Eubranchus alexeii ( Martynov, 1998 )

Ekimova, Irina A., Grishina, Darya Yu. & Nikitenko, Ekaterina D., 2024, Nudibranch molluscs of Sakhalin Island, Northwestern Pacific: new records and descriptions of two new species, Ruthenica, Russian Malacological Journal 34 (2), pp. 69-91 : 82

Eubranchus alexeii ( Martynov, 1998)

( Fig. 8A, H )

Material studied: MIMB 48067, 1 specimen, dissected, Sea of Okhotsk, Aniva Bay, Moguchi River, Hirano ridge, 46°05’06.5”N, 142°12’39.4”E, 0.1–1 m in depth, on Zostera sp. with Obelia longissima colonies, 7 August 2023, coll. I. Ekimova, D. Grishina.

Diagnosis: Body elongate, up to 4 mm in length, yellowish semitransparent with reddish-brown round spots. Rhinophores and oral tentacles elongated, smooth. Cerata finger-shaped, bearing thickening in middle part with digestive gland bulging, arranged in 5 rows with 2–3 cerata per row. Jaws rectangular, with masticatory border bearing one row of small conical denticles. Radular formula 29 × 1.1.1. Triangular rachidian tooth, central cusp cylindrical, with pointed tip, 4–6 lateral denticles on each side of cusp, innermost denticles smallest. Lateral teeth two times wider than rachidian teeth, with rectangular base, triangular cusp. Penis absent, copulative apparatus formed by everting atrium with complex stylet system.

Molecular data: A BLAST-n search of COI sequences resulted in 99.17% identity to sequences of Eubranchus alexeii from GenBank thus confirming identification of the studied specimen.

Distribution: Eubranchus alexeii was described and further mentioned as an endemic species of the Peter the Great Bay, northern Sea of Japan [Martynov, 1998; Martynov et al., 2015b]. Subsequent works suggested it has wider geographic range at least in the Sea of Japan [ Chichvarkhin, 2016, as Eubranchus misakiensis Baba, 1960 ]. Its discovery in Aniva Bay support this view and extends the known range of this species to southern parts of the Sea of Okhotsk.

Figures

FIG. 8. External and internal morphology of studied representatives of the family Fionidae.A. Eubranchus alexeii, MIMB48067, living specimen. B. Eubranchus rupium, MIMB48068, living specimen. C. Eubranchus rupium, egg masses on Obelia longissima colonies.D. Cuthonella soboli, MIMB48071, living specimen.E. Cuthonella soboli, MIMB48073,living specimen. F, G. living specimen of Cuthonella soboli MIMB48071 (F) and its egg-mass (G) on Sertularia sp. colony. H. Eubranchus alexeii, MIMB48067, middle radular portion, SEM. I. Eubranchus rupium, MIMB48070, posterior radular portion, SEM. K. Cuthonella soboli, MIMB48073, anterior radular portion, SEM. Scale bars: H, K = 20 µm; I = 10 µm. Photo credits: A–C, E. Darya Grishina; D, F, G. Irina Ekimova. РИС. 8. ВнешнЯЯ и внутреннЯЯ морфологиЯ иЗученных представителей семейства Fionidae. A. Eubranchus alexeii, MIMB48067. B. Eubranchus rupium, MIMB48068. C. Eubranchus rupium, кладка на колонии Obelia longissima. D. Cuthonella soboli, MIMB48071. E. Cuthonella soboli, MIMB48073. F, G. Cuthonella soboli MIMB48071 (F) и ее кладка (G) на колонии Sertularia sp. H. Eubranchus alexeii, MIMB48067, среднЯЯ часть радулы, СЭМ. I. Eubranchus rupium, MIMB48070,ЗаднЯЯ часть радулы, СЭМ.K.Cuthonella soboli, MIMB48073, переднЯЯчасть радулы, СЭМ.Масштабные линейки: H, K = 20 µm; I = 10 µm. Авторство фотографий: A–C, E. ДарьЯ Гришина; D, F, G. Ирина Екимова.