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Article

Report on Eight Unrecorded Species of Freshwater Oligochaetes in Korea

by
Jeounghee Lee
Marine Biological Resources Institute, Sahmyook University, Seoul 01795, Republic of Korea
Taxonomy 2025, 5(1), 9; https://doi.org/10.3390/taxonomy5010009
Submission received: 22 August 2024 / Revised: 3 February 2025 / Accepted: 13 February 2025 / Published: 17 February 2025
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Abstract

:
Freshwater oligochaetes, belonging to the class Oligochaeta, are vital components of aquatic ecosystems globally, contributing significantly to nutrient cycling, sediment dynamics, and overall ecosystem function. Despite their ecological importance, freshwater oligochaetes in Korea remain relatively understudied compared to other aquatic organisms. This study aimed to enhance our understanding of the diversity and ecological roles of freshwater oligochaetes in Korean aquatic ecosystems. We identified nine species across six genera and two subfamilies from samples collected from various freshwater environments in Korea. This research contributes valuable taxonomic knowledge and highlights the ecological significance of freshwater oligochaetes in Korean aquatic ecosystems.

1. Introduction

Freshwater oligochaetes, a group of segmented worms belonging to the class Oligochaeta, are integral components of aquatic ecosystems worldwide [1,2]. Their ecological significance stems from their roles in nutrient cycling, sediment dynamics, and overall ecosystem functioning [3,4,5,6,7,8,9,10,11]. These organisms exhibit diverse adaptations and ecological strategies, contributing substantially to the biodiversity and functioning of freshwater habitats. Research on freshwater oligochaetes encompasses various aspects, including taxonomy, ecology, physiology, and responses to environmental change. These organisms inhabit many freshwater environments, including rivers, streams, lakes, ponds, wetlands, and groundwater systems [12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29]. Their distribution and abundance are influenced by factors such as water chemistry, substrate type, flow regime, and habitat complexity. One remarkable characteristic of freshwater osligochaetes is their ability to thrive in diverse habitats and tolerate a broad range of environmental conditions [30,31,32,33,34,35,36,37,38,39]. This adaptability makes them valuable indicators of environmental health and water quality, frequently employed in biomonitoring programs to assess the ecological status of freshwater ecosystems and detect environmental disturbances [40,41,42,43,44,45,46].
In Korea, the aquatic ecosystem is diverse, under the influence of the country’s varied topography and climate. Major rivers, lakes, wetlands, streams, and coastal ecosystems contribute to the diversity, shaping the distribution and abundance of freshwater oligochaetes across different regions [47,48,49,50,51,52,53,54,55,56]. The current study is among the pioneering efforts to extensively analyze the diversity of species and ecological roles of freshwater oligochaetes in diverse aquatic environments across Korea. Despite the ecological diversity of freshwater habitats in Korea, morphological studies on aquatic oligochaetes remain scarce [57,58,59,60,61,62,63,64,65,66]. We identified nine species of oligochaetes from samples collected in various freshwater habitats, marking the first records of the species from freshwater environments in Korea. The current study provides valuable taxonomic information that confirms the species diversity within the subfamilies Tubificinae and Rhyacodrilinae, offering crucial baseline data for future assessments of the functional health of aquatic ecosystems. The findings have important implications for predicting a high diversity of aquatic oligochaetes in Korea and guiding environmental conservation efforts for aquatic oligochaetes in Korea.

2. Materials and Methods

2.1. Study Areas

Sampling was conducted at eight freshwater locations across Korea encompassing various aquatic environments, such as streams, brooks, farm waterways, ponds, and wetlands. The study sites are listed in Table 1.
The sites were selected to cover diverse freshwater habitats, ensuring a comprehensive study of oligochaete diversity across different types of aquatic environments.

2.2. Sample Collection

Samples were collected from fine, organic-rich sand and mud sediments at the aforementioned locations. Sampling was conducted along the water edge, using shovels and hand nets, and the specimens were transported alive to the laboratory. Transporting live specimens enabled accurate identification and differentiation of oligochaetes from other similar organisms in the sample, as well as observation of their biological characteristics in a live state.

