulbeniaceae family. Laboulbeniomycetes are a group of ascomycete fungi that utilizes arthropods for nutrition and/or dispersal. The species *Rhachomyces aphaenopsis* is known on a large number of Pyrenean cave Trechinae (see [67] for a complete list of the hosts) and was found on three species of Trechinae occurring in the Coume Ouarnède system: *Aphaenops cerberus bruneti*, *A. tiresias tiresias* and *A. ehlersi* [67,68].

#### **6. Species Richness in a Regional Context**

The Coume Ouarnède system is the second hotspot of subterranean biodiversity of France; it is second to Cent Fonts spring in total species richness and second to Grotte de Lestelas in number of troglobionts (Table 3). Aside from its 21 stygobionts and 17 troglobionts, the system is also home to 20 troglophiles, including three species of endogean beetles (Table 2).

**Table 3.** Number of obligate subterranean species of the richest caves and aquifers of France, and of three rich caves close to the Coume Ouarnède system (Grotte de la Buhadère, Grotte de Lestelas and Grotte de Gourgue). Tb, number of troglobionts; Sb, number of stygobionts; \*, caves close to Coume Ouarnède.


The Coume Ouarnède system shares most of its species with three karstic systems geographically close to it [2], that is, the Grotte de Gourgue, 200 m from Goueil di Her, but not related to the Coume Ouarnède system; the Lestelas cave of the Cassaunous system; and the Buhadère cave of the Source Bleue system. The differences in faunistic richness are primarily due to undersampling of these three caves, in particular for aquatics. None of the three cited caves have been sampled for microcrustaceans, which account for more than half of the total number of species in the Coume Ouarnède system. More generally, inventories of the hyporheic fauna would be required to precise the distribution limits and the degree of endemicity of the microfauna of subterranean aquifers in most parts of the Pyrenees. The few stygobionts encountered in the three caves are present in the Coume Ouarnède, except *Echinophrya stenaselli*, ectoparasite on *Stenasellus* pleopods [72].

The troglobiont dataset is less uneven. In the Grotte de Gourgue, very close to the Goueil di Her, all troglobionts, but one, are shared with the Coume Ouarnède system. The missing one is a rare Pseudoscorpion, *Neobisium* (*Blothrus*) sp. The Grotte de Lestelas, more thoroughly sampled, shares 11 species with the Coume Ouarnède, but has 8 species not found in this system. Among them, two species have vicariants in the Coume Ouarnède. *Tomocerus problematicus* is replaced by *Tritomurus falcifer* (though very different morphologically, they are mutually exclusive in the region). *Speonomus* (*M.*) *infernus infernus* is represented by a different subspecies, *S.* (*M.*) *infernus arbasanus* in the Coume Ouarnède. The other six species, that live in habitats undersampled in the Coume Ouarnède system, could be potentially found in the system, like Pseudoscorpiones or *Ischyropsalis pyrenaea*, a Pyrenean endemic, which occurs in several caves close to the system: Lestelas, Lespugue among others [46]. Comparisons with Grotte de la Buhadère would require deeper taxonomic investigations, but this cave has at least two rare troglobiotic micro-arthropods that could be potentially present in the Coume Ouarnède system: a Rhagidiidae and a new Oncopodura springtail of a species group different from *O. tricuspidata*. Discoveries of additional troglobionts in the Coume Ouarnède system is therefore foreseeable.

Conversely, several of the troglobionts (*Spelaeoglomeris jeanneli* for instance) present in the Coume Ouarnède system and lacking in Lestelas, are susceptible to be found in this last cave. Given the lack of all-taxa inventories in most caves of Pyrenees, it is likely that caves of richness similar to that of the Coume Ouarnède exist in the central and western Pyrenees. In the same line, some groups which are widespread in subterranean ecosystems of the Pyrenean range have not been reported from the Coume Ouarnède system nor from the three caves of its surroundings mentioned above, such as Diplura, frequent troglobionts in Pyrenean caves [73], or Ostracoda among stygobionts. This last group is widespread and diversified in groundwaters of central Pyrenees, but its species have been rarely identified.

