ZOOSYSTEMATICA ROSSICA, 19(1): 18–22 15 JULY 2010 A new species of diatom-feeding Asperotobrilus (Nematoda: Triplonchida: Tobrilidae) from Lake Baikal Новый вид диатомоядного Asperotobrilus (Nematoda: Triplonchida: Tobrilidae) из озера Байкал A.V. SHOSHIN А.В. ШОШИН A. V. Shoshin, Zoological Institute, Russian Academy of Sciences, 1 Universitetskaya Emb., St. Petersburg, 199034, Russia. E-mail: [email protected] A new species, Asperotobrilus holophagus sp. nov. is described, which is a specialised diatom- feeder. A simple funnel-shaped stoma, a high vestibulum, and reduction of lips are typical of diatom-feeding tobrilids. A key to species of the genus Asperotobrilus is given. Приводится описание нового вида Asperotobrilus holophagus sp. nov., специализирую- щегося на поедании диатомей. Для этого вида, как и для всех диатомоядных тобрилид, характерны простая воронковидная стома, образование высокого вестибулюма и ре- дукция губ. Дается определительная таблица видов рода Asperotobrilus. Key words: freshwater free-living nematodes, morphology, Triplonchida, Tobrilidae, Aspero- tobrilus, new species, Baikal Ключевые слова: пресноводные свободноживущие нематоды, морфология, Triplonchida, Tobrilidae, Asperotobrilus, новый вид, Байкал INTRODUCTION In such a giant and stable water body as Lake Baikal, the evolution of tobrilids has Nutrition of free-living nematodes is led them to occupy the ecological niches still insufficiently studied. The probable uncommon of most continental water bod- feeding objects may be inferred from the ies. This phenomenon is reflected in a va- morphology of the stoma and anterior body riety of adaptations to different nutrition end. Having applied this approach, Wieser types. For example, a diverse group of large (1953) distinguished four general types of predatory tobrilids feeding on oligochaetes nematode nutrition; this system was later occurs in Lake Baikal (their food object modified by Jensen (1987). However, the can be easily determined by the presence of feeding of tobrilids is difficult to character- numerous chaetae in their intestine) (Tsa- ise on the sole basis of the stoma structure. lolikhin, 1980). Diatoms constitute anoth- According to Jensen’s system, they can be er favourite food object of some tobrilids classified either as predators (presence of in Lake Baikal. Diatoms are occasionally onchia in the voluminous stoma) or as de- found in the digestive systems of tobrilids posit-feeders (a voluminous stoma, allow- outside Lake Baikal as well, for example, in ing food to be swallowed without destroy- Brevitobrilus stefanskii (Tsalolikhin, 2001), ing it by the onchia, which are hidden deep but the stoma of the species is of an ordinary in recesses). The feeding objects of tobrilids structure. Most of diatom-feeding tobrilids can seldom be determined with confidence of Lake Baikal have a specifically modified from the contents of their intestine. stoma (discussed below), by which they © 2010 Zoological Institute, Russian Academy of Scienсes A.V. SHOSHIN. A NEW ASPEROTOBRILUS FROM LAKE BAIKAL 19 may be recognised even if there are no dia- lature and between body wall and internal toms in their intestine. Different tobrilid organs, absent at anterior end and cylindri- taxa of Lake Baikal have acquired this nu- cal tail portion. Crystalloids rod-like, 2.4– trition type independently. Diatom-feeding 2.8 × 0.8 μm. Anterior body end rounded, tobrilids have been found in the genera As- not offset. Labial papillae small, indistinct. perotobrilus, Tobrilus and Eutobrilus. Cephalic setae in two slightly separated In this paper, a diatom-feeding species circles; setae of anterior circle shorter than of Asperotobrilus is described. The diatom- those of posterior circle. Cephalic setae of feeding tobrilids from other taxa as well as anterior circle 15% of head width, papilloid; the transformations of their head end and those of posterior circle resembling common stoma will be described in detail in a sepa- body setae. Cheilostome high, surrounded rate publication. with plump cheeks, true lips reduced, buc- cal cavity with thick-walled, funnel-shaped, TAXONOMIC ACCOUNT followed by two largely overlapping pock- ets. Pockets rounded, poorly distinguished The diagnosis of the genus Asperotobri- from buccal