2.3. Specimen Processing and Preservation

In the laboratory, live oligochaetes were sorted using a dissecting microscope (Laica S9i) equipped with a Leica DMC2900 camera and software kit (Leica, Wetzlar, Germany). Following the initial sorting, the specimens were fixed in a solution of 10% formalin and 80% ethanol to preserve their morphological integrity.
A multi-step staining process was then applied:
Initial Staining were first stained with a 10% formalin solution containing Rose Bengal.
Secondary Staining was followed by borax carmine staining, administered through a graded series of alcohol solutions.
Mounting specimens were mounted in Canada balsam for long-term preservation, in accordance with the protocol described by Erséus (1993) [67].

2.4. Voucher Specimen Deposition

Detailed morphological observations and measurements were carried out using a BX41 research microscope fitted with an Olympus DP73 camera system (Olympus, Tokyo, Japan). The images obtained were processed using InnerViewTM-i series image analysis software (Innerview Co. Ltd., Seongnam-si, Republic of Korea).

2.5. Morphological Analysis

Voucher specimens were deposited in national repositories for future reference:
National Institute of Biological Resources (NIBR): NIBRIV0000903040, NIBRIV0000910361, NIBRIV0000753818, NIBRIV0000901663, NIBRIV0000903043, NIBRIV0000903041.
Nakdonggang National Institute of Biological Resources (NNIBR): NNIBRIV111423, NNIBRIV111424.
These specimens are preserved as reference material for future taxonomic and ecological studies.