At the scale of France, between-site comparisons are more biased. The highest species richness known so far, that of Cent-Fonts spring, and the third and fourth richest ones, those of Triadou and of the Baget, do not compare with the Coume Ouarnède dataset as they are focused on interstitial aquatic fauna based on exceptional sampling efforts during years. The current pattern at country level is that hotspots of subterranean diversity are located in two regions: central and western Pyrenees on one hand for both stygobionts and troglobionts, and southeastern Massif Central on the other hand for stygobionts. Within these regions, the richest caves are clearly also the most heavily sampled.

#### **7. Conclusions and Perspectives**

The faunistic inventory presented in this paper has to be put in perspective. Its limitation is due to several ecological, spatial, taxonomical and methodological gaps. Guano and shallow subterranean habitats have been only marginally sampled or not sampled at all. A single site, the Goueil di Her, has been the object of thorough all-taxa sampling; however, its cave habitats, though highly interesting, are quite unusual among the caves of the region due to frequent flooding of all the sampled passages. Taxonomic coverage has been insufficient for snails and springtails, while Ostracoda and Diplura have not been identified. At least, mostly basic sampling techniques have been used for terrestrial and aquatic fauna except at Goueil di Her. Overall, this may explain why several cave species encountered in the surroundings of the Arbas massif have not yet been recorded from the Coume Ouarnède system. Filling these gaps would significantly increase the subterranean biodiversity of the system. On the other hand, sampling surrounding karsts, finetuning distribution limits, and testing genetic differentiation of populations of stygobionts would obviously help in understanding the observed geographical patterns. In this respect, investigations are currently being carried out, that focus on the major biodiversity issues raised above, that is, sampling a few targeted caves located in the upper Coume Ouarnède system and shallow subterranean habitats of its basin, and caves in the three surrounding systems identified by the BRGM [2]. The basic sampling methods were completed by bait-based techniques in oligotrophic and aquatic habitats.

In spite of these limitations, this work confirms the richness of the Coume Ouarnède system and of its most interesting cave, the Goueil di Her. Springtails and beetles are dominant, like in all central and western Pyrenean karsts (Table 1) [18], and these two groups clearly illustrate its difference with non-Pyrenean regions of France in terms of global subterranean species richness. The presence of two relictual troglomorphic species in the Coume Ouarnède fauna is a second illustration of its exceptional biodiversity. This raises fascinating questions about the origin and age of this fauna, as much as similarly isolated relicts do not exist in the karsts of central Pyrenees surrounding the Arbas and its satellite massifs.

**Author Contributions:** Both authors A.F. and L.D. participated in all phases of the realization and writing of the paper. Both authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Institutional Review Board Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Acknowledgments:** We thank Christian Vanderbergh and Sunbin Huang for providing pictures of some hypogean taxa. We are particularly grateful to Sylvestre Clément (Speleo-Club du Comminges, Arbas) for sharing with us his deep knowledge of the Coume Ouarnède system and for providing an up-to-date general topography of the system.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


## *Communication* **Biodiversity of the Huautla Cave System, Oaxaca, Mexico**

**Oscar F. Francke, Rodrigo Monjaraz-Ruedas † and Jesús A. Cruz-López \*,‡**

Colección Nacional De Arácnidos, Departamento de Zoología, Instituto de Biología, Universidad Nacional Autónoma de México, Ciudad Universitaria, Coyoacán, Mexico City C. P. 04510, Mexico; offb@ib.unam.mx (O.F.F.); roy\_monrue@hotmail.com (R.M.-R.)

**\*** Correspondence: thelyphonidito@gmail.com


**Abstract:** Sistema Huautla is the deepest cave system in the Americas at 1560 m and the fifth longest in Mexico at 89,000 m, and it is a mostly vertical network of interconnected passages. The surface landscape is rugged, ranging from 3500 to 2500 masl, intersected by streams and deep gorges. There are numerous dolinas, from hundreds to tens of meters in width and depth. The weather is basically temperate subhumid with summer rains. The average yearly rainfall is approximately 2500 mm, with a monthly average of 35 mm for the driest times of the year and up to 500 mm for the wettest month. All these conditions play an important role for achieving the highest terrestrial troglobite diversity in Mexico, containing a total of 35 species, of which 27 are possible troglobites (16 described), including numerous arachnids, millipedes, springtails, silverfish, and a single described species of beetles. With those numbers, Sistema Huautla is one of the richest cave systems in the world.

