Manayunkia kolhymensis Gagaev, Regel & Sitnikova 2025, sp. nov.

Manayunkia kolhymensis Gagaev, Regel & Sitnikova sp. nov. urn:lsid:zoobank.org:act: 2950B7E0-A48A-4D9B-9847-A4BEB912C307 (Figures 1–4) Manayunkia sp. – Pudovkina et.al., 2014: 32–40 (Lakes Serdyakhskie; COI sequences of M. ‘ kolymensis ’); Manayunkia sp. ‘kolymensis’ – Atkinson et al., 2020: 310-320 (COI sequences, nomen nudum) Holotype. Male, ZIN 1/50802 (in 70% alcohol), Russia, Northeast, Magadan region, Ola River basin, near Klyopka village, Lake “Uglozub” of the Lakes Serdyakhskie (59°46'12.66"N, 151°21'8.69"E), about 1 m depth, on submerged wood, 28 July, 1996, leg. K. Regel. Paratypes. ZIN 2 /50803, 8 specimens in 70% alcohol, the same collection data as the holotype. Paratypes. ZIN 3/50804, 11 specimens in 70% alcohol, Russia, Northeast, Magadan region, Ola River basin, Serdyakhskie Lakes, underside of submerged wood, depth down to 1 m, 6 August, 1998, leg. K. Regel. Paratypes. ZIN 4/50805, 6 specimens in Canadian balsam slides, Russia, Northeast, Magadan region, Kolyma River basin, 1417 km from the mouth, Lakes Namakanskie (Oymyakonskie, local name) (63°46'42.78"N, 153°35'56.00"E), at a depth of about 1.0 m, 14 August, 1996, leg. K. Regel. Paratypes. ZIN 5/50806, 26 specimens in 70% alcohol, Russia, Northeast, Magadan region, Ola River basin, near Klyopka village, Serdyakhskie Lakes, underside of submerged wood, about 1 m depth, June 2010, leg. K. Regel. Description. Holotype. Male, 8 thoracic and 3 abdominal chaetigers (Figs 2 Aa, Ba, C, D). Total length 3.2 mm, including radiolar crown length of 0.7 mm, and width of 0.3 mm. Size of paratypes: total length with radiolar crown 3.05 ± 0.38 (2.2 – 3.3) mm (n=13), body width (on 3th chaetiger) 0.30 ± 0.06 (0.20–0.36) mm (n=13), radiolar crown with two pairs of radioles and one pair of the unbranched vascularized ventral filamentous appendages. Body cylindrical, flattened in abdominal region, with an oval pygidium. Body colour of live specimens from dark yellow to light brown, transparent, without distinct pigmentation. Two pairs of radiolar lobes with 22–32 branches (Figs 2 Ab, Bb, E, 3A). Two vascularised unbranched filamentous appendages ventrally (Fig. 2C), containing a green blood vessel, somewhat thicker than radiolar branches. One light yellow ‘spot’ visible on the ventral side of the base of radioles in some individuals. Separation into anterior and posterior rings of peristomium not visible; peristomium with rectangular ventral lobe (Fig. 2 Aa, Ab, C). Eyes present on peristomium (Fig. 4A) but fading in alcohol and becoming barely visible in fixed specimens (Fig. 2A–C); no eyes on pygidium. Faecal groove (Fig. 2B) running along dorsal side and turning left at the upper part of 8th thoracic chaetiger, continuing onto ventral part of the first abdominal chaetiger and then crossing remaining abdominal chaetigers (Fig. 1 Ac). First thoracic chaetiger with notochaetae only. The 2 nd –5 th thoracic chaetigers with no more than six (usually two to five) uncini with a few small teeth per chaetiger in males and females; notopodia with max. six long capillary chaetae with small faint teeth, and max. three–four short capillary chaetae, also with small faint teeth (Fig. 3D, E, F). The 6–8 th longer thoracic chaetigers in males, with four–five neuropodial uncini (Fig. 3D), notopodia with four–fives short and elongate, narrowly hooded chaetae. The 6 th –8 th thoracic chaetigers in females with same number of notopodial chaetae, but neuropodia 6 th –7 th thoracic chaetigers with three transitional chaetae (Fig. 4B), which were not visible on 8 th chaetiger. Noticeable dorsal wide transverse fold (genital ridge) in females between the 5 th and 6 th thoracic chaetigers (Fig. 3C), the brood chamber located between 6 th and 7 th chaetigers has about 10 thin transverse folds on the dorsal part (Fig. 3C). Neuropodia of thoracic chaetigers of juveniles (less than 1 mm length) without uncini and chaetae. Abdomen flattened dorso-ventrally, and pygidium oval (Fig. 2 Ac). Abdominal uncini with long manubrium; abdominal notopodia with more than 20 uncini on chaetiger 9, with 17-19 uncini on chaetiger 10, and with 13–15 uncini on chaetiger 11. Uncini with 6–7 rows of equal-sized teeth. Abdominal neurochaetae thin; capillaries with shortened proximal part and very long distal part with double edging; two chaetae per neuropodium, one slightly longer than the other (Fig. 3D). Tube delicate, semi-transparent, muddy, and covered with sand grains (Fig. 1A). Reproduction. The breeding period coincides with the warm season (June and possible July). Six to seven developing embryos were detected in the brood chambers in June 2010. Examination of histological sections of the worms whose sex could not be identified (Fig. 2F) revealed simultaneous presence of oocytes in 3 rd –5 th thoracic chaetigers (Fig. 5A) and spermatids in in 6 th –8 th thoracic chaetigers (Fig. 5B, C). The diameter of oocytes varied from 8.9 to 24.8 µm (mean 17.4 ± 6.0 µm, n=6), and was thus more than 10 