3. Results

Systematic accounts.
Phylum Annelida Linnaeus, 1758
Class Clitellata Linnaeus, 1740
Subclass Oligochaeta Grube, 1850
Order Haplotaxida Grube, 1850
Suborder Tubificina Jamieson, 1988
Family Naididae Ehrenberg, 1828
Subfamily Tubificinae Vejdovský, 1884
Genus Haber Holmquist, 1978
Haber speciosus (Hrabĕ, 1931) (Figure 1A–C)
Tubifex speciosus Hrabĕ, 1931: 24–27, figure 4a–e. [68]
Haber speciosus f. fluminialis: Milligan, 1986: 406–416, figures 1–2, tables 2–4. [69]
Material examined. Two individuals: one individual was deposited at NIBR (NIBRIV0000903040), and the other individual was deposited at Sahmyook University. The specimens were collected from Site 2.
Description. Length 8.92–10.11 mm in fixed state. Body smooth. Clitellum covering X–XII. Dorsal anterior bundles with 2–3 very fine pilose hair chaetae and 2–4 pectinated chaetae with lateral teeth of equal length and up to 2–5 intermediate denticles (Figure 1A). Ventral chaetae bifid; anterior bundle with 3–6 bifid with upper tooth approximately as long as thinner than lower. Posterior bundle with 1–2 bifid, with almost equal teeth. Spermathecal ampulla small, sac-like. Spermathecal chaetae modified, single per bundle, with thin, hollow-tipped, embedded in glandular sac in IX (Figure 1B). Spermathecal pore dorsal to ventral chaeta. Male funnel small. Vas deferens long, bipartite. Ectal region partly ciliated, enters atrium of similar width. Glandular prostate attached medially to relatively elongate, spindle-shaped atrium. Ejaculatory duct present, terminating in bulb-like penial apparatus. Basal membrane lining the penial canal forming a tube longer than wide in XI (Figure 1C).
Habitat. Freshwater (Stream).
Distribution. Europe, North America, Korea.
Remarks. The hollow-tipped spermathecal chaetae and penis thickened short cylindrical penial membrane and long hairs in the posterior segments distinguish this species from all other tubificids. Milligan [69] reviewed this genus, delineating two forms, i.e., the “fluminialis” form and the “simsi” form, for the North American fauna. The distinction between these forms lies in the number and morphology of the anterior pectinate and ventral bifid chaetae: the differentiations are more distinct, and the upper tooth of the ventral chaetae is stouter in “simsi”. The Korean type of spermathecal chaetae and penis are very similar to the “simsi” form in Milligan’s description; however, according to Timm and Patrick [70], the shape of the spermathecal chaetae showed slight differences, and the shape of the penis could not be confirmed from the information provided.
Taxonomy 05 00009 g001
Figure 1. Haber speciosus. (A). Dorsal bundle; (B). spermathecal chaeta; (C). basement membrane. Scale bars. (A) = 10 μm; (B,C) = 20 μm.
Figure 1. Haber speciosus. (A). Dorsal bundle; (B). spermathecal chaeta; (C). basement membrane. Scale bars. (A) = 10 μm; (B,C) = 20 μm.
Taxonomy 05 00009 g001
Genus Isochaetides Hrabĕ, 1966
Isochaetides arenarius (Michaelsen, 1926) (Figure 2A–C)
Limnodrilus arenarius Michaelsen, 1926: 153–173. [71]
Isochaetides arenarius (Michaelsen, 1926): Brinkhurst, 1981: 10–11. [72]
Material examined. Three individuals: one individual was deposited at NIBR (NIBRIV0000910361), and other individuals were deposited at Sahmyook University. The specimens were collected from Site 1.
Diagnosis. Length 11.5–13.2 mm in fixed state. Prostomium short, with small conical prostomia. Chaetae absent. Prostomium broad anterior end with small conical (Figure 2A). Chaetae 4–9 per bundle, upper tooth slightly equal or longer than lower (Figure 2B). Atrial duct longer than the vasa deferentia. Penis elongated, thin, and conical shaped. Spermathecal chaeta present in X, with long, thin, and grooved distal portion and curved proximal end, in a glandular sac (Figure 2C).
Taxonomy 05 00009 g002
Figure 2. Isochaetides arenarius. (A). Entire body; (B). ventral chaetae; (C). spermathecal chaeta. Scale bars. (A) = 100 μm, (B,C) = 40 μm.
Figure 2. Isochaetides arenarius. (A). Entire body; (B). ventral chaetae; (C). spermathecal chaeta. Scale bars. (A) = 100 μm, (B,C) = 40 μm.
Taxonomy 05 00009 g002
Habitat. Freshwater (Stream).
Distribution. Russia; Lake Baikal and Angara River.
Remarks. This species differs from I. baicalensis in that it has fewer chaetae (5 per bundle), and the teeth are nearly equal in length. The atria may be longer than those of I. baicalensis. This is the first record of this species from Korean water.
Isochaetides suspectus (Sokolskaya, 1964) (Figure 3A–C)
