Myja karin, Martynov, Alexander, Mehrotra, Rahul, Chavanich, Suchana, Nakano, Rie, Kashio, Sho, Lundin, Kennet, Picton, Bernard & Korshunova, Tatiana, 2019

Martynov, Alexander, Mehrotra, Rahul, Chavanich, Suchana, Nakano, Rie, Kashio, Sho, Lundin, Kennet, Picton, Bernard & Korshunova, Tatiana, 2019, The extraordinary genus Myja is not a tergipedid, but related to the Facelinidae s. str. with the addition of two new species from Japan (Mollusca, Nudibranchia), ZooKeys 818, pp. 89-116 : 89

Myja karin sp. n. Figs 2, 4B, 5

Type material.

Holotype, ZMMU Op-610, ca. 12 mm long (alive), Japan, Osezaki, 10 Sept 2016, depth 7-15 m, stones, rocks, hydroids, collector Tatiana Korshunova, Alexander Martynov. Paratype, ZMMU Op-611, Japan, Uchiura, 09 Sept 2016 depth 20 m, collector Hiroshi Takashige.

Type locality.

Japan.

Etymology.

In honour of Karin Fletcher (Port Orchard, Washington), who has made considerable recent efforts in uncovering hidden diversity and understanding of the nudibranch fauna of the NE Pacific.

Diagnosis.

Up to ten ceratal rows, ground colour translucent greyish, ceratal cores light to dark greyish, ceratal tops dull reddish, apices with white spot, anterior cerata with brownish basal spot, no sparse white spots in the first half of the dorsal part, white gonad spherules moderately dense, cerata moderately widened at top without smaller separate cupola-shaped tip, central tooth narrowly triangular with very sharp non-pitted top and numerous lateral denticles, up to 20-30 small irregular in size denticles, very distinct ridges and furrows on the teeth surface, no accessory penial gland, penis unarmed.

Description.

Body very elongate, holotype ca. 12 mm alive (Fig. 2 A–D). Rhinophores ca. 1.5 times longer than oral tentacles, smooth. Dorsal papillae cylindrical to spindle-shaped, forming nine or ten ceratal rows along dorsal edges. Apices of papillae form moderate oval swellings, without cupola-shaped appendage (Fig. 2E). Notal edge absent. Anal opening acleioproctic on right side before first posterior ceratal rows. Reproductive openings lateral