Seaweed-Associated Diatoms (Bacillariophyta) in Dokdo of South Korea: I. Subphyla Melosirophytina, Coscinodiscophytina, and Class Mediophyceae

Abstract

Dokdo is an island located in the easternmost part of Korea, which has high levels of biodiversity of birds and fish, especially marine invertebrates. However, the biodiversity of microalgae, especially diatoms (Bacillariophyta), is relatively unknown, despite their ecological importance as primary producers of the marine food web and bioindicators of environmental conditions associated with climate change. To understand the biodiversity of seaweed-associated diatoms from Dokdo, we collected macroalgae present at a depth 5–15 m by SCUBA diving on 17 October 2017. There were a large number of diatoms (over 130 species), even though it was a one-time survey. As it includes too many taxa to cover at once, voucher flora for other taxonomic groups will be provided through the continuous serial papers. This is the first series of seaweed-associated diatoms, with 26 species belonging to the subphyla Melosirophytina and Coscinodisophytina, and the class Mediophyceae. Among these, seven species including one new taxon were reported for the first time in Korea, which, along with the geopolitical characteristics of the survey area, proved that there is no domestic interest in seaweed-related diatoms. In particular, the appearance of species that have been reported in subtropical waters, such as the order Ardissoneales, requires continuous monitoring of marine seaweed-associated diatoms to confirm whether their colonization in Dokdo waters was due to climate change or species-specific water temperature tolerance.

1. Introduction

Dokdo is recognized for its geographical importance and the need for conservation, and has been designated as a national natural monument and national marine park. Geologically, the island is composed of basalt and trachyte, which is known for its durability, and forms the foundation of its unique topography. Moreover, the coastal cliffs exhibit distinct geological features. Overall, these ecological, geographical, and geological characteristics emphasize the significance of Dokdo and its preservation and management practices. Dokdo has a high level of biodiversity, particularly of birds, fish, and invertebrates. Myoung et al. [1] found 72 fish species by SCUBA investigation, and Song et al. [2] documented 578 marine invertebrate species belonging to various phyla, including Mollusca, Arthropoda, Annelida, Cnidaria, Echinodermata, and Porifera. Ninety-six algal species, categorized into Chlorophyta, Phaeophyta, Rhodophyta, and Spermatophyta, were documented by Choi et al. [3]. Additionally, Kim and Park [4] investigated phytoplankton dynamics, reporting 72 species in the summer. Despite the richness of biodiversity in Dokdo, knowledge of microalgal biodiversity remains relatively limited, despite its ecological significance as a primary producer and bioindicator.

Diatoms play significant roles in marine ecosystems, and serve as primary sources of energy for aquatic food webs. These microscopic organisms are responsible for capturing sunlight and converting it into organic matter through photosynthesis, making them crucial for sustaining the entire food chain in aquatic environments. In particular, periphytic diatoms that live on seaweeds have been recognized as environmental bioindicators because of their sensitivity to changes in water quality and habitat conditions (e.g., [5]). Their abundance, diversity, and health provide valuable insights into the overall ecological health and integrity of aquatic ecosystems. Despite their importance, the study of diatoms in the Dokdo has been relatively neglected, requiring further research to better understand their ecological roles and contributions to the island’s marine environment. In particular, consistent, reliable identification of the diatoms is fundamental to correctly understanding their ecological roles in the marine ecosystem. For consistent and reliable identification, the provision of voucher flora, which is a collection of light microscope images, has been suggested in the Diatoms of North America project [6].

The first study on epiphytic diatoms in the Korean waters was conducted by Cho et al. [7]. They analyzed diatoms attached to Porphyra tenera Kjellman collected from various regions along the Korean coast. Their research identified 18 species, documenting the ecological distribution, and a brief description of the diatoms with accompanying micrographs. Since then, several studies on epiphytic diatoms have focused on the community structure associated with seaweeds [8,9] or seagrass [10,11], but have not provide diatom images to guarantee data quality. Cho [12] provided images of epiphytic diatoms, but the study was conducted in Lake Gocheonam, not in marine environments. Thus, there have been few floristic studies on epiphytic diatoms in Korean coastal waters.

This is the first study on seaweed-associated diatoms in Dokdo, South Korea. There were a large number of diatoms (over 130 species), which was high, even though it was a one-time survey. As a large number of taxa had to be studied simultaneously; voucher flora for other taxonomic groups will be provided in continuous serial papers. This is the first series of species in the subphyla Melosirophytina, Coscinodiscophytina, and class Mediophyceae. Voucher flora for other taxonomic groups, such as the subclass Fragilariophycidae, orders Mastogloiales and Naviculaes, and subclass Bacillariophycidae, will be successively prepared. This study was conducted to investigate the biodiversity of diatoms in Dokdo and highlight their contributions to underwater biodiversity and the overall health of the marine ecosystem. The findings of this research are expected to provide valuable information for the conservation and management of marine environments in Dokdo.

2. Materials and Methods

2.1. Study Area

Dokdo is a South Korean territory located in the southwestern part of the East Sea, approximately 87.4 km southeast of Ulleungdo (Figure 1A). Dokdo consists of two main islets, Seodo (western islet) and Dongdo (eastern islet), with approximately 89 smaller rocky islets (Figure 1B). The total area of Dokdo is approximately 187,554 km², with Dongdo covering approximately 73,297 km² and Seodo covering approximately 88,740 km². Dongdo hosts residential facilities and related infrastructures, whereas Seodo is a nature reserve. Dokdo island is a volcanic island and is geologically mainly composed of basalt, trachyte, and trachyandesite. The coastline of Dokdo is characterized by steep cliffs and sea caves, and the surrounding waters are deep, with strong ocean currents. A maritime climate was observed, with an average annual temperature of around 12 °C. This area experiences strong winds and frequent fog. The waters surrounding Dokdo are rich fishing grounds that are home to a variety of marine organisms. However, some areas are restricted from preserving their natural environments. Dokdo holds significant geographical, ecological, and historical value, and serves as a symbol of South Korea’s territorial sovereignty.

2.2. Sample Collection and Morphological Analysis

To understand the biodiversity of seaweed-associated diatoms in Dokdo, we collected macroalgae at a depth of 5–15 m by SCUBA diving on 17 October 2017 (Figure 1). The collected algal samples were placed in a 1 L polyethylene bottle with 500 mL filtered seawater and vigorously shaken to detach the diatoms from the thalli of the seaweeds and it was then screened through a 200 µm sieve. The filtrates obtained were fixed in Lugol’s solution. Cell organelles and organic matter in the materials were removed to observe the fine structure and specific characteristics of the diatoms [13]. Briefly, equal amounts of saturated potassium permanganate and 33% hydrochloric acid were added to the raw diatom materials and boiled in 80 °C until the color became clear. The acid was then removed using distilled water. The cleaned samples were rinsed until free of acid. The cleaned material was mounted in Pleurax and observed at 400× to 1000× magnification, using a light microscope (Olympus 80i; Nikon, Tokyo, Japan) with a digital camera. For species identification, we provided various literature sources that referred to identifying the species found in Dokdo. Dimensions are provided for all species and a brief description is provided for the newly recorded species in Korean waters. The species found in Dokdo were checked against the National Institute of Biological Resources checklist and other relevant checklists [14,15].

3. Results

A total of 26 diatom taxa in the subphyla Melosirophytina and Coscinodisophytina, and class Mediophyceae were observed (Figure 2, Figure 3, Figure 4, Figure 5, Figure 6, Figure 7, Figure 8, Figure 9, Figure 10, Figure 11, Figure 12, Figure 13, Figure 14, Figure 15, Figure 16, Figure 17 and Figure 18). Brief descriptions were only documented for 21 species that were unrecorded or rarely recorded in South Korea (

Table 1. List of newly reported species in South Korea.

No.	Species	Remarks
1	Hyalodiscus ambiguous (Grunow) Tempère & H. Peragallo, 1889	New record in South Korea
2	Podosira hormoides (Montagnei) Kutzing, 1844	New record in South Korea
3	P. montagnei Kutzing, 1844	New record in South Korea
4	Actinocyclus pruinosus Castracane, 1886	New record in South Korea
5	Ardissonoepsis dokdoensis J.S. Park	New to science
6	A. fulgicans Lobban & Ashworth, 2022	New record in South Korea
7	Grunowago pacifica Lobban & Ashworth, 2022	New record in South Korea

), and the figures and dimensions for all diatoms are provided because the truth of species occurrences may be re-evaluated in future. Higher classification rank into subclass followed Mann in Adl et al. [16], and the lower rank including order and family followed Algaebase [17].

Subphylum Melosirophytina D.G. Mann in Adl et al., 2019

Subclass Melosirophycidae E.J. Cox, 2015

Order Melosirales R.M. Crawford in Round et al., 1990

Family Hyalodiscaceae R.M. Crawford in Round et al., 1990

Genus Hyalodiscus Ehrenberg, 1845

Hyalodiscus ambiguus (Grunow) Tempère & H. Peragallo, 1889 [Figure 7A–C (SEM)]

Basionym: Podosira ambigua Grunow, 1879.