**Keywords:** troglobitics; arachnids; insects; millipedes

### **1. Introduction**

Caves are some of the most adverse environments on earth, as the restricted access to food and the extreme conditions of darkness and humidity make these habitats very challenging for living organisms [1]. Despite this, many taxa have colonized these subterranean environments, including arthropods and vertebrates [2,3]. Some of these animals have become fully established in cave systems; therefore, they have evolved specific morphological adaptations (troglomorphisms) such as cuticular depigmentation, elongation of appendages, and reduction or loss of eyes [4]. Troglobitic animals (those who exhibit troglomorphisms) are excellent models for studies of the evolution, e.g., cave adaptations such as morphological convergences among distant lineages [5–8].

Caves are extremely important in terms of diversity, due to the number of endemic species inhabiting these environments [2]. For example, in the class Arachnida, troglobites are known for 9 of the 11 extant orders; only Thelyphonida (vinegaroons) and Solifugae (camel spiders or wind scorpions) lack cave representatives [9], and the Huautla System has a fair representation of all other Arachnida groups.

Cave explorations in the Systema Huautla have taken place since the mid-1960s, and multiple discoveries in terms of speleology but also in biological diversity have been reported [1–8]. However, this exploration is yet to be completed as new expeditions recently have concluded in extending the length of the system (see http://www.mexicancaves.org/ maps/0104 (accessed on 27 August 2021)) and have also increased the number of new species inhabiting the system [1,2,8,9].

As part of the Special Issue "Hotspots of Subterranean Biodiversity" in the Diversity journal, we have decided to put together a list of the numerous taxa inhabiting one of the most important cave systems in Mexico revisiting all of the available literature. This list exemplifies the large biological diversity of the system, and furthermore, this resource can

**Citation:** Francke, O.F.; Monjaraz-Ruedas, R.; Cruz-López, J.A. Biodiversity of the Huautla Cave System, Oaxaca, Mexico. *Diversity* **2021**, *13*, 429. https://doi.org/ 10.3390/d13090429

Academic Editors: Tanja Pipan, David C. Culver and Louis Deharveng

Received: 29 June 2021 Accepted: 25 August 2021 Published: 6 September 2021

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

be used as reference for future works not only on diversity but evolution, conservation, and inspiration for cave exploration in the several unexplored cave systems in Mexico.

#### **2. Sistema Huautla**

This is the deepest cave system in the Americas at 1560 m, and the fifth longest in Mexico at 89,000 m, and it is a mostly vertical network of interconnected passages, a few of which can be traversed by humans, but there are undoubtedly many narrow cracks and crevices inaccessible to cavers. Located at Huautla de Jiménez, on the Sierra Mazateca in the northwestern part of the State of Oaxaca, the caves have been explored by skilled speleologists since 1966, with international and local support. The topography of the cave is formed by multiple pits, stretch passages, and crevices (See http://www.mexicancaves. org/maps/0104 (accessed on 27 August 2021).

The Sierra Mazateca is basically a massive Cretaceous karst formation, up to 5000 m thick in some areas, overlain by older intrusive volcanic rocks. The landscape is rugged, ranging from 3500 to 2500 masl, intersected by streams and deep gorges. There are numerous dolinas, from hundreds to tens of meters in width and depth. Rural land in Mexico is mostly owned by the community living there (called "ejidos"), and permission is required from the local authorities before any exploration.

The weather is basically temperate subhumid, with a marked dry season from March to May. The average yearly rainfall is approximately 2500 mm, with a monthly average of 35 mm for the driest times of the years and up to 500 mm for the wettest month. The average monthly temperature is 23.6 ◦C, and the minimum average monthly temperature is 9.4 ◦C for the coldest month of the year and 13 ◦C for the warmest month, whereas the monthly daily average is of 14 ◦C for the coldest month and 29 ◦C for the warmest month [10].