Limnodrilus suspectus Sokolskaya, 1964: 1071–1074. [73]
Isochaetides suspectus: Hrabĕ, 1966: 71. [74]
Material examined. Two individuals: one individual was deposited at NNIBR (NNIBRIV111423), and the other individual was deposited at Sahmyook University. The specimens were collected from Site 3.
Description. Length 7.54–9.81 mm in fixed state. Hair chaetae are absent. Anterior bundle chaetae bifid, 3–5 per bundle, with upper tooth slightly longer than lower; in tail region two per bundle. Spermathecal chaeta in X, with narrow, grooved distal portion and sharp tip (Figure 3B). Penial sheaths thin-walled, cylindrical, length about 2.7 times width (Figure 3C). Esophagus chloragogen tissue beginning in VI; dissepiment 3/4 thicker than the subsequent ones.
Habitat. Freshwater (Stream).
Distribution. Russia (Sakhalin Island).
Remarks. Information about Isochaetides suspectus is minimal. The only area where the species has been reported so far is Russia. However, the morphological characteristics of our sample were consistent with the species diagnosis in Timm and Martin [70].
Taxonomy 05 00009 g003
Figure 3. Isochaetides suspectus. (A). Entire body; (B). spermathecal chaeta; (C). penial sheaths. Scale bars. (A) = 1 mm, (B,C) = 100 µm.
Figure 3. Isochaetides suspectus. (A). Entire body; (B). spermathecal chaeta; (C). penial sheaths. Scale bars. (A) = 1 mm, (B,C) = 100 µm.
Taxonomy 05 00009 g003
Potamothrix hammoniensis (Michaelsen, 1901) (Figure 4A–E)
Ilyodrilus hammoniensis Michaelsen, 1901: 66–68. [75]
Tubifex camerani De Visart, 1901: 1–4+, figures 1–5. [76]
Psammoryctes fossor Ditlevsen, 1904: 398–480 + plates XVI–XVII. [77]
Material examined. Two individuals: one individual was deposited at NIBR (NIBRIV0000753818), and the other individual was deposited at Sahmyook University. The specimens were collected from Site 2.
Description. Length 9.98–11.69 mm in fixed state. Anterior ventral chaetae 3–4 per bundle, 130–150 μm long, with upper tooth slightly longer and thinner. (Figure 4A,B). Anterior dorsal chaetae 1–2 hair chaetae with 2–3 pectinate chaetae, 80–129 μm long, with upper tooth slightly longer than lower, and several slightly shorter intermediate denticles (Figure 4C). Spermathecal chaetae large, surrounding compacted gland cell in X, with a thick and blunt, uncurved tip, 293–296 μm long; edges are parallel, but the wide groove is oblique (Figure 4D). Sperm in spermathecae organized into spermatozeugmata (Figure 4E). The atrium is long, club-shaped in XI.
Taxonomy 05 00009 g004
Figure 4. Potamothrix hammoniensis. (A). Anterior ventral chaetae; (B). posterior chaeta; (C). pectinate chaetae in dorsal bundle; (D). spermathecal chaetae; (E). genital organs in X–XI. Scale bars. (A,B,E) = 100 μm, (C) = 40 μm; (D) = 20 μm. aa: atrial ampulla, ce: coelomocyte, pr: prostate glands, sf: sperm fennel, sd: spermathecal duct, sa: spermathecal ampulla, smz: spermatozeugmata, vs: vas deferens.
Figure 4. Potamothrix hammoniensis. (A). Anterior ventral chaetae; (B). posterior chaeta; (C). pectinate chaetae in dorsal bundle; (D). spermathecal chaetae; (E). genital organs in X–XI. Scale bars. (A,B,E) = 100 μm, (C) = 40 μm; (D) = 20 μm. aa: atrial ampulla, ce: coelomocyte, pr: prostate glands, sf: sperm fennel, sd: spermathecal duct, sa: spermathecal ampulla, smz: spermatozeugmata, vs: vas deferens.
Taxonomy 05 00009 g004
Habitat. Freshwater (Stream).
Distribution. America, Korea.
Remarks. Potamothrix hammoniensis can be recognized by its large and thick modified spermathecal chaetae in X. The spermathecal chaetae of P. Vejdovský may resemble that of P. hammoniensis in size and shape, but in P. Vejdovský the dorsal chaetae are bifid with short hairs, while in P. hammoniensis the dorsal chaetae are pectinate with relatively long hairs. According to Timm [78], this species could be the earliest and most successful invader in the European freshwater bodies before human transport, although it has not yet reached its potential limits in Scandinavia and Siberia.
Psammoryctides deserticola deserticola (Grimm, 1876) (Figure 5A–C)
Tubifex deserticola Grimm, 1876: 108–110. [79]
Material examined. One individual mounted on a slide was deposited at NNIBR (NNIBRIV111424). The specimen was collected from Site 4.
Diagnosis. Length 7.35 mm in fixed state. Dorsal bundles 1–3 pectinate chaetae with equally long or slightly longer upper tooth, with several sort intermediate denticles, 1–2 pilose hair chaetae (Figure 5B). Ventral chaetae bifid, 2–3 per bundles, with upper tooth equally longer and thinner than lower (Figure 5C). No ventral chaetae in X–XI in mature. In tail segments, both ventral and dorsal bundles with a single thick, curved bifid chaeta with very thick, curved lower tooth, the upper small and erect. Spermathecal pores in X, without ventral chaetae. No coelomocytes.