Reference: Peragallo and Peragallo [18], pl. 119, fig. 19; Al-Handal et al. [19], figs 35, 36; Al-Handal et al. [20], p. 4, pl. 1, fig. 7.

Samples: Dokdo9.

Dimensions: 120.6 µm in diameter; areolae 14 in 10 µm.

Description: Valve is lens-shaped with a slightly convex valve face (Figure 7A). Hyaline central area is relatively narrow (Figure 7B). Areolation is spiral arrangement of the row of areolae (Figure 7A,B). Small rimoportulae are scattered on the whole of the valve (Figure 7C).

Korean occurrence: This is the first report from Korean waters.

World distribution: Hyalodiscus ambiguus has been rarely reported worldwide: St-Paul [18]; South Africa [21,22]; South Iraq [19]; Reunion and Rodrigues Islands [20].

Hyalodiscus scoticus (Kützing) Grunow, 1879 [Figure 2A–C (LM); Figure 7D–I (SEM)]

Basionym: Cyclotella scotica Kützing, 1844.

Reference: Witkowski et al. [23], p. 34, pl. 7, figs 3, 4; Lee et al. [24], p. 580, fig. 5e.

Samples: Dokdo2, Dokdo9, Dokdo11, Dokdo14.

Dimensions: Valves 17.5–25.3 µm in diameter; areolae 28–30 in 10 µm.

Description: Valves are lens-shaped with a moderate convex valve face (Figure 7D,E,G,H). Hyaline central area is variable, but usually occupying half of the valve (Figure 7D,E,G,H). Areolae are fine, and invisible in LM. Areolation is spiral arrangement of the row of areolae (Figure 7E,H). One ring of rimoportulae located in the valve margin (Figure 7H,I); externally opened as the perpendicularly elliptical pores that are difficult to distinguish from the ambient areolae (Figure 7F); internally slit as the horizontally simple sessile (Figure 7I). There were no additional rimoportulae on the valve face (Figure 7H).

Korean occurrence: Shim [25] mentioned this species has been found throughout Korean waters. Lee et al. [24] found this species from Baeknyeong Island in the Yellow Sea.

World distribution: Hyalodiscus scoticus has been widely reported worldwide: Tanzanian coast [26]; South Africa [27]; Eastern Australia [28]; Black Sea [29]; the Revillagigedo Archipelago, Mexico [30].

Comment: Hyalodiscus species has mainly been reported based on only LM images. There is little information on the fine structure under SEM. Due to the distinct hyaline area, which is certainly observed under LM, SEM examination may be somewhat overlooked.

Genus Podosira Ehrenberg, 1840

Podosira hormoides (Montagne) Kützing, 1844 [Figure 2D–G (LM); Figure 7J–M (SEM)]

Basionym: Melosira hormoides Montagne, 1839.

References: Peragallo and Peragallo [18], p. 444, pl. 120, fig. 12; Hustedt 1927–1930, figs 123–125; Lobban et al. [31], p. 248, pl. 4, figs 1, 2.

Sample: Dokdo4; Dokdo6; Dokdo9; Dokdo14.

Dimensions: Valves 44.8–67.7 µm in diameter; areolae 11–12 in 10 µm.

Description: Valves are lens-shaped with a moderate convex valve face (Figure 7J,L). Areolae are loculate and externally cribra (Figure 7K), and internally poroid (Figure 7M), and arranged in fasciculate rows in the radial sectors (Figure 7K). Small rimoportulae are scattered in the whole of the valve and replace the areolae (Figure 7K,M), which are externally opened by a simple pore (Figure 7K), and internally slit in a sessile (Figure 7M).

Korean occurrence: This is the first report from Korean waters of the temperate northern Pacific.

World distribution: Podosira hormoides was originally described from Callao, Peru [32]. Subsequently, this species has been rarely reported worldwide: Normandie, France [18]; South Africa [33]; Guam [31].

Comment: Park et al. [34] reported this species from Chuuk, FSM., but the figures and the areolae density of the Micronesian specimens are not P. hormoides.

Podosira montagnei Kützing, 1844 [Figure 2H (LM); Figure 7N,O (SEM)]

References: Hustedt [35], p. 281, fig. 122.

Sample: Dokdo7; Dokdo9.

Dimensions: Valves 26.4–44.5 µm in diameter; areolae 22–24 in 10 µm.

Description: Valves are lens-shaped with a moderate convex valve face (Figure 7N). Areolae structures are indistinct in our study, and arranged in decussate rows in the radial sectors (Figure 7O). Small rimoportulae are scattered in the whole of the valve, externally opened by depressed simple pores (Figure 7O).

Korean occurrence: This is the first report from Korean waters of the temperate northern Pacific.

World distribution: Podosira montagnei was originally described from the Antillean Sea [36]. Subsequently, this species has been rarely reported worldwide: Normandie, France [18]; Guam [31].

Subphylum Coscinodiscophytina Medlin & Kaczmarska, 2004

Class Coscinodiscophyceae Round & R.M. Crawford in Round et al., 1990

Subclass Coscinodiscophyceae Round & R.M. Crawford in Round et al., 1990

Order Asterolamprales Round & R.M. Crawford in Round et al., 1990

Family Asterolampraceae H.L. Smith, 1872

Genus Asteromphalus Ehrenberg, 1844

Asteromphalus heptactis (Brébisson) Ralfs in Prichard, 1861 [Figure 2I (LM)]

Basionym:Spatangidium heptactis Brébisson, 1857

References: Chung [37], p. 174, pl. 31, fig. 238; Shim [25], p. 184, pl. 18, figs 50a, b; Hasle and Syvertsen [38], p. 137, pl. 24; Tiffany and Hernández-Becerril [39], p. 254, pl. 2, fig. 2; pl. 15, 1–4; pl. 16, 1–4; pl. 17, 1–6; pl. 18, 1–6; pl. 19, 1–6.

Samples: Dokdo15.

Dimensions: Valve 47.2 µm in diameter; seven areolae in 10 µm; seven hyaline rays.

Korean occurrence: Asteromphalus heptactis has been frequently reported in Korean coastal waters [25,37]. Choi [40] frequently found this species surrounding the coastal waters of South Korea.

World distribution: Asteromphalus heptactis was originally described from the Peruvian coast [41]. Subsequently, this species has been frequently reported worldwide [38].

Order Coscinodiscales Round & R.M. Crawford in Round et al., 1990

Family Coscinodiscaceae Kützing, 1844

Genus Coscinodiscus Ehrenberg, 1839

Coscinodiscus asteromphalus Ehrenberg, 1844 [Figure 2J (LM)]

References: Hasle and Lange [42], p. 42, figs 1–14; Shim [25], p. 167, fig. 57; Hasle and Syvertsen [38], p. 104, pl. 15.

Sample: Dokdo13.

Dimensions: Valve 157 µm in diameter; three areolae in 10 µm.

Korean occurrence: This species has been frequently reported in Korean coastal waters (e.g., [25]).

World distribution: Coscinodiscus asteromphalus has a wide distribution with a wide temperature tolerance [38].

Coscinodiscus marginatus Ehrenberg, 1843 [Figure 2K (LM)]

References: Sancetta [43], p. 240, pl. 1, figs 1–13; Hasle and Syvertsen [38], p. 107, pl. 18; Nikolaev et al. [44] p. 17, pl. 16, figs 1–6.

Sample: Dokdo9.

Dimensions: Valve 26.9 µm in diameter; five areolae in 10 µm.

Korean occurrence: This species has been frequently reported in Korean coastal waters (e.g., [25]).

World distribution: Coscinodiscus marginatus is widely distributed worldwide [38].

Coscinodiscus radiatus Ehrenberg, 1840 [Figure 2L,M (LM)]

References: Chung [37], p. 163, pl. 28, fig. 212; Shim [25], p. 175, pl. 15, fig. 44; Hasle and Syvertsen [38], p. 107, figs 6d and 6e, pl. 18; Sar et al. [45], p. 517, figs 33–50.

Samples: Dokdo4; Dokdo15.

Dimensions: Valves 47.6–68.8 µm in diameter; 3–4 areolae in 10 µm.

Korean occurrence: Coscinodiscus radiatus has been frequently reported in Korean coastal waters [25,37,46].

World distribution: Coscinodiscus radiatus has been widely distribute worldwide [38], and distribute from tropical waters such as in Thailand [47,48] and the Philippines [49] to cold waters [50].

Family Hemidiscaceae Hendey ex Hasle, 1966

Genus Actinocyclus Ehrenberg, 1837

Actinocyclus pruinosus Castracane, 1886 [Figure 3A–E (LM), Figure 8A–F (SEM)]

References: Castracane [51], p. 144, pl. 4, fig. 2; Mann [49], p. 13.

Samples: Dokdo6; Dokdo7; Dokdo9; Dokdo11; Dokdo12.

Dimensions: Valves 37.9–49.8 µm in diameter; 6–7 areolae in 10 µm.