cavity; each pockets armed lus is given in earlier publications (Shoshin, with small onchium, distance between two 1991, 1998). onchia 3.0–3.5 μm, total width of pocket lumen 6.0 μm. Amphid large, its aperture Asperotobrilus holophagus sp. nov. more or less oval, situated at middle level (Figs 1–7) of stoma pockets. Oesophagus (pharynx by some authors) relatively short, strongly Measurements: Table. muscular, slightly expanded anteriorly and Holotype. Male, Russia, Lake Baikal, posteriorly. Cardial glands 3 in number, Krestovskiy Cape, depth 20 m, fine grey globular, 14 × 18 μm. Intestine polycytous sand, 25.10.00 (leg. Shoshin); slide A-6740, with irregular cells 8.0–12.0 μm in diam- deposited at the Zoological Institute, St. Pe- eter, enlarging posteriorly (13.5 × 20.0 μm tersburg, Russia. in size). Intestine about 50% of body width, Paratypes. Two males and one female compressed by reproductive system. Some from same locality as holotype, and three diatoms discernible in intestinal lumen. young females: Russia, Lake Baikal, Pe- Testes are two, ventral to intestine, with schanaya Bay, depth 5.0 m, fine sand, 11.08.86 main portions of both testes on left side (leg. Shoshin), all slides deposited at the Zo- of intestine. Spermia closely packed in ve- ological Institute, St. Petersburg, Russia. siculae seminales; they are oblong, rod-like, Description. Male. Relatively small to- 5.7–6.3 μm long and 1.0–1.2 μm wide. Male brilid, body spindle-shaped, thick. Body af- sexual duct very thick, about 50% of body ter fixation C-shaped or irregularly shaped, width. Vas deferens separated from ejacula- maximum body width in the middle, taper- tory duct by hard constriction. Supplemen- ing distinctly to anterior and posterior ends. tary organs absent. Spicules slender, slightly Cuticle 2.2–2.4 μm thick, two-layered un- curved with rounded proximal capitulum. der light microscope, outer layer 1.4–1.6 μm Tail thick, conical with pointed terminus. thick, inner layer 0.8 μm thick. Cuticle fine- Female. Similar to male except for slight- ly transversely striated, striae more promi- ly shorter somatic setae (4.5–4.7 μm), sexu- nent near anterior end, 1.0 μm apart, at mid- al characters, size of amphids, and numerous body 0.8 μm apart. Somatic setae numerous, crystalloids between body wall and internal hard, 7.5–8.0 μm long in precloacal region organs. Crystalloids of at least two forms: and 5.5–6.5 μm long on rest of body. Sub- larger rod-like ones 10.5–11.8 × 0.9–1.3 μm cuticular metanemes indistinct. Numerous and small rod-like ones 5.3–6.3 × 0.6– crystalloids associated with somatic muscu- 0.7 μm, absent at anterior end and cylindri- © 2010 Zoological Institute, Russian Academy of Scienсes, Zoosystematica Rossica 19(1): 18–22 20 A.V. SHOSHIN. A NEW ASPEROTOBRILUS FROM LAKE BAIKAL Table. Absolute and relative measurements of Asperotobrilus holophagus sp. nov. All absolute lengths are in μm. Male Female paratypes Krestovskiy Cape Krestovskiy Cape Peschanaya Bay holotype paratypes (n=2) (n=1, adult) (n=3, young) Total length 1114 1040, 1098 998 1116–1160 (1142) Body length/body width 16.6 15.5, 18.0 12.0 18.7–20.9 (20.0) Body length/oesophagus length 4.8 4.6, 4.9 3.9 4.6–5.0 (4.8) Body length/tail length 8.7 9.0 8.5 8.5–9.3 (8.9) Tail length/anal body width 3.7 2.7, 3.2 2.6 3.7–4.2 (3.9) Cephalic setae length (anterior circle/ 4.0/6.5 4.0/6.5 4.0/6.5 3.0–3.5/4.8–5.0 posterior circle) Head end width 26 23, 26 26 20 Stoma width 10 13 15 10–11 Total stoma length (vestibulum/ 30 30 32 24–26 buccal cavity/pockets) (6.5/13/10) (6.5/13/10) (6.5/15/10) (4.0–4.5/10–11/8–12) Amphidial aperture width 6.5 6.5 ? 4.5–4.7 Distance between anterior end of body 41.7% 41.4%, ? 