The major caves are each associated with one of the larger dolinas, and the walls of the dolinas also have side entrances and pits at varying depths. There are at least 28 entrances and 6 major caves. The system continues to be pushed to new depths and lengths on a yearly basis. The six major caves in the system are: Sótano de San Agustín, Sótano del Río Iglesia, La Grieta, Sótano de Agua de Carrizo, Li Nita, and Nita Nanta (Figure 1). The Huautla system contains 50 species, with possibly 27 or more troglobites total (16 described), including arachnids, millipedes, springtails, silverfish, and a beetle [11–14]. In Li Nita, a new troglobitic scorpion of the genus *Typhlochactas* Mitchel, 1971 was collected in 2014. A small colony of vampire bats roosts not far inside one of the three entrances to Sótano del Río Iglesia and another unidentified bat colony roost just inside Sótano de San Agustín. In its deepest parts (~1000 m), Huautla system contains long, deep sumps, which makes the exploration and collecting even harder, as diving experience is needed. Three caves were impacted by garbage dumping, including medical waste with syringes, but the dumping has ceased, and the PESH cavers (Proyecto Espeleológico Sistema Huautla) have begun a clean-up effort [11–14].

**Figure 1.** Sistema Huautla maps. (**A**) profile view, (**B**) plan view, (**C**) Sótano de San Agustín entrance. I, Nita Nanta entrance. II, Li Nita entrance. III, La Grieta entrance. IV, Sótano de Carrizo entrance. V, Sótano de San Agustín entrance. VI, Río Iglesia entrance. VII, Sump 9. Cave maps provided by William Steele. Photo on Figure C taken by Liz Rogers, 2015.

#### **3. Biological Diversity**

Although troglobionts have been collected in the Huautla Cave System since the beginning explorations, it has only been in the past eight years that real collecting efforts have been conducted by a group of arachnologists from the National Arachnid Collection at Universidad Nacional Autónoma de México (CNAN, UNAM). We were kindly invited by the organizers of the PESH expeditions on a yearly basis, exploring over 40 caves in the area. The PESH Expeditions are organized in the months of April–May of each year, during the dry season to minimize the risk of being trapped underground by flooding;

the entire expedition lasts one month, with about 100 cavers from around the world participating. We usually participate during the third week of the expedition, when the speleologists have already rigged several of the deeper caves; therefore, we can concentrate our efforts strictly on collecting with minimal time spent securing ropes. Usually, three to four members go into a cave with the support of several speleologists, and two or three remain collecting on the surface so that we can understand better the origin and evolutions of the troglobionts inside a given cave. Some species, such as the highly troglomorphic scorpion *Alacran tartarus* Francke, 1982, have been collected as shallow as −60 m and as deep as −920 m in several different caves; therefore, presumably, it moves into the deeper, common passage region of the system and can then disperse upwards through the numerous shafts. However, others, such as the troglobitic tarantula *Hemirrhagus grieta* (Gertsch, 1982), are confined to the middle depths (from −300 to −600 m) of a single cave (Cueva de La Grieta).

It is remarkable that no stygobionts have been collected in the system, even though there have been numerous diving expeditions to explore the deepest sumps, from both ends, i.e., from inside the cave and from the resurgence 7 km away. There are several reports, photographs and at least one video showing *Alacran* walking/running underwater; and the only other living animals reported at those depths are tadpoles (immature anurans), undoubtedly carried down during the flash floods of the rainy season. The tips of the pedipalp fingers in *Alacran* have sharp elongated hooks, which presumably can be an adaptation to catch those slippery prey items underwater, although nobody has seen a scorpion feeding on a tadpole underwater (or anywhere else).

The Arachnid collection in Mexico City has 19 species of the 27 known troglobionts in the Huautla Cave System, earning it one of the top ranks as the most diverse explored underground biotas in the world [15,16]. Sistema Huautla is the third-most diverse cave in Mexico, after Cueva de la Mina, Tamaulipas (24 troglobionts out of 60 species), and Sistema Purificación, also in Tamaulipas (19 troglobionts out of 103 species), followed in fourth place by Sistema de los Sabinos, San Luis Potosi, with 14 troglobionts out of 127 species (William Elliott, pers. comm.). A detailed list of the troglobionts in Sistema Huautla is given in Table 1. Underground collecting during the rainy season may increase the number of recorded troglobionts for all cave systems, as many terrestrial arthropods are known to overwinter in inactive stages during the drier and colder winter months, such as mites and ticks (Acari); however, due to the risk of flooding, this task is basically impossible to achieve.