Description: Valves are flat with rounded edges (Figure 8A). Areolae are very coarse, externally covered by a raised cribrum that are connected together by one to three extended subsidiary siliceous (Figure 8B,C), and internally opened by foramina (Figure 8D,E). Areolae are arranged in fascicles separated by complete radial rows of areolae extending from the valve center to the mantle, and the areolae within the fascicle are decussate or sparsely irregular (Figure 8B,C). Central annulus forms a hyaline area by a collection of the first areola in each complete radial row of areolae, and which has one to three separate areolae (Figure 8B,E). Pseudonodulus is large and present near the junction of the flat part of valve face and mantle (Figure 8C,F). Rimoportulae are located at the distal end of each complete areolae row, and externally opened in the mantle (Figure 8E,F).

Korean occurrence: This is the first report from Korean waters of the temperate northern Pacific.

World distribution: Actiniocyclus pruinosus was collected in the Pacific Ocean during the expedition of H.M.S. Challenger 1873–1876 [51]. Subsequently, this species was reported from Philippine Islands by Mann [49], but there was no illustration for the species. This species is rare worldwide.

Comment: The elegant areolae arranged in fasciculate separated by complete radial rows of A. pruinosus is similar to that of A. octonarius var. tenellus. However, the areolae arrangement within fascicle of A. pruinosus are decussate or sparsely irregular, while A. octonarius var. tenellus are parallel and dense [52]. In addition, both species can be distinguished by areolae density (6–7 versus 9–11 in 10 µm, respectively).

Actinocyclus subtilis (Gregory) Ralfs in Prichard, 1861 [Figure 3F–L (LM); Figure 8G–L (SEM)]

Basionym: Eupodiscus subtilis Gregory, 1857.

Synonym: Actinocyclus tenuissimus Cleve, 1878.

References: Cleve [53], p. 21, fig. 34; Navarro [54], p. 429, figs 28, 29; Lobban et al. [31], p. 249, pl. 5, figs 1, 2; Lee et al. [55], p. 259, fig. 28; Park et al. [34], p. 7, figs 7a–b, 73.

Samples: Dokdo6; Dokdo7; Dokdo9; Dokdo11; Dokdo13.

Dimensions: Valves 39.9–88.3 µm in diameter; 12–18 areolae in 10 µm.

Description: Valves are flat to slightly depressed (Figure 8G). Areolae are fine, arranged inconspicuous bundles and more or less wavy (Figure 8H,K). Central annulus is large and filled with many areolae (Figure 8H,K). Pseudonodule is distinct and present on the valve face in the margin (Figure 8I,L). Rimoportulae are located at the distal ends of radial areolae rows (Figure 8J,L).

Korean occurrence: Lee et al. [55] frequently found A. subtilis surrounding the coastal waters of South Korea.

World distribution: Actinocyclus sutbilis was firstly described as Eupodiscus subtilis Gregory from the Firth of Clyde and Loch Fyne in Scotland [56]. Subsequently, it has been found in various regions: Plymouth [57]; northern South Sea [58]; south African waters [33,59]; Huwaiza marsh, South Iraq [19]; Chuuk, Micronesia [34].

Comment: Actinocyclus subtilis and A. tenuissimus are almost similar in overall morphology; both species are only distinguished by the density of areolae (12–15 in 10 µm versus 17–18, respectively) (e.g., [34]). However, the range of areolae density showed in the cultural specimen by Anderson et al. (1986) included the minimum and maximum range of areolae (namely, 12–17 in 10 µm). Therefore, the areolae density cannot be the distinguishing characteristic for both species. In Dokdo specimens, the small-sized A. subtilis has coarse areolae (12–15 in 10 µm), while the larger one has finer (16–18 in 10 µm); therefore, areolae density is a variation in the growth stage. In our SEM examination, there were no additional differences besides the areolae density. In addition, the co-occurrence of two morphotypes in Dokdo also support that both species are identical. Therefore, based on the morphological similarity and ecological co-occurrence, we regard both species, A. subtilis and A. tenuissimis, as identical, and propose A. tenuissimus is synonymized to A. subtilis according to the priority.

Subclass Rhizosoleniophycidae P.C. Silva, 1962

Order Rhizosoleniales P.C. Silva, 1962

Family Rhizosoleniaceae De Toni, 1890

Genus Pseudosolenia B.G. Sundström, 1986

Pseudosolenia calcar-avis (Schultze) B.G. Sundström, 1986 [Figure 3M (LM)]

Basionym: Rhizosolenia calcar-avis Schultze, 1858

References: Chung [37], p. 182, pl. 32, fig. 251; Shim [25], p. 193, pl. 20, fig. 59; Yun and Lee [60], p. 304, figs 2A–F.

Sample: Dokdo1.

Korean occurrence: This species has been mainly reported from the South Sea of South Korea [25]. Yun and Lee [60] found P. calcar-avis from the oceanic waters of Jeju Island and the Yellow Sea.

World distribution: Pseudosolenia calcar-avis is a circumglobally distributed species [61] and occurs in warm waters and occasionally in temperate waters [38].

Subphylum Bacillariophytina Medlin & Kaczmarska 2004

Class Mediophyceae (Jousé & Proshkina-Lavrenko) Medlin & Kaczmarska, 2004

Subclass Chrysanthemodiscophycidae D.G. Mann in Adl et al., 2019

Order Ardissoneales Round in Round et al., 1990

Family Ardissoneaceae Round, 1996

Genus Ardissonea De Notaris, 1870

Ardissonea formosa (Hantzsch) Grunow, 1880 [Figure 4A–C (LM), Figure 9A–K (SEM)]

Basionym: Synedra formosa Hantzsch, 1863

References: Shim [25], p. 283, fig. 266 (as Synedra formosa); Lobban et al. [31], p. 259, pl. 15, figs 4, 5, pl. 16, figs 1, 2; Park et al. [62], p. 105, fig. 5; Joh [63], p. 11, figs 46–49; Lobban et al. [64], p. 110, figs 1A–J.

Samples: Dokdo7; Dokdo9; Dokdo14.

Dimensions: Valves 269.8–370.5 µm long, 13.4–19.1 µm wide; transapical striae 8–10 in 10 µm.

Description: Valves are linear with rounded apices in large specimens (Figure 4A–C and Figure 9A,H). Striae are uniseriate, and consist of apically elongated areolae (Figure 9D–G). Two longitudinal costae are entirely positioned at the halfway point between the centerline and valve margin (Figure 9D–G), and correspond to the location of the bifacial annulus (Figure 9D–G). Central longitudinal line is not costa, which is formed by the offset striae meet in LM (Figure 4A–C). Pseudosepta are present at both poles (Figure 9I,K). Valves are double-walled and internally covered by the silica plate with three longitudinal small circular foramina lines (Figure 9I–J), and a large opening at the poles (Figure 9I,K).

Korean occurrence: This species was reported as Synedra formosa from the Goheung in the South Sea of South Korea [25]. Recently, Joh (2021) found this species from the Seogwipo coast of Jeju Island.

World distribution: Aridssonea formosa was originally described in the East Indies as Synedra formosa [65]. Since then, this species has been found in mainly warm water regions. In Northwestern Pacific, A. formosa has been reported from warm waters: Guam [31,64]; Chuuk, FSM [62]; Jeju Island of South Korea (Joh [63].

Ardissoneaaff. formosa (Hantzsch) Grunow, 1880 [Figure 4D–F (LM), Figure 10A–H (SEM)]

Samples: Dokdo3; Dokdo6; Dokdo7; Dokdo9; Dokdo13.

Dimensions: Valves 140.5–222.6 µm long, 12.7–16.7 µm wide; transapical striae 8–10 in 10 µm.

Description: Valves are linear-lanceolate in large specimens (Figure 4D and Figure 10A,E), and lanceolate with narrowed obtusely rounded apices (Figure 4E,F). Striae are uniseriate, and consist of apically elongated areolae (Figure 10A–D). Two longitudinal costae are entirely positioned at the halfway between centerline and valve margin (Figure 9A–H), and correspond to the location of the bifacial annulus (Figure 9A–D). Central longitudinal line is not costa, which is formed by the offset striae meet in LM (Figure 4E–F). Pseudosepta are present at both poles (Figure 10F,H). Valves are double-walled that internally covered by the silica plate with four longitudinal small circular foramina lines (Figure 10E–H), and a large opening at the poles (Figure 10F,H).

Korean occurrence: Recently, Joh [63] found this morphotype species as an unidentified Ardissonea species from Seogwipo coast of Jeju Island.

Taxonomic comment: Since Lobban et al. [64] reappraised the big-stick diatoms in the order Ardissoneales, four Ardissonea species were retained in this genus: A. densistriata Lobban; A. formosa (Hantzsch) Grunow; A. pulcherrima (Hantzsch) Grunow; A. robusta (Ralfs) De Notaris. The newly described A. densistratia by Lobban was characterized by its small size, lanceolate shape, and relatively high stria density [64]. Joh [63] found also found the lanceolate Ardissonea species from Jeju Island, but he retained the species in an unidentified state because of the smaller valve size, and the convex margin was different from the other Ardissonea species. The lanceolate valve of Ardissonea aff. formosa is similar to A. densistriata, but the valve size is longer (141–223 µm vs. 46–103 µm) and the striae are sparser than A. densistriata (8–10 vs. 16–17 in 10 µm, respectively). The lanceolate form of A. formosa has been also found; therefore, it is premature to describe it as a new species only based on the valve outline. We postpone the reporting as a new species until certain evidence such as phylogenetic relationship is revealed.