Taxonomy 05 00009 g005
Figure 5. Psammoryctides deserticola deserticola. (A). Entire body; (B). dorsal bundle; (C). ventral chaeta. Scale bars. (A) = 1mm, (B,C) = 40 µm.
Figure 5. Psammoryctides deserticola deserticola. (A). Entire body; (B). dorsal bundle; (C). ventral chaeta. Scale bars. (A) = 1mm, (B,C) = 40 µm.
Taxonomy 05 00009 g005
Habitat. Freshwater (Stream).
Distribution. Europe, Ponto-Caspian Basin, Danube River, Balkan Peninsula, Near East.
Remarks. Psammoryctides deserticola deserticola could be confused with Varichaetadrilus harmani (Loden, 1979). The tail of V. harmani is similar to that of P. deserticola deserticola, but it is distinguished from this species by its smooth and very fine hairs. Our specimens match the morphological characteristics of P. deserticola deserticola.
Genus Tubifex Lamark 1816
Tubifex conicus He, Cui & Wang, 2012 (Figure 6A–C)
Tubifex conicus He, Wang & Cui, 2012: 160–162, figure 1A–E, table 1. [80]
Material examined. Two individuals: one individual was deposited at NIBR (NIBRIV0000901663), and the other individual was deposited at Sahmyook University. The specimens were collected from Site 5.
Description. Length 1.58–1.65 mm in fixed state. Prostomium obtuse. Clitellum inconspicuous (Figure 6A). No coelomocytes. Dorsal bundle 1–3 hairs and 1–3 pectinated needles with upper tooth subequal lower, 2–3 fine intermediated teeth (Figure 6B). Ventral chaetae bifid, 3–4 per bundle anteriorly, 2 per bundle posteriorly, upper tooth slightly longer than lower. Ventral chaetae unmodified in X. Ventral chaetae in XI absent. Genitalia paired in X–XI. Sperm funnel cup-shaped. Vas deferens 3–4 times as long as atrium, with cilia throughout and entering atrium apically. Atrium spindle-shaped, attached prostate gland in ental portion. Penial sheath short truncated conical with distal end oblique (Figure 6C). Spermathecae absent.
Taxonomy 05 00009 g006
Figure 6. Tubifex conicus. (A). Entire body; (B). dorsal bundle; (C). penial sheath. Scale bars. (A) = 200 µm; (B,C) = 300 µm.
Figure 6. Tubifex conicus. (A). Entire body; (B). dorsal bundle; (C). penial sheath. Scale bars. (A) = 200 µm; (B,C) = 300 µm.
Taxonomy 05 00009 g006
Habitat. Freshwater (Farm waterway).
World distribution. China: Lake Yamzho Yumco in Tibet.
Remarks. In the original description [80], this species was described as having spermathecae, whereas in the redescription, this was revised to no spermathecae [81]. In addition, this species was found in the salty lakes in the highlands of Tibet. Our specimens differ slightly from the description, but the overall main morphological features are the same; however, the discovered area exhibited environmental differences from the original description. It was originally discovered in a lake in a highland area, but we found it in farm waterways containing organic and inorganic substances.
Genus Tasserkidrilus Holmquist, 1985
Tasserkidrilus americanus (Brinkhurst & Cook, 1966) (Figure 7A–D)
Tubifex kessleri americanus Brinkhurst & Cook, 1966: 13–14, figure 6B–E. [82]
Material examined. Two individuals: one individual was deposited at NIBR (NIBRIV0000903043), and the other individual was deposited at Sahmyook University. The specimens were collected from Site 6.
Description. Length 1.08–1.64 mm in fixed state. Ventral chaetae 3–5 per bundle anteriorly, with upper tooth thinner than lower tooth (Figure 7A). Posteriorly 1–2 chaetae, with teeth equal. Dorsal chaetae 1–3 finely pilose hair chaetae besides 3–4 pectinates with equal obtuse teeth and 2–3 intermediate denticles (Figure 7B). Vasa deferentia short, three times the length of the atrial, entering atria apically. Atria spindle-shaped, symmetrical with large prostate gland. Penial sheaths tubular, funicular proximal bit distally cupped and acute in XI (Figure 7C).
Taxonomy 05 00009 g007
Figure 7. Tasserkidrilus americanus. (A). Entire body; (B). ventral chaeta; (C,D). penial sheath. Scale bars. (A) = 200 µm; (B,C) = 20 µm; (D) = 40 µm.
Figure 7. Tasserkidrilus americanus. (A). Entire body; (B). ventral chaeta; (C,D). penial sheath. Scale bars. (A) = 200 µm; (B,C) = 20 µm; (D) = 40 µm.
Taxonomy 05 00009 g007
Habitat. Freshwater (Farm waterway).
World distribution. America, Europe, Korea.