It is important to emphasize that in the Huautla Cave System, seven different species of troglomorphic tarantulas of the genus *Hemirrhagus* Simon, 1903 have been collected thus far (several still undescribed): two of them from the same cave (La Grieta), though they are found at different depths and have not been collected together. The other five species are from separate entrances into the system and do not appear to be closely related, presumably representing independent invasions from epigean ancestor(s).

At least two species of bats (Mammalia: Chiroptera) have been photographed from caves in the Sistema Huautla; one is a leaf-nosed bat belonging to the family Phyllostomatidae. We have not been able to collect in the guano piles associated with those roosting areas, and it is likely there might be some additional troglobionts there.

#### **4. Class Arachnida**

#### *4.1. Order Araneae*

Spiders are among the most common arachnids into caves. According to available estimates, around 1000 troglomorphic spiders worldwide have been classified as troglobionts [9,17,18]. They are located from the entrance to the deepest galleries of the caves, although not all the species found in the caves are exclusive to the underground environments. Around 25–30% are accidental (trogloxenes) and appear in the entrance area; about 50% are regularly found in caves but also in the epigean environment (troglophiles); and between 20–25% are strictly cave inhabitants (troglobitic) [19,20]. In the Huautla Cave

System seven families, eleven genera and thirteen species of spiders have been recorded. The most diverse spider families are Pholcidae and Theraphosidae. In Mexico, pholcids spiders are the most common spider family found in caves, mainly in the entrances and median-depth zone of caves and grottoes. Huber (2018) [21] reported about 86 species of troglomorphic pholcid species worldwide, including 21 eyeless species and 21 species with strongly reduced eyes. Most troglomorphic pholcids spiders are representatives of only two genera: *Anopsicus* Chamberlin and Ivie, 1938 and *Metagonia* Simon, 1893 [21] with *Metagonia* represented in the Huautla Cave System (Table 1). Mexico is by far the richest country in terms of troglomorphic pholcids. This apparent dominance may partly be due to collectors' and taxonomists' biases [21], mainly towards the northern cave systems (e.g., Purification and Cuetzalan Cave Systems). Mygalomorph spiders are generally poorly represented in the cave faunas of the world. Mexico holds the second highest diversity of species of tarantulas worldwide behind Brazil, and Mexico is the richest country in tarantula cave species. The genus *Hemirrhagus* is endemic to Mexico and has 27 described species, with epigean, troglophile, and troglobitic species [22]. In the Huautla Cave System, *Hemirrhagus grieta* Gertsch, 1982 and *H. billsteelei* Mendoza and Francke, 2018 have been described; however, an additional five probably new species have been recently collected (Table 1; Figures 2A,D and 3D). Although important contributions about the spider fauna of the cave system have been published in the last five years, the Huautla Cave System is still poorly known and explored with respect to their spider fauna diversity.

#### *4.2. Order Opiliones*

In Mexico, a total of 265 species of harvestmen (Opiliones) has been recorded, representing the four extant suborders ([23] and Cruz-López, pers. obsv.). The Mexican fauna of Opiliones is remarkable because about 20% are troglobites or troglophiles, and estimates indicate that there is still a high percentage of undescribed species for the country, especially those that inhabit different cave systems [23].

Until 2003, only two species of Stygnopsidae were reported for the Huautla system; *Hoplobunus mexicanus* (Roewer, 1915) and *Karos gratiosus* Goodnight and Goodnight, 1971 [24]. However, recent taxonomic revisions of several genera of Stygnopsidae have revealed that the original taxonomic determinations made by Goodnight and Goodnight during 1953–1973 are erroneous [25,26]. Currently, *H. mexicanus* is in fact an undescribed species of *Stygnopsis* Sørensen, 1932 (Figure 2C), and all previous records published of *K. gratiosus* now correspond to *Huasteca kardia* Cruz-López and Francke, 2019. Additionally, Cruz-López et al. (2019) [27] described the endemic genus *Minisge* Cruz-López, Monjaraz-Ruedas and Francke, 2019, which includes two highly troglomorphic species: the shallower *Minisge sagai* Cruz-López, Monjaraz-Ruedas and Francke, 2019 (Figure 3F) and the deeper *Minisge kanoni* Cruz-López, Monjaraz-Ruedas and Francke, 2019. It is hypothesized [27] that both species of *Minisge* colonized the Huautla System independently, with *M. kanoni* being the oldest one, colonizing the caves about 3.3 Mya ago, whereas the colonization by *M. sagai* occurred very recently. It is remarkable that this species inhabiting the shallow regions of the cave system does not present a large genetic diversification or structure, among the populations in the many caves it has been found.