Genus Ardissoneopsis Lobban & Ashworth, 2022

Ardissoneopsis dokdoensis J.S. Park sp. nov. [Figure 4G–J (LM); Figure 11A–J, Figure 12A–G (SEM)]

Sample: Dokdo6; Dokdo7; Dokdo14.

Dimensions: Valves 216.2–301.7 µm long, 10.7–12.7 µm wide, transapical striae 14–16 in 10 µm.

Diagnosis: Spathulate heteropolar valve; gradually appearance of apical costae near both poles in the internal valve view.

Description: Valves are linear, heteropolar with obtusely rounded apices (Figure 4G–J, Figure 11A and Figure 12A). Striae are parallel throughout and slightly radiate at apical pole (Figure 11B–E). Bifacial annulus located near valve margin (Figure 11B–E,H–J). Pseudosepta are absent at both poles (Figure 12B–E).

Korean occurrence: This species is new to science.

Holotype: Slide no. B-S-PP-00011226, deposited at the Library of Marine Samples, KIOST, Republic of Korea. Figure 4J.

Isotype: Slide no. B-S-PP-00011227, deposited at the Library of Marine Samples, KIOST, Republic of Korea. Figure 4H.

Type locality: Dokdo: seaweed samples from subtidal area at a depth of 5–15 m (37°14′15″ N 131°52′07″ E).

Etymology: This species is named for Dokdo of South Korea, where the specimen was collected.

Taxonomic comment: Lobban et al. [64] characterized the genus Ardissoneopsis by the single-walled valve and the absence of pseudosepta in the apical poles. The pseudosepta can be verified by the presence of a notch in the valvocopula. If the structure of valvocopula is not observed, the presence or absence of pseudosepta is vague, and it can be differently shown by the observation angles. The other morphological characteristic of Ardissoneopsis species is shown by the gradual appearance of the transverse costae in the apical poles, although Lobban et al. [64] did not emphasize this as a genus characteristic. In the Dokdo specimens, it is difficult to decide the presence or absence of pseudosepta. Instead of the pseudosepta, we use the gradual appearance of apical costae to put the Dokdo specimens in the genus Ardissoneopsis, and this characteristic is also observed from the basal pole of Climcosphenia species. Both genera can be easily distinguished by the presence or absence of a craticular bar. There are four Ardissoneopsis species: A. appressata Lobban & Ashworth, A. fulgicans, A. gracilis Lobban, and A. undosa. All four species have an isopolar valve or undulate valve outline, Ardissoneopsis dokdoensis is different from the four other Ardissoneopsis species due to its spatulate heteropolar valve. Rather, Synedrosphenia bikarensis Lobban is similar to A. dokdoensis due to its spathulate heteropolar valve. However, both species are basically different in the transverse costae in the apical pole, and A. dokdoensis is narrower (10.7–12.7 µm vs. 12–14 µm), and the striae are sparser than S. bikarensis (14–16 vs. 21 in 10 µm, respectively). In addition, A. dokdoensis has a distinct bifacial annulus near the valve margin as well as irregularly distributed marginal spines along the junction between the valve face and mantle.

Ardissoneopsis fulgicans Lobban & Ashworth in Lobban et al., 2022 [Figure 4K,L (LM), Figure 13A–F (SEM)]

References: Lobban et al. [64], p. 133, figs 14A–J.

Samples: Dokdo1; Dokdo6; Dokdo11.

Dimensions: Valves 210–350.1 µm long, 9.1–13.4 µm wide; transapical striae 13–17 in 10 µm.

Description: Valves are usually slightly widened in the middle and bluntly rounded at poles (Figure 4K,L and Figure 13A,D). Areolae are circular (Figure 13B,C). Transapical striae are uniseriate (Figure 13C). Transverse costae are present on most vimenes, but thinning toward the margin and absent at the poles (Figure 13E). Annulus is present along the valve–margin junction, underlain by longitudinal costae (Figure 13C,F). Pseudosepta are absence at the both poles (Figure 13E).

Korean occurrence: This is the first report from Korean waters of the temperate northern Pacific.

World distribution: Ardissoneopsis fulgicans was firstly described in Guam by Lobban et al. [64], it was also found from Chuuk, FSM, and Marshall Island [64].

Genus Climacosphenia Ehrenberg, 1841

Climacosphenia moniligera Ehrenberg, 1843 [Figure 5A–C (LM); Figure 14A–H (SEM)]

References: Round [66], figs 1–3, 7, 8, 9; Shim [25], p. 291, fig. 282; Witkowski et al. [23], p. 44, pl. 18, fig. 1.

Samples: Dokdo3; Dokdo6; Dokdo11; Dokdo13; Dokdo14.

Dimensions: Valves 158.9–259.3 µm long, 18.8–24.1 µm wide at the apex, 19–24 µm at the apical pole.

Description: Valves are clavate with gradually tapered basal pole nearly half the valve (Figure 5A–C and Figure 14A,E). Striae are uniseriate, and consist of apically elongated areolae (Figure 14B,C). Two longitudinal costae are positioned at the halfway point between the centerline (Figure 14A,C,E,G) and valve margin of apical poles (Figure 14B,F), and absent at the basal pole (Figure 14D,H). Annulus is bifacial and corresponds to the location of the longitudinal costae (Figure 14A–H).

Korean occurrence: This species has been reported from the northwestern parts of South Korea [25].

World distribution: Climacosphenia moniligera is widespread in warm water regions [23]. In the northwest Pacific, this species was observed in Guam [67]; Chuuk, FSM [34].

Comment: Valve margins are gradually sloping (in contrast to C. elongata, which is strongly spathulate), often curved.

Genus Grunowago Lobban & Ashworth in Lobban et al., 2022

Grunowago pacifica Lobban & Ashworth, 2022 [Figure 5D,E (LM); Figure 15A–G (SEM)]

References: Lobban et al. [64], p. 144, figs 20A–J, 21A–E.

Samples: Dokdo3; Dokdo7; Dokdo9; Dokdo14.

Dimensions: Valves 135.1–354.3 µm long, 13.2–17.6 µm wide, 8–10 transapical striae in 10 µm.

Description: Valves are linear, isopolar with rounded or often slightly rostrate poles (Figure 5D,E and Figure 15A,D). Striae are uniseriate, and consist of circular or apically slightly elongated areolae (Figure 15B–D). Several small spines are usually present on each pole (Figure 14B,D). Valves are single-walled (Figure 15D–G), internally costate with a distinctive central costa in the entire length of valve and thickened vimines (Figure 15D–E).

Korean occurrence: This is the first report from Korean waters of the temperate northern Pacific.

World distribution: Grunowago pacifica was originally described in Guam by Lobban et al. [64].

Comment: There are two species in the genus Grunowago, namely G. pacifica and G. bacillaris (This name is changed as G. superba in this study, the formal taxonomic revision is provided below). Both species can be distinguished by the valve width and length [64]: namely, that of Grunowago pacifica is smaller.

Genus Synedrosphenia (H. Peragallo) Azpeitia, 1911

Synedrospheniacrystallina (C.A. Agardh) Lobban & Ashworth, 2022 [Figure 5F,G (LM); Figure 16A–H (SEM)]

Basionym: Diatoma crystallina C.A. Agardh, 1824.

Synonyms: Exilaria crystallina (C.A. Agardh) Greville, 1827; Synedra crystallina (C.A. Agardh) Kützing 1844; Synedra gallionii var. crystallina (C.A. Agardh) Rabenhorst 1847; Ardissonea crystallina (C.A. Agardh) Grunow 1880.

References: Peragallo and Peragallo [18], p. 310, pl. 79, fig. 1; Navarro [68], p. 260, figs 59, 60; Poulin et al. [69], figs 28–30; Pickett-Heaps et al. [70], figs 9–19; Shim [25], p. 282, fig. 265; Lobban et al. [31], p. 259, pl. 15, figs 1–3; Lobban et al. [64], p. 114, figs 3A–J.

Samples: Dokdo13; Dokdo14.

Dimensions: Valves 405.4–410.2 µm long, 17.5–22.9 µm wide, transapical striae 13–16 in 10 µm.

Description: Valves are linear, isopolar with cuneate apices (Figure 5C,D and Figure 16A,E). Striae are parallel throughout and slightly radiate at apical pole (Figure 5C,D and Figure 16A–H). Longitudinal costae entirely positioned at the halfway point between the centerline and valve margin (Figure 16A–H), and which correspond to the location of the bifacial annulus (Figure 16B–D). Pseudosepta are present at both poles (Figure 16F,H).

Korean occurrence: According to the monograph by Shim [25], this species had been reported from the Yellow Sea by Skvortzov [71], and the East Sea by Choi and Noh [72].

World distribution: Greville [73] was originally described this species from Appin of Scotland. Since then, Synedrosphenia crystallina has been widely reported from warm water regions: Philippine [49]; Puerto Rico [68]; Micronesian waters [31,64].