Remarks. Tasserkidrilus americanus has originally been described from North America [82]. It is found in cold, oligotrophic, profundal areas [83]. However, our specimen and a European specimen were found in the lotic environments of eutrophic brooks and rivers [84]. This species exhibits differences in shape and length of the penial sheath between North American and European species. Compared to those native to North America, the European species are proportionally larger and wider [85].
Our sample was most likely American, based on the length and shape of the penile cover.
Tasserkidrilus variabilis (Semernoy, 1982) (Figure 8A–D)
Tubifex kessleri variabilis Semernoy, 1982: 69–70, figure 7A–D. [86]
Material examined. Three individuals: one individual was deposited at NIBR (NIBRIV0000903041), and the other individual was deposited at Sahmyook University. The specimens were collected from Site 8.
Description. Length 9.98–11.69 mm in fixed state. Prostomium small and triangular. Chloragogen begins with VI. Hair chaetae absent. Chaetae 4–5 per bundle, with upper tooth slightly longer than lower tooth (Figure 8B). Clitellum not distinctly defined. Spermatheca in lateral line of X. Spermatheca short, and ampulla rounded. Atrium narrow and tubular, with compact prostatic gland attached into middle part. Penis sheaths cone-shaped and vent, with later oblique opining. Distal end acutely attenuated, fairly thick walls, and finely barbed on surface (Figure 8C,D).
Taxonomy 05 00009 g008
Figure 8. Tasserkidrilus variabilis. (A). Entire body; (B). ventral chaeta; (C,D). penial sheath. Scale bars. (A) = 600 µm; (B) = 40 µm; (C,D) = 100 µm.
Figure 8. Tasserkidrilus variabilis. (A). Entire body; (B). ventral chaeta; (C,D). penial sheath. Scale bars. (A) = 600 µm; (B) = 40 µm; (C,D) = 100 µm.
Taxonomy 05 00009 g008
Habitat. Brackish zone (wetland).
World distribution. Russia: Lake Baikal and Angara River in Siberia
Remarks. Tasserkidrilus variabilis (Semernoy, 1982) is an oligochaete worm species originally described from freshwater environments in Siberia [86]. The species has been reported from lakes and rivers, usually inhabiting sediments, and has been found in deep sandy areas, particularly in Lake Baikal, at depths of more than 200 m [87].
Subfamily Rhyacodrilinae Hrabě, 1963
Genus Rhyacodrilus Bretscher, 1901
Rhyacodrilus stephensoni Černosvitov, 1941 (Figure 9A–E)
Rhyacodrilus stephensoni Černosvitov, 1942: 284, figure 8, 31E–H. [88]
Material examined. Four individuals: one individual was deposited at NIBR (KOSPIV0000241742), and the other individuals were deposited at Sahmyook University. The specimens were collected from Site 7.
Description. Length 8.12–9.25 mm in fixed state. Prostomium bluntly conical, marked off by a distinct groove, external annulation distinct, double in anterior segments, anterior ring considerably narrower (Figure 9A). Chaetae bifid, 2–4 chaetae per bundle, more numerous in ventral bundles, 80–85 μm long, both teeth about same length, or distal slightly longer, proximal more strongly curved in posterior chaetae than in anterior chaetae (Figure 9B,C). Dorsal and ventral chaetae of same form begin in II, very thin, slightly curved with indistinct nodulus at junction of middle and distal third. Clitellum in XI–XII, sometimes extending to X and XIII, slightly developed ventrally. Testes and ovaries paired in X and XI. Atrial ampulla ovoid, or pear-shaped, continuing into narrower and shorter, efferent duct. No pseudopenis, penial chaetae 3–4, with rudimentary teeth (Figure 9E). Spermethecae a pair in X, consisting of a large thin-walled rounded or bilobed ampulla and a very short, narrow duct.
Taxonomy 05 00009 g009
Figure 9. Rhyacodrilus stephensoni. (A). Entire body; (B). anterior chaetae; (C). posterior chaetae; (D) dorsal chaetae; (E). penial chaetae. Scale bars. (A–D) = 100 μm; (E) = 40 μm.
Figure 9. Rhyacodrilus stephensoni. (A). Entire body; (B). anterior chaetae; (C). posterior chaetae; (D) dorsal chaetae; (E). penial chaetae. Scale bars. (A–D) = 100 μm; (E) = 40 μm.
Taxonomy 05 00009 g009
Habitat. Freshwater (Farm waterway).
World distribution. Lake Baikal, Smaller Siberian lakes, East China, Tibet.
Remarks. In most previous studies, this species has been found in a high alpine lake or deep within Lake Baikal [89,90,91,92,93]. According to Snimschikova and Akinshina [94], Rhyacodrilus stephensoni has been found in high mountain lakes in Tibet. Moreover, this species is assumed to be an ancestor of R. issosimovi, which is widely distributed in the lake as an endemic species.