**Figure 2.** Troglobiont fauna of Sistema Huautla. (**A**) *Hemirrhagus* sp., (**B**) *Paraphrynus grubbsi* Cokendolpher and Sissom, 2001, (**C**) *Stygnopsis* sp., (**D**) *Hemirrhagus billsteelei* Mendoza and Francke, 2018, (**E**) *Rhachodesmus digitatus* Causey, 1971. Photos by Jean Krejca, Figure (**D**) taken and modified from Mendoza and Francke, 2018.

**Figure 3.** Troglobiont fauna of Sistema Huautla. (**A**) *Typhlochactas* sp., (**B**) *Sphaeriodesmus iglesia* Shear, 1986, (**C**) *Typhlochactas* sp. collecting method, (**D**) *Hemirrhagus grieta* Gertsch, 1982, (**E**) *Alacran tartarus* Francke, 1982, (**F**) *Minisge sagai* Cruz-López et al. 2019. Photos by Jean Krejca and CNAN-IBUNAM historic record.

Another remarkable species inhabiting the Huautla System is an undescribed species of an undescribed genus of the subfamily Gagrellinae (Eupnoi: Sclerosomatidae), which seems to be closely related to *Parageaya* Mello-Leitão, 1933, but currently, only considered as a member of Gagrellinae with troglomorphic features such as body pale color. Other record of troglomorphic species in the system is an undescribed *Stygnomma* Roewer, 1912. However, since the revision of Stygnommatidae by Pérez-González (2009) [28], *Stygnomma* was recognized as a polyphyletic assemblage, with the fauna of southern Mexico belonging in fact to members to the family Biantidae, but, unfortunately, a taxonomic act has not yet been formalized for this group.

Finally, near of the entrances of several caves, we have found specimens of an undescribed species of *Isaeus* Sørensen, 1932, but this species does not exhibit any troglomorphic features.

#### *4.3. Order Amblypygi*

Whip spiders are conspicuous arachnids that are very common in caves and most of the times very easy to spot by people visiting caves. The case of the Sistema Huautla is not different, and there are at least two different species reported in several caves of the system belonging to the genus *Paraphrynus* Moreno, 1940. The only species described as *Paraphrynus grubbsi* Cokendolpher and Sissom, 2001 (Figure 2B) was reported for several caves including Nita Lajao, Sótano de San Agustín, Cueva del Escorpión, etc., suggesting that this species is widely distributed in the system and can probably move from cave to cave. Additionally, Cokendolpher and Sissom (2001) [29] reported an undescribed species of *Paraphrynus* which was considerably smaller and morphologically distinct from *P. grubbsi* which has well-developed eyes, suggesting that it could be an epigean species not adapted to caves which lives in the surroundings of Sistema Huautla and can be found occasionally in the caves [29].

#### *4.4. Order Pseudoscorpiones*

A few small pseudoscorpions have been collected in the Huautla Cave System [13], but they have not been fully identified yet.

#### *4.5. Order Schizomida*

Short-tailed whip scorpions are represented in the cave system by a single species *Baalrog magico* (Monjaraz-Ruedas and Francke, 2018), which was originally described under the genus *Stenochrus* Chamberlin, 1922 and then transferred to the newly described genus *Baalrog* Monjaraz-Ruedas, Prendini and Francke, 2019, which harbors species restricted to the cave environment. Other members of genus *Baalrog* are distributed in cave systems in Valle Nacional, Oaxaca, and Atoyac, Veracruz [30]. Although there are other undescribed species of schizomids in the Sierra Mazateca outside the caves, those belong to different genera and are apparently not related to the species distributed inside Sistema Huautla [31].