Synedrosphenia cf. gomphonema (Janish & Rabenhorst) Hustedt, 1932 [Figure 5H–J (LM); Figure 17A–G (SEM)]

Basionym: Synedra gomphonema Janisch & Rabenrhost, 1863.

References: Joh [63], p. 12, figs 66–68; Lobban et al. [64], p. 128, figs 11A–G, 12A–G.

Samples: Dokdo6; Dokdo9; Dokdo11; Dokdo13; Dokdo14.

Dimensions: Valves 286.4–327.7 µm long, 19.5–24.7 µm wide; transapical striae 13–16 in 10 µm.

Description: Valves are long, broadly clavate with subrostrate head pole (Figure 5E–G and Figure 17A). Striae are parallel throughout (Figure 17B–G) and slightly radiate at apical pole (Figure 17B,E). Longitudinal costae entirely positioned at the valve margin (Figure 17B–G), and which correspond to the location of the bifacial annulus (Figure 17B–D). Pseudosepta are present at both poles (Figure 17E,G).

Korean occurrence: Joh [63] reported this species as an epiphyte from the rock pools along the Seogipo coast in Jeju Island. Lee et al. [55] provided the figure on Ulnaria pseudogaillonii (H. Kobayasi & M. Idei) M. Idei in their checklist of unrecorded diatoms species in South Korea ([55], fig. 25). However, the clavate shape and robust size of the specimen match those of the genus Synedrosphenia rather than the genus Ulnaria, but it is difficult to identify the species level because they provided only a single figure.

World distribution: Synedrosphenia gomphonema was originally described in Honduras [74]. Since then, this species has been widely reported in warm water regions: Micronesian waters [34,64]; Jeju Island of South Korea [63].

Taxonomic comment: The Dokdo specimens are similar to S. gomphonmea in the valve length and the density of striae, but the width (never exceed 25 µm) are narrower than the previous reports which showed the greatest width (27–36 µm). In addition, Lobban et al. [64] described the striae abruptly changed from parallel to radiate near the apical pole narrows, but the abrupt change of striation was not found in Dokdo specimen. The striation and valve outline of Dokdo specimen are also similar to S. parva, but both are distinctly different in the dimensions. We temporally identify the Dokdo specimen to S. gomphonema.

Genus Toxarium Bailey, 1854

Toxarium hennedyanum(Gregory) Pelletan, 1889 [Figure 5K (LM)]

Basionym: Synedra hennedyana Gregory, 1857

References: Gregory [56], p. 534, pl. 14, fig. 108; Witkowski et al. [23], p. 83, pl. 30, fig. 11, pl. 31, fig. 7; Lobban et al. [31], p. 260, pl. 17, figs 6–8; Lee and Park [75], p. 552, figs 14–16; Park et al. [62], p. 105, figs 7a, b; Lobban et al. [64], p. 147, figs 22A–K.

Sample: Dokdo6.

Dimensions: Valve 582.7 µm long, 9.3 µm wide in the middle; transapical striae 9 in 10 µm.

Description: Valves are extremely long with inflated center and sub-capitate, rounded apices. Areolae scattered over much of the edge of the valve face and on the mantle.

Korean occurrence: Lee and Park [75] was found this species from Jeju Island.

World distribution: Gregory [56] originally reported from Lamlash Bay, Scotland. This species was described from the European coasts [23]. In the northwest Pacific, T. hennedyanum was observed from Guam [31,64]; Chuuk, FSM [62]; Jeju Island of South Korea [75].

Toxarium undulatum Bailey, 1854 [Figure 5L–N (LM)]

Synonym: Synedra undulata (Bailey) W. Smith, 1956

References: Shim [25], p. 285, fig. 272 (as Synedra undulata); Hasle and Syvertsen [38], p. 253, pl. 52; Lobban et al. [64], p. 149, figs 23A–M, 24A–H.

Samples: Dokdo3; Dokdo6; Dokdo7; Dokdo11; Dokdo14.

Dimensions: Valve 256.1–350.8 µm long, 6.4–7.7 µm wide in the middle; transapical striae 9–10 in 10 µm.

Description: Valves are linear or curved with undulating margins, slightly sub-capitate, rounded apices. Areolae are scattered over much of the edge of the valve face and on the mantle.

Korean occurrence: This species was reported as Synedra unulata from Gori in the East Sea of South Korea [25].

World distribution: Toxarium undulatum was originally described from Narragansett Bay [76]. This species is common to warm water regions [77], but are occasionally found from temperate regions [38]. In the northwestern Pacific, T. undulatum has been reported from Micronesian waters [62,64].

Order Biddulphiales Krieger, 1954

Family Biddulphiaceae Kützing, 1844

Genus Biddulphiopsis Stosch & Simonsen, 1984

Biddulphopsis titiana (Grunow) von Stosch & Simonsen, 1984 [Figure 6A–C (LM); Figure 18A–I (SEM)]

Basionym: Cerataulus titianus Grunow, 1863.

References: von Stosch and Simonsen [78], p. 12, figs 1–35; Lobban et al. [31], p. 251, pl. 7, fig. 3; Park et al. [34], p. 8, figs 9, 76.

Samples: Dokdo6; Dokdo7; Dokdo11; Dokdo14.

Dimensions: Valves 51.9–67.7 µm diameter; 9–11 areolae in 10 µm.

Description: Valves are triangular with small central annulus packed by areolae (Figure 18A,D,G). Striae are radiate from central annulus (Figure 18D,G). Areolae are externally covered by rotae (Figure 18E), and some simple small pores are scattered (Figure 18E). Central annulus formed a distinct ring and filled with many areolae (Figure 18E,H). Rimoportulae are positioned adjacent to the pseudocelli (Figure 18I), which are externally difficult to distinguish from adjacent simple small pores (Figure 18L), internally shortly stalked (Figure 18I).

Korean occurrence: According to the checklist of Korean diatoms by Shim and Park [79] and Choi [40], B. titiana was reported from southwestern coast of Korean waters.

World distribution: Bidduphiopsis titiana was originally described from the Adriatic Sea [80]. In the northwestern Pacific, this species has been reported from Micronesian waters [31,34].

Figure 2. Light microscope images of seven diatoms in Dokdo. (A–C) Hyalodiscus scoticus, (D–G) Podosira hormoides, (H) Podosira montagnei, (I) Asteromphalus heptactis, (J) Coscinodiscus asteromphalus, (K) Coscinodiscus marginatus, (L,M) Coscinodiscus radiatus. Scale bar = 10 µm.

Figure 2. Light microscope images of seven diatoms in Dokdo. (A–C) Hyalodiscus scoticus, (D–G) Podosira hormoides, (H) Podosira montagnei, (I) Asteromphalus heptactis, (J) Coscinodiscus asteromphalus, (K) Coscinodiscus marginatus, (L,M) Coscinodiscus radiatus. Scale bar = 10 µm.

Figure 3. Light microscope images of four diatoms in Dokdo. (A–E) Actinocyclus pruinosus, (F–L) Actinocyclus subtilis, (M) Pseudosolenia calcar-avis. Scale bar = 10 µm.

Figure 3. Light microscope images of four diatoms in Dokdo. (A–E) Actinocyclus pruinosus, (F–L) Actinocyclus subtilis, (M) Pseudosolenia calcar-avis. Scale bar = 10 µm.

Figure 4. Light microscope images of four diatoms in Dokdo. (A–C) Ardissonea formosa, (D–F) Ardissonea aff. formosa, (G–J) Ardissoneopsis dokdoensis, (G) Isotype image, (J) Holotype image, (K,L) Ardissoneopsis fulgicans. Scale bar = 10 µm.

Figure 4. Light microscope images of four diatoms in Dokdo. (A–C) Ardissonea formosa, (D–F) Ardissonea aff. formosa, (G–J) Ardissoneopsis dokdoensis, (G) Isotype image, (J) Holotype image, (K,L) Ardissoneopsis fulgicans. Scale bar = 10 µm.

Figure 5. Light microscope images of six diatoms in Dokdo. (A–C) Climacosphenia moniligera, (D,E) Grunowago pacifica, (F,G) Synedrosphenia crystallina, (H–J) Synedrosphenia cf. gomphonema, (K) Toxarium henndeyanum, (L–N) Toxarium undulatum. Scale bar = 10 µm.

Figure 5. Light microscope images of six diatoms in Dokdo. (A–C) Climacosphenia moniligera, (D,E) Grunowago pacifica, (F,G) Synedrosphenia crystallina, (H–J) Synedrosphenia cf. gomphonema, (K) Toxarium henndeyanum, (L–N) Toxarium undulatum. Scale bar = 10 µm.

Figure 6. Light microscope images of five diatoms in Dokdo. (A–C) Biddulphiopsis titiana, (D,E) Trigonoium cf. arcticum, (F,G) Lampriscus shadboltianus, (H) Pseudictyota reticulata, (I–K) Thalassiosira sp. Scale bar = 10 µm.