4. Discussion

Aquatic oligochaetes inhabit a broad spectrum of aquatic environments, including freshwater, seawater, brackish water, and groundwater, with approximately 8100 species documented globally (WoRMS, 2024). In Korea, prior research has identified 81 species of freshwater oligochaetes distributed across 6 orders, 6 families, and 36 genera. In the current study, we identified 9 species belonging to 6 genera and 2 subfamilies from various freshwater environments, including rivers, streams, agricultural waterways, ponds, and wetlands. The sampling sites were characterized by slow water flow and elevated organic matter contents, with some locations exhibiting abundant aquatic vegetation or situated in the lower reaches of rivers influenced by seawater (Table 1).
The current study is a valuable contribution to the understanding of the biodiversity and distribution of freshwater oligochaetes in Korea. The identification of diverse species within a relatively constrained geographic range underscores the potential for greater species diversity within Korea’s freshwater ecosystems; as a result, further extensive exploration is warranted.
Taxonomic identification of aquatic oligochaetes remains challenging due to the limited morphological features available for species differentiation. Reproductive structures, essential for precise species identification, are typically observable only during specific phases of sexual maturity, such as the development of the clitellum. The current study documented critical morphological traits, including external features (e.g., chaetal structure, ventral chaetae, hair chaetae, and needle forms), as well as additional characteristics visible during reproductive phases. Illustrations and photographic documentation were provided to support more accurate species identification. However, due to constraints in specimen availability, genetic data were not acquired during the current study. Future research will prioritize the collection of additional specimens to facilitate genetic analyses and improve taxonomic resolution.
The detailed morphological analyses conducted in this study considerably enhance the accuracy of oligochaete identification and classification, which is vital for elucidating their ecological roles within freshwater habitats. Such foundational knowledge supports ecological investigations and informs conservation strategies aimed at mitigating the impacts of environmental change. Expanding future research to encompass broader geographic areas and a wider range of habitats will provide a more comprehensive understanding of oligochaete diversity in Korea. Moreover, integrating molecular approaches, such as DNA barcoding, will enhance the accuracy of species identification and elucidate evolutionary relationships within the group. In addition, investigations into the ecological roles of oligochaetes, including their interactions with other biotic components and responses to environmental stressors, are critical for a holistic understanding of their ecological significance.

5. Conclusions

The findings of the current study provide valuable insights into freshwater polychaetes in Korean aquatic ecosystems. Identifying seven species across five genera and one subfamily from various freshwater habitats broadens our understanding of the diversity and taxonomy of these important organisms. The results suggest that Korean freshwater ecosystems harbor even greater species diversity than currently recognized, highlighting the importance of continued exploration in this field.
The findings of the current study lay a solid foundation for future taxonomic and ecological research, offering a baseline for further studies on freshwater polychaetes. Additionally, they contribute to ongoing efforts to conserve and manage Korea’s aquatic biodiversity, providing essential knowledge to address the challenges posed by environmental change.
This study advances our understanding of the biodiversity of freshwater oligochaetes in Korea and serves as a foundational resource for future taxonomic and ecological studies. The findings lay essential groundwork for freshwater ecosystem research and conservation initiatives, contributing to the broader understanding and management of aquatic biodiversity in Korea.

Funding

This research was supported by a National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIT) (NRF-2021R1A6A3A01088490), a grant from the National Institute of Biological Resources (NIBR) funded by the Ministry of Environment (MOE) of the Republic of Korea (NIBR201501201 NIBR201601201, NIBR202304201, NIBR202203106), and a grant from the Nakdonggang National Institute of Biological Resources (NNIBR), funded by the Ministry of Environment (MOE) of the Republic of Korea (NNIBR202301201).

Data Availability Statement

The data presented in this study can be accessed in the manuscript.

Conflicts of Interest

The authors declare no conflict of interest.

References

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Table 1. Sample collection sites.
Table 1. Sample collection sites.
No.GPSEnvironment
St. 1N: 37°34′55.22″ E: 127°04′36.14″Stream
St. 2N: 37°34′52.87″ E: 127°04′41.74″Stream
St. 3N: 37°34′51.09″ E: 127°04′38.08″Stream
St. 4N: 36°03′76.27″ E: 126°78′06.71″Brook
St. 5N: 36°04′7.27″ E: 126°51′41.96″Farm waterway
St. 6N: 34°57′26.26″ E: 128°39′16.67″Pond
St. 7N: 34°48′52.38″ E: 128°40′31.15″Pond
St. 8N: 33°31′04.07″ E: 126°57′03.17″wetland
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