times smaller than the embryos of the first stages of development. All male gametes were at different stages of development, but some cythophores consisted of mature spermatozoa. Two adults with a length of 3.0– 3.2 mm had neuropodial uncini on thoracic chaetigers 6–8 (Fig. 4D), so they can be characterized as functional males, and the translucent white matter inside the thoracic chaetigers 6–8 were probably male gametes. These two individuals had oocytes that were 9.2–12.8 µm in size (Fig. 4C) and can therefore be characterized simultaneously as adult males and juvenile females. Remarks. Morphological differences between individuals from Serdyakhskie and Namakanskie(Oymyakonskie) Lakes were not detected. The sexual dimorphism that occurs in the chaetae pattern, the longer 6 th –8 th chaetigers in males, and the presence of transversal thin folds between the 6 th and 7 th thoracic chaetigers in females indicate that M. kolhymensis sp. nov., is characterized by sexual reproduction, which is consistent with Rouse & Fitzhugh (1994) for most Manayunkia species. The questions of how, why, and for what reason males have neuropodial uncini in the 6 th –8 th chaetigers while females have transitional chaetae in the 6 th –7 th or 8 th chaetigers remain unanswered. Note, Pettibone (1953) indicated separate sexes for Manayunkia speciosa inhabiting Lake Erie, while Croskery (1978) thought that worms of this species found in the Ottawa River were hermaphroditic. Perhaps, within Manayunkia there are the gonochoristic (most described species), simultaneous hermaphrodite (Manayunkia athalassia), and presumably protandric hermaphrodite individuals (M. kolhymensis sp. nov.) like within the genus Ophryotrocha Claparède & Mecznikow, 1869 (Polychaeta; Dorvilleidae). The investigation of the life history and fitness in Ophryotrocha species has shown that the presence of the different forms of sexuality is probably correlated with the density of this species in natural habitats (Prevedelli, N’Siala & Simonini, 2006). The detailed studies of the development of gametes in adult individuals should be performed to resolve the question of hermaphroditism in the new species. Manayunkia kolhymensis sp. nov. differs slightly from the North American species M. occidentalis described from the Klamath River (California, USA), and from the type species M. speciosa from the Schuylkill River (Pennsylvania, USA). The new species has a longer peristomal ring than found in these species, and noticeable one transversal fold between the 5 th and 6 th thoracic chaetigers and several thin folds on the dorsal part of the 6 th –7 th thoracic chaetigers in females only. Manayunkia kolhymensis sp. nov., differs from M. occidentalis in the absence of four–five yellow-white spots on the ventral radiolar branch and a greater number of radiolar branches (22–32 versus 12). This species differs from M. speciosa Leidy, 1859 by a smaller number of radiolar branches (22–32 versus 36–40). The similar difference in the branches number (22–32 versus 36–40) is found between Manayunkia kolhymensis sp. nov. and M. zenkewitschii Sitnikova, Sherbakov & Kharchenko, 1997 from Lake Baikal, besides, M. zenkewitschii specimens have brown pigmented bodies up to 5 mm in length versus non-pigmented bodies less than 4 mm long in the new species. Manayunkia kolhymensis sp. nov. differs from other species found in Lake Baikal, M. godlewskii (Nusbaum, 1901) and M. baicalensis (Nusbaum, 1901), in size, as these species are significantly larger; they are up to 10 mm long, they have pointed pygidia, their radiolar crown contains a larger number of branches (from 40–60 to 104), and the females have two dorsal transverse folds (genital ridges formed the brood chamber), one between the 5 th and 6 th thoracic chaetigers and another between the 8 th thoracic and 1st abdominal chaetigers. The pygidium shape of Manayunkia kolhymensis sp. nov. is similar to M. baicalensis hydani Slastnikov, 1940, which is larger (up to 8 mm in length) and has two genital ridges. Molecular remarks. In the Bayesian tree (Fig. 6) based on COI sequences including four individuals from Serdyakhskie Lakes and one individual from the Namakanskie (Oymyakonskie) Lakes named as M. sp. ‘ kolymensis’ sp. nov., is similar to the Maximum Likelihood tree published by Atkinson et al. (2020) and Bick & Bastrop (2025). The worms from Serdyakhskie and Namakanskie Lakes differ in one substitution only. The new species forms a well-supported monophyletic clade that is the sister group to the clade of the North American M. occidentalis and M. speciosa. The bPTP and ASAP analyses also confirmed that M. kolhymensis sp. nov. is a separate species. The minimal uncorrected p -distance (13.8%) was revealed between M. kolhymensis sp. nov. and M. occidentalis (Table 1). Note, the smallest genetic distance, 8.3%, was found between Baikal species M. godlewskii and M. zenkewitschii, which are strictly different in morphology. The nucleotide sequenc

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