Figure 6. Light microscope images of five diatoms in Dokdo. (A–C) Biddulphiopsis titiana, (D,E) Trigonoium cf. arcticum, (F,G) Lampriscus shadboltianus, (H) Pseudictyota reticulata, (I–K) Thalassiosira sp. Scale bar = 10 µm.

Figure 7. Scanning electron microscope images of four species in the family Hyalodiscaceae in Dokdo. (A–C) Hyalodiscus ambiguus, (A) Internal valve view, (B) Central hyaline area, (C) Internal structure of valve face rimoportula, (D–I) Hyalodiscus scoticus, (D,E) External valve view, (F) External opening of marginal rimoportula (arrowhead), (G,H) Internal valve view, (I) Internal structure of marginal rimoportula, (J–M) Podosira hormoides, (J) External valve view, (K) Fasciculate areolation and central area covered by mucilage pad, (L) Internal valve view, (M) Fasciculate areolation and central annulus (N,O) Podosira montagnei, (N) External valve view, (O) Decussate areolation and scattered valve face rimoportulae. Scale bar = 10 µm (A,J,L,N); 5 µm (D,E,G,H,K); 1 µm (B,C,F,I,M,O).

Figure 7. Scanning electron microscope images of four species in the family Hyalodiscaceae in Dokdo. (A–C) Hyalodiscus ambiguus, (A) Internal valve view, (B) Central hyaline area, (C) Internal structure of valve face rimoportula, (D–I) Hyalodiscus scoticus, (D,E) External valve view, (F) External opening of marginal rimoportula (arrowhead), (G,H) Internal valve view, (I) Internal structure of marginal rimoportula, (J–M) Podosira hormoides, (J) External valve view, (K) Fasciculate areolation and central area covered by mucilage pad, (L) Internal valve view, (M) Fasciculate areolation and central annulus (N,O) Podosira montagnei, (N) External valve view, (O) Decussate areolation and scattered valve face rimoportulae. Scale bar = 10 µm (A,J,L,N); 5 µm (D,E,G,H,K); 1 µm (B,C,F,I,M,O).

Figure 8. Scanning electron microscope images of two Actinocyclus species in Dokdo. (A–F) Actinocyclus pruinosus, (A) External valve view, (B) Fasciculate areolation and covered by cribra, (C) external opening of pseudonodule and mantle rimoportula, (D) Internal valve view (E) Internal opening of areolae and fasciculate areolation, (F) Internal structure of rimoportula and pseudonodule, (G–L) Actinocyclus subtilis, (G) External valve view, (H) Spicate areolation (I) External opening of pseudonodule and mantle rimoportula, (J) Internal valve view, (K) spicate areolation, (L) Internal structure of rimoportula and pseudonodule. Scale bar = 10 µm (A,D,G,J); 5 µm (B,E,H,K); 1 μm (C,F,I,L).

Figure 8. Scanning electron microscope images of two Actinocyclus species in Dokdo. (A–F) Actinocyclus pruinosus, (A) External valve view, (B) Fasciculate areolation and covered by cribra, (C) external opening of pseudonodule and mantle rimoportula, (D) Internal valve view (E) Internal opening of areolae and fasciculate areolation, (F) Internal structure of rimoportula and pseudonodule, (G–L) Actinocyclus subtilis, (G) External valve view, (H) Spicate areolation (I) External opening of pseudonodule and mantle rimoportula, (J) Internal valve view, (K) spicate areolation, (L) Internal structure of rimoportula and pseudonodule. Scale bar = 10 µm (A,D,G,J); 5 µm (B,E,H,K); 1 μm (C,F,I,L).

Figure 9. Scanning electron microscope images of Ardissonea formosa in Dokdo. (A–C) Gridle view, (A,C) Apical poles showing valve, valvocopula, and copula, (B) Valve middle showing valve, valvocopula, and copula, (D) External valve view, (E,G) External apical poles (F) External valve middle with bifacial annulus in the half between the center and margin, (H) Internal valve view, (I,K) Internal apical poles covering the valve chamber with three longitudinal rows of foramina and apical openings, (J) Internal chamber with three longitudinal rows of small circular foramina lines. Scale bars = 10 µm (D,H); 5 µm (A–C,E–G,I–K).

Figure 9. Scanning electron microscope images of Ardissonea formosa in Dokdo. (A–C) Gridle view, (A,C) Apical poles showing valve, valvocopula, and copula, (B) Valve middle showing valve, valvocopula, and copula, (D) External valve view, (E,G) External apical poles (F) External valve middle with bifacial annulus in the half between the center and margin, (H) Internal valve view, (I,K) Internal apical poles covering the valve chamber with three longitudinal rows of foramina and apical openings, (J) Internal chamber with three longitudinal rows of small circular foramina lines. Scale bars = 10 µm (D,H); 5 µm (A–C,E–G,I–K).

Figure 10. Scanning electron microscope images of Ardissonea aff. formosa in Dokdo. (A) External valve view, (B,D) Apical ends in the external valve view, (C) Valve middle with grooved annulus (arrowhead), (F,H) Apical ends in the internal valve view, (G) Internal chamber with four longitudinal rows of small circular foramina lines. Scale bars = 10 µm (A,E); 5 µm (B–D,F–H).

Figure 10. Scanning electron microscope images of Ardissonea aff. formosa in Dokdo. (A) External valve view, (B,D) Apical ends in the external valve view, (C) Valve middle with grooved annulus (arrowhead), (F,H) Apical ends in the internal valve view, (G) Internal chamber with four longitudinal rows of small circular foramina lines. Scale bars = 10 µm (A,E); 5 µm (B–D,F–H).

Figure 11. Scanning electron microscope images of Ardissoneopsis dokdoensis in Dokdo. (A) External valve view, (B,C) External apical poles with numerous marginal spines and radiate striae, (D,E) External basal poles with marginal spines and radiate striae, (F,G) Marginal spines along valve edges (arrowheads in subfigure (G)), (H–J) Valve middle with grooved annulus (arrowheads) lines. Scale bars = 10 µm (A); 1 µm (B–J).

Figure 11. Scanning electron microscope images of Ardissoneopsis dokdoensis in Dokdo. (A) External valve view, (B,C) External apical poles with numerous marginal spines and radiate striae, (D,E) External basal poles with marginal spines and radiate striae, (F,G) Marginal spines along valve edges (arrowheads in subfigure (G)), (H–J) Valve middle with grooved annulus (arrowheads) lines. Scale bars = 10 µm (A); 1 µm (B–J).

Figure 12. Scanning electron microscope images of Ardissoneopsis dokdoensis in Dokdo. (A) Internal valve view, (B,C) Internal apical poles with blurring transverse costae and absence of pseudosepta, (D,E) Internal basal poles without transverse costae and radiate striae, (F,G) Valve middle with grooved annulus (arrowhead in subfigure (F)). Scale bars = 10 µm (A); 1 µm (B–G).

Figure 12. Scanning electron microscope images of Ardissoneopsis dokdoensis in Dokdo. (A) Internal valve view, (B,C) Internal apical poles with blurring transverse costae and absence of pseudosepta, (D,E) Internal basal poles without transverse costae and radiate striae, (F,G) Valve middle with grooved annulus (arrowhead in subfigure (F)). Scale bars = 10 µm (A); 1 µm (B–G).

Figure 13. Scanning electron microscope images of Ardissoneopsis fulgicans in Dokdo. (A) External valve view, (B) External apical pole with radiate striae, (C) External valve middle showing marginal bifacial annulus with groove (arrowhead), (D) Internal valve view, (E) Internal apical poles with weakened costae and without pseudosepta (arrowhead), (F) Internal valve middle showing transverse costae and marginal bifacial annulus with groove (arrowhead), Scale bars = 10 µm (A,D); 1 µm (B,C,E,F).

Figure 13. Scanning electron microscope images of Ardissoneopsis fulgicans in Dokdo. (A) External valve view, (B) External apical pole with radiate striae, (C) External valve middle showing marginal bifacial annulus with groove (arrowhead), (D) Internal valve view, (E) Internal apical poles with weakened costae and without pseudosepta (arrowhead), (F) Internal valve middle showing transverse costae and marginal bifacial annulus with groove (arrowhead), Scale bars = 10 µm (A,D); 1 µm (B,C,E,F).

Figure 14. Scanning electron microscope images of Climacosphenia moniligera in Dokdo. (A) External valve view, (B) External apical pole with radiate striae and apical spines, (C) External bifacial annulus with groove (arrowhead), (D) External basal pole with sparse areolae, (E) Internal valve view, (F) Internal apical pole with craticular bar, (G) Internal transapical bifacial costae with groove (arrowhead), (H) Internal basal pole without costae. Scale bars = 10 µm (A,E); 1 µm (B–D,F–H).

Figure 14. Scanning electron microscope images of Climacosphenia moniligera in Dokdo. (A) External valve view, (B) External apical pole with radiate striae and apical spines, (C) External bifacial annulus with groove (arrowhead), (D) External basal pole with sparse areolae, (E) Internal valve view, (F) Internal apical pole with craticular bar, (G) Internal transapical bifacial costae with groove (arrowhead), (H) Internal basal pole without costae. Scale bars = 10 µm (A,E); 1 µm (B–D,F–H).