#### *4.6. Order Scorpiones*

The Huautla Cave System is unusual among Mexican caves as it is the only one that harbors two species of troglobiont scorpions, both belonging to the family Typhlochactidae. *Typhlochactas* n. sp. is a small species about 2 cm long, totally eyeless and unpigmented, known from a single specimen collected about −100 m depth in one of the major caves (Figure 3A,C). The genus *Typhlochactas* has eight described troglomorphic species and a couple of new ones waiting to be described. This is the first record for the Huautla System [32].

The second one is *Alacran tartarus* (Figure 3E), which is about 8 cm long and is known from more than a dozen specimens collected in several caves in the system. It is a tan brown in color and is also completely eyeless. Two other species are known in the genus: one from the state of Puebla from a cave system that was presumably at one time interconnected to Systema Huautla and the other one from Oaxaca about 50 km away, both known from single collection events, as some caves in Mexico are not visited very frequently [33].

#### **5. Class Diplopoda**

Five endemic troglobitic species of millipedes belonging to four genera, four families, and four separate orders have been collected inside the Huautla Cave System. Perhaps, the most spectacular is a large yellow and bluish-green rhachodesmid which is quite abundant in the aptly named Millipede Cave (Figures 2E and 3B).

#### **6. Class Insecta**

There are five orders of insects reported from the Huautla Cave System, none of which are very abundant. There are two species of ground-dwelling beetles, two species of springtails, and one of each cave cricket, silverfish and dipluran (Table 1). Videos taken inside several of the caves in the system show flying insects glowing from the lights of the cavers, but as far as we know, they have not been collected and identified.

**Table 1.** A detailed list of the troglobionts in Sistema Huautla is given. Abbreviations: TB, troglobiont; SB, stygobiont; TP, troglophile.



**Table 1.** *Cont.*

: ? = there is no enough data about the biological ecology of the species. Yes? = there is no enough data about geographical distribution.

#### **7. Class Crustacea**

As mentioned earlier, no stygobionts have been observed in the Huautla Cave System, and there are two unidentified species of rolly-pollies (Isopoda) found in the cave, neither one with troglomorphic features.

#### **8. Conservation**

As we have shown, caves are important harbors of biodiversity and should be considered as habitats of high priority for conservation due the high number of endemic species inhabiting them. Their importance for evolutionary studies makes them natural laboratories for the study of evolution. Caves are important water reservoirs, which help maintain stability on the entire environments, as they store and provide water during the dry seasons. In recent years, we have noticed an increase in cave conservation activities in three areas. First, several cave entrances were used to dump garbage by the local residents; however, that practice has been actively discouraged by members of the PESH Expeditions, and in many cases, it has ceased altogether. Second, these same expedition participants have played a significant role in cleaning up some of those cave entrances that had been severely affected. Finally, some of the cave entrances are now protected by locked fences, and access is only allowed with the proper authorization from the local authorities. None of the cave entrances are commercially exploited, and very few speleologists visit the caves, as they are technically challenging and remote. Cave explorations as well as research should emphasize the importance of conservation, combating the myths behind caves as they are highly perpetuated in the communities as places of evil, which only results in the destruction of these important environments. Finally, an extensive multidisciplinary effort is necessary to incorporate Huautla System into conservation agendas, in order to propose the system as a protected area given the biological relevance mentioned above. In addition, one of the most important points for the conservation of cave species is the wide gap of knowledge that exists on the species inhabiting these systems, since generally only very few observations during explorations were reported from Huautla System.

**Author Contributions:** The three authors participated equally during the field work and speleological expeditions, curation of the material, and in writing the manuscript. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Acknowledgments:** We are thankful to W. Steele, T. Shifflet, M. Minton, I. Romms, and S. Davlantes for assistance in entering the caves: to J. Mendoza, G. Contreras. D. Barrales, and A. Guzman for collecting in the caves; J. Krejka provided some photos and collected with us; A. Valdez assisted with the identification of spiders; and W. Elliott, G. Roewer, J. Reddell, and C. Jewel shared with us information on the cave inhabitants. Finally, we express our gratitude to T. Pipan for inviting us to contribute to this Special Issue on subterranean biodiversity.

**Conflicts of Interest:** Authors declare no conflict of interest.

#### **References**