31% 39.6–44.7 (42.6)% and nerve ring (% oesophagus length) Oesophageal length 230 224, 224 256 230–240 (235) Maximum body width 67 61, 67 83 55–62 (57) Distance between anterior end of body – – 56.1% 51.7–57.0 (53.6)% and vulva (% body length) Distance between anterior end of body 43.0% 39.3%, 43.2% and testis joint (% body length) Length of anterior genital branch – – 160 ? of female Length of posterior genital branch – – 144 ? of female Length of anterior testis 118 80, 112 – – Length of posterior testis 80 106, 112 – – Distance from vulva to anus – – 320 335–435 (395) Male sexual duct length 506 477, 544 – – Ejaculatory duct length 362 349, 352 – – (% male sexual duct length) (71.5%) (64.7%, 73.2%) Spicules length/width 37/6 38/8 – – Gubernaculum length 20 20 – – Rectal length 49 51, 52 42 33–36 (35) Tail length 128 115, 122 118 125–135 (128) © 2010 Zoological Institute, Russian Academy of Scienсes, Zoosystematica Rossica 19(1): 18–22 A.V. SHOSHIN. A NEW ASPEROTOBRILUS FROM LAKE BAIKAL 21 Figs 1–7. Asperotobrilus holophagus sp. nov.: 1, head end of male; 2, tail of male; 3, tail of female; 4, oesophagus of male; 5, spicules and gubernaculum; 6, total view of female; 7, total view of male. Scale bars: 10 μm (1, 5), 35 μm(2–4, 6, 7). © 2010 Zoological Institute, Russian Academy of Scienсes, Zoosystematica Rossica 19(1): 18–22 22 A.V. SHOSHIN. A NEW ASPEROTOBRILUS FROM LAKE BAIKAL cal tail part. Female reproductive system – the onchia and recesses of the stoma didelphic, amphidelphic. Ovaries ventral to are considerably reduced in most species; intestine, reflexed, with main portions on – the vestibulum is significantly modi- left side of intestine. Cells in germinal zone fied: the true lips are reduced, so that the in multiple rows, oocytes first in tandem and oral fissure is closed not by the lips but by then in continuously enlarged single row. the inner walls of vestibulum, formed by Oviduct indistinct. Spermatheca filled with soft tissues. sperm. Uterus 37.0–64.0 μm long when one Owing to the above transformations, the egg present, twisted when eggs absent. Egg stoma of diatom-feeding tobrilids becomes oval, 44.0 × 36.0 μm. Vagina perpendicu- similar to that of Monhysterida, among lar to body axis, vagina length about 50% which diatom-feeding species are quite of body width at its level. Vulva transver- common. sal, slit-like, just anterior to mid-body, lips slightly protuberant. ACKNOWLEDGEMENTS Comparison. Differs from type species I am grateful to S.Ya. Tsalolikhin (Zoological A. asper Shoshin, 1992 by conical stoma, Institute, St. Petersburg) for discussion, O.A. Ti- high cheilostome and reduction of lips. moshkin and T.Ya. Sitnikova (Limnological In- Etymology. From holo- and phagos stitute, Irkutsk) for the expedition facilities and (Greek: whole, entire and eating) because collecting of material. This study was carried out of its mode of feeding. within the framework of Project 301/20 “Inter- relation in development of bottom-dwelling and pelagic associations of Lake Baikal” (headed by CONCLUSION O.A. Timoshkin). Key to species of the genus Asperotobrilus REFERENCES 1(6) Buccal cavity narrow, nearly cylindrical. 2(5) Somatic setae extremely long and hard, lon- Jensen, P. 1987. Feeding ecology of free-living ger than 7 μm (usually 10–20 μm). aquatic nematodes. Marine Ecology Progress 3(4) True supplementary organs absent, ventral Series, 35: 187–196. precloacal setae indistinguishable from so- Shoshin, A.V. 1991. Two new species of Baikal matic setae . . . . . . . A. asper Shoshin, 1991 nematodes of the order Enoplida. Zoolog- 4(3) Supplementary organs in form of shortened icheskiy Zhurnal, 70: 132–135. (In Russian). setae . . . . . . . . . A. aculeatus Shoshin, 1998 Shoshin, A.V. 1998. Two new species of Baikal 5(2) Somatic setae weak, not longer than 4–5 μm nematodes of the genus Asperotobrilus. Zoo- systematica Rossica, 7: 223–227. . . . . . . . . . . . . . . . . A. investis Shoshin, 1998 Tsalolikhin, S.Ya. 1980. [Free-living Nematodes 6(1) Buccal cavity wide, funnel-shaped . . . . . . . . . of Baikal]. Nauka, Novosibirsk. 118 pp. . . . . . . . . . . . . . . . . . A. holophagus, sp. nov. Tsalolikhin, S.Ya. 2001. Notes on composition of the genus Brevitobrilus Tsalolikhin, 1981 The following transformations of the (Nematoda, Enoplida: Tobrilidae). Zoosys- stoma are characteristic of diatom-feeding tematica Rossica, 9: 25–35. tobrilids of Lake Baikal: Wieser, W. 1953. Beziehungen zwischen Mund- – the buccal cavity is funnel-shaped, höhlengestalt, Ernährungsweise und Vorkom- whereas the stoma of other species of the men bei freilebenden, marinene Nematoden. genera Asperotobrilus, Tobrilus and Eutobri- Arkiv für Zoologie, 2: 439–484. lus is wineglass- or goblet-shaped; Received 22 March 2010 / Accepted 18 June 2010 © 2010 Zoological Institute, Russian Academy of Scienсes, Zoosystematica Rossica 19(1): 18–22