Figure 15. Scanning electron microscope images of Grunowago pacifica in Dokdo. (A) External valve view, (B,D) External apical poles with marginal spines (arrowheads), (C) External valve middle with distinct trace of central long costa, (E) Internal valve view, (F,H) Internal apical poles with distinct costae, (G) Internal valve middle with distinct central long costa and transverse costae. Scale bars = 10 µm (A,E); 1 µm (B–D,F–H).

Figure 15. Scanning electron microscope images of Grunowago pacifica in Dokdo. (A) External valve view, (B,D) External apical poles with marginal spines (arrowheads), (C) External valve middle with distinct trace of central long costa, (E) Internal valve view, (F,H) Internal apical poles with distinct costae, (G) Internal valve middle with distinct central long costa and transverse costae. Scale bars = 10 µm (A,E); 1 µm (B–D,F–H).

Figure 16. Scanning electron microscope images of Synedrosphenia crystallina in Dokdo. (A) External valve view, (B,D) External apical poles with radiate striae, (C) External valve middle showing bifacial annulus with groove (arrowhead), (E) Internal valve view, (F,H) Internal apical poles with weakened costae and pseudosepta (arrowhead). (G) Internal valve middle showing transapical costae with groove (arrowhead), Scale bars = 10 µm (A,E); 1 µm (B–D,F–H).

Figure 16. Scanning electron microscope images of Synedrosphenia crystallina in Dokdo. (A) External valve view, (B,D) External apical poles with radiate striae, (C) External valve middle showing bifacial annulus with groove (arrowhead), (E) Internal valve view, (F,H) Internal apical poles with weakened costae and pseudosepta (arrowhead). (G) Internal valve middle showing transapical costae with groove (arrowhead), Scale bars = 10 µm (A,E); 1 µm (B–D,F–H).

Figure 17. Scanning electron microscope images of Synedrosphyenia cf. gomphonema in Dokdo. (A,B) External valve view and both individuals showing the different valve face undulation, (C) External apical pole with rostrate apex and slightly radial striae toward apex, (D) External valve middle showing the marginal bifacial annulus with groove (arrowhead), (E) External basal pole with rounded apex, (F) Internal valve view, (G) Internal apical pole with slightly radiate costae and pseudosepta (arrowhead), (H) Internal valve middle showing the marginal bifacial annulus with grooved annulus (arrowhead) and transverse costae, (I) Internal basal pole without costae and with pseudosepta (arrowhead). Scale bar = 10 µm (A,B,F); 1 µm (C–E,G–I).

Figure 17. Scanning electron microscope images of Synedrosphyenia cf. gomphonema in Dokdo. (A,B) External valve view and both individuals showing the different valve face undulation, (C) External apical pole with rostrate apex and slightly radial striae toward apex, (D) External valve middle showing the marginal bifacial annulus with groove (arrowhead), (E) External basal pole with rounded apex, (F) Internal valve view, (G) Internal apical pole with slightly radiate costae and pseudosepta (arrowhead), (H) Internal valve middle showing the marginal bifacial annulus with grooved annulus (arrowhead) and transverse costae, (I) Internal basal pole without costae and with pseudosepta (arrowhead). Scale bar = 10 µm (A,B,F); 1 µm (C–E,G–I).

Figure 18. Scanning electron microscope images of two multipolar diatoms in Dokdo. (A–I) Biddulphiopsis titiana, (B) Girdle view, (C) Valvocopula, (D) Valve mantle and pseudocelli in the corner, (E) External valve view, (E) Central annulus in the external valve view, (F) Pseudoocelli in the external valve view, (G) Internal valve view, (H) Central annulus in the internal valve view, (I) Internal structure of two rimoportulae, (J–O) Lampriscus shadboltianus, (J) External valve view, (K) Central annulus in the external valve view, (L) Ocelli in the external valve view, (M) Internal valve view, (N) Central annulus in the internal valve view, (O) Internal structure of two rimoportulae. Scale bar = 10 µm (A,B,D,G,J,M); 1 µm (C,E,F,H,I,K,L,N,O).

Figure 18. Scanning electron microscope images of two multipolar diatoms in Dokdo. (A–I) Biddulphiopsis titiana, (B) Girdle view, (C) Valvocopula, (D) Valve mantle and pseudocelli in the corner, (E) External valve view, (E) Central annulus in the external valve view, (F) Pseudoocelli in the external valve view, (G) Internal valve view, (H) Central annulus in the internal valve view, (I) Internal structure of two rimoportulae, (J–O) Lampriscus shadboltianus, (J) External valve view, (K) Central annulus in the external valve view, (L) Ocelli in the external valve view, (M) Internal valve view, (N) Central annulus in the internal valve view, (O) Internal structure of two rimoportulae. Scale bar = 10 µm (A,B,D,G,J,M); 1 µm (C,E,F,H,I,K,L,N,O).

Family Trigoniumaceae Glezer, 2019

Genus Trigonium Cleve, 1867

Trigonium cf. arcticum (Brigthwell) Cleve, 1868 [Figure 6D,E (LM)]

Basionym: Triceratium arcticum Brightwell, 1853

References: Hoban [81], p. 277, figs 37–42.

Sample: Dokdo9.

Dimensions: Valves 83.408 µm in a side; 3–4 areolae in 10 µm.

Description: Valves are triangular with smoothly rounded corners (Figure 6D,E). Areolae are loculate and coarse, gradually decreased at the pseudocellus (Figure 6D,E). Areolation is radial.

Korean occurrence: Lee [82] listed this species as the fossil diatom in his checklist. There was no information of the location. Lee and Park [75] reported the variety japonica of this species from the Jeju Island.

World distribution: This species was originally described from Beechy Island, Arctic region [83]. Since then, this species has mainly been found in the Antarctic region. When Hoban [81] investigated the fine structure of T. arcticum, he used culture samples collected from Antarctic.

Taxonomic comment: The distribution from the Arctic to Antarctic of this species cast doubt on it being a single species, and the presence of many varieties also necessitate the biogeographical and phylogenetical investigation of T. arcticum.

Subclass Odontellophycidae D.G. Mann in Adl et al., 2019

Order Eupodiscales V.A. Nikolaev & D.M. Harwood, 2005

Family Eupodiscaceae Ralfs in Pritchard, 1861

Genus Lampriscus A.W.F. Schmidt, 1882

Lampriscus shadboltianus(Greville) H. Peragallo & M. Peragallo, 1902 [Figure 6F,G (LM), Figure 18J–O (SEM)]

Basionym: Triceratium shadboltianum Greville, 1862

References: Navarro [84], p. 618, figs 33a–36; Park et al. [34], p. 10, figs 14, 15.

Samples: Dokdo8.

Dimensions: Valves 65.4 µm in diameter; 6–7 areolae in 10 µm.

Description: Valves are circular with conspicuous pseudocelli which superimpose a quadrangular pattern (Figure 6F,G and Figure 18J,M). Striae is radiate from the center and continue without a break down the deep valve mantle (Figure 18J,M). Areolae are elliptical rotae (Figure 18K,N). Central annulus forms a distinct ring filled with many areolae (Figure 18K,N). Pseudocelli are located on the slight elevations of the corner (Figure 18J,L,M,O); separated into inner and outer portions by a circular hyaline rim located towards the inner margin of the pseudocellus (Figure 18L,O), which can appear in LM as a circular spot with an outer arc (Figure 6F,G).

Korean occurrence: This species was included as Triceratium shadboltianum in the checklist by Lee et al. [15], but the certain locality was not provided.

World distribution: Lampriscus shabdoltianus has been reported from temperate to tropical waters [84]. Park et al. [34] also found this species from Chuuk, FSM.

Family Odontellaceae P.A. Sims, D.M. Williams & Ashworth, 2018

Genus Pseudictyota P.A. Sims & D.M. Williams in Sims et al., 2018

Pseudictyota reticulata (Roper) P.A. Sims & D.M. Williams in Sims et al., 2018 [Figure 6H (LM)]

Basionym: Biddulphia reticulata Roper, 1859.

Synonyms: Odontella reticulata (Roper) De Toni, 1894; Triceratium dictyotum Sims & Ross, 1990.

Reference: Chung [37], p. 232, pl. 45, fig. 343 (as Biddulphia reticulata); Lobban et al. [31], p. 250, pl. 6, figs 1, 2 (as Triceratium dictyoum); Joh [63], p. 16, figs 12, 18, 19, 170, 181.

Sample: Dokdo9.

Dimensions: Valve 76.6 µm long; 50.2 µm wide; 3–4 areolae in 10 µm.

Description: Valves are broadly rhomboic, bipolar with apically raised elevation having two ocelli. Valve face is irregularly covered by pseudoloculi that structured by basal layer with poroid areolae. Rimoportulae located at the proximal base of each elevation; externally opened by short spines; internally by a slit between raised lips.

Korean occurrence: This species was reported as Biddulphia reticulata in the Yellow Sea and South Sea [37]. Shim [25] also described this species, but the figure by him was not P. reticulata. Recently, Joh [63] found this species as an epiphyte from in the Seogwipo coast.

World distribution: Pseudictyota reticulata has been reported in warm water regions: Guam [31]; Philippine [49]; Sri Lanka [85]; Puerto Rico [84].

Subclass Thalassiosirophycidae Round & R.M. Crawford in Round et al., 1990

Order Thalassiosirales Glezer & I.V. Makarova, 1986

Family Thalassiosiraceae Lebour, 1930

Genus Thalssiosira Cleve, 1873

Thalassiosira sp. [Figure 6I–J (LM)]

Sample: Dokdo13.

Dimensions: Valves 69 µm diameter; 6 areolae in 10 µm.

Description: Valves are flat with a shallow valve mantle (Figure 6I–J). Areolae are arranged in a fasciculate manner (Figure 6I–J). Portules are scattered on the valve face (Figure 6J,K). Central portule is uncertain in LM. One ring of marginal portules is present (Figure 6K). One rimoportula is located close to a marginal fultoportula (Figure 6K).

Comment: The fasciculate areolation and the scattered valve face portules of Dokdo specimens is most similar to Thalassiosira wongii Mahood. However, the areolae of Dokdo specimen are coarser than those of T. wongii (6 vs. 9–11 in 10 µm, respectively). Additionally, T. wongii has been mainly reported from the coastal regions as a brackish species [86,87]. The other similar species is Thalassiosira caspica Makarova due to its fasciculate areolae and areolae density [88], but this species was transferred to the genus Actinocyclus based on the further SEM examination by same author [89]. The scattered portules in the valve face and dense marginal portules in the valve mantle of the Dokdo specimen seem to be more suitable to Thalassiosira than Actinocyclus. The morphological dimensions and the ecology of the Dokdo specimen suggest a potential new species. Until the fine structures of the specimen are revealed, we retain the species’ unidentified status.

4. Discussion

This is the first study on seaweed-associated diatoms in Dokdo, South Korea. A large number of seaweed-associated diatoms, over 130 species, were observed in a one-time survey. In this first serial study, we have only provided an illustration of 26 diatoms that classified into the subphyla Melosirophytina, Coscinodiscophytina, and the class Mediophyceae. Of 26 species, 7 species are newly described in Korean waters (Table 1).

A voucher flora is a collection of light microscope images that not only guarantees data quality but also supports future work on ecological monitoring [6]. We found some misidentified diatom records from previous Korean works. For examples, Shim [25] mentioned Pseudictyota reticulata as Bidddulphia reticulata in Korean waters, but the figure by him was not P. reticulata but rather a Triceratium species due to its tripolar valve with three ocellus in the corner; Lee et al. [55] provided a figure on Ulnaria pseudogaillonii in their checklist of unrecorded diatoms in South Korea. However, the clavate shape and robust size of the specimen matched the genus Synedrosphenia rather than the genus Ulnaria. However, it was difficult to identify the species level because they provided only a single figure. Many ecological studies using diatoms have been conducted without voucher images for the species; therefore, readers simply trust the species records and ecological interpretation based on the provided species list. We do not intend to criticize or point out misidentifications or data quality. Rather, we wish to emphasize the importance of providing species images before conducting ecological studies, as even misidentified species can be corrected and improved in the future if image information is available.

Periphytic diatoms have often been defined as functional groups that are used to describe communities and can be more clearly related to environmental factors than when examined at the species level alone (e.g., [90,91]). Fránková et al. [92] divided epiphytic diatoms into five functional groups based on their lifestyle: Group 1—planktonic taxa represented by centric diatoms; Group 2—typically periphytic taxa adhering to the surface directly by a mucous film or with a mucilaginous stalk; Group 3—facultatively periphytic araphid taxa, passively moving diatoms able to attach; Group 4—facultatively periphytic raphid taxa with fibulae, actively moving diatoms able to attach; and Group 5—epipelic taxa with raphe, actively moving, and mainly symmetrical biraphid pennate diatoms. Our first series of studies on seaweed-associated diatoms in Dokdo comprised Groups 1 and 2. Members of the subphylum Coscinodiscophytina consist of typical planktonic taxa in Group 1, and the rarely occurrence of Asteromphalus heptactis, and three Coscinodiscus species in the samples, implies that the dead cells of this planktonic group are temporally sunken. Two Actinocyclus species, however, were abundant within seaweed-associated diatoms despite their planktonic-centric forms. Actinocyclus subtilis extrudes polysaccharide secretion from a ring of marginal rimoportulae, which plays a role in locomotion and attachment to substrates such as Melosira and Aulacoseira [93]. The occurrence of Actinocyclus species at the seaweeds may indicate the living individuals naturally settled and then colonized the seaweed by extruded secretion. Members of the class Mediophyceae, except the subclass Odontellophycidae, were included in functional Group 2. Except Biddulphosis tittiana, two multipolar species, Lampriscus shadboltianus and Pseudictyota reticualta, were also temporally settled, similar to members of the subphylum Coscinodisciophytina; the low abundance and rare observations support that these species are not common epiphytes. Biddulphiopsis titiana has frequently been observed in seaweed samples [31,34], which is typical of a periphytic taxon in Functional group 2.

The big-stick taxa in the order Ardissoneales were the best-represented group in Dokdo, with six genera and ten species. The stick-like taxa, over 300 µm long, were found to be abundant on seaweeds as an epiphyte [64], which were characterized by their valve morphogenesis from a bifacial annulus at or near the edge of the valve face, forming striae internally and externally [94]. These characteristics make their phylogenetic position more suitable for the class Mediophyceae than for the class Fragilariophyceae [95]. Lobban et al. [64] emended the description of big-stick taxa and divided them into six genera, which formed a phylogenetically monophyly, and placed the groups in the order Ardissoneales. In addition, Lobban et al. [64] collected all their specimens from Guam. Phylogenetic closeness and co-occurrence within a single area suggests that this group forms an assemblage that shares ecological and physiological characteristics. Although the ecological features of these species are still unknown, the occurrence of these tropical species in Dokdo indicates that the epiphytic species may detach in a tychoplanktonic form and be transferred to the Kuroshio Current.

Although Dokdo is located in a temperate region, we found several tropical and subtropical species, namely Podosira hormoides, P. montagnei, Actinocyclus pruinosus, Ardissonea formosa, Ardissoneopsis fulgicans, Grunowago pacifica, Synedrosphenia cf. gomphonema, and Pseudictyota reticulata. There are two possible explanations for the occurrence of tropical species in Dokdo. First, the eurythermal nature of emerging species is related to water temperature. However, until the temperature range of each species is verified through culture and physiological experiments, it is premature to conclude that a species is eurythermal solely based on its distribution in a given area. Second, owing to global warming, there is a northward movement and spread of subtropical species in areas where they can appear. The Dokdo waters are temperate in terms of latitude and tend to maintain relatively low water temperatures owing to the influence of the southward-flowing North Korean Cold Current. However, along with climate change, problems have been identified with respect to the acceleration of subtropicalization of the waters around the Korean Peninsula [96]. In particular, the frequency of occurrence of subtropical marine species in the temperate waters of the Korean Peninsula is increasing (e.g., [97,98]). Diatom species reported in subtropical waters can also be seen as an example, disproving the phenomenon of the subtropicalization of waters around the Korean Peninsula. However, as no research has been conducted on seaweed-related diatoms in the Dokdo area, it is necessary to verify whether their appearance has existed in the past or is an opportunistic one-time appearance through continuous monitoring of seaweed-related diatoms in the surrounding waters.

In conclusion, this is the first series of studies on the flora of marine algae-associated diatoms on Dokdo. It reports 26 species of diatoms belonging to the subphyla Melosirophytina and Coscinodisophytina, and the class Mediophyceae. Among these, seven species, including one new species, were reported for the first time in South Korea, which, along with the geopolitical characteristics of the survey area, proved that there is no domestic interest in marine seaweed-associated diatoms. In particular, the appearance of species that have been reported in subtropical waters, such as big-stick diatoms, requires continuous monitoring, to confirm if their settlement in the Dokdo waters is due to climate change or species-specific water temperature tolerance.

Taxonomic revision

Grunowago superba (Kützing) J.S. Park, comb. nov. Lectotype: Synedra superba Kützing, 1844.

Synonyms: Synedra crystallina var. bacillaris Grunow, 1877; Ardissonea crystallina var. bacillaris (Grunow) Grunow, 1880; Synedra bacillaris (Grunow) Hustedt, 1932; Grunowago bacillaris (Grunow) Lobban & Ashworth, nom. illeg.

Taxonomic note: When Lobban et al. [64] established the genus Grunowago based on the type Synedra crystallina var. bacillaris Grunow, several previous names were synonymized into the type species, G. bacillaris. Of the synonymized names, Synedra superba was also regarded as G. bacillaris, and they incorrectly regarded the authority of this species as S. superba Pergallo, 1900. However, the name S. superba did not originate from Peragallo, and was originally named by Kützing in 1844. According to the nomenclatural priority, the lectotype for the genus Grunowago should be changed to Synedra superba Kützing and a new combination name Grunowago superba (Kutinzg) J.S. Park, therefore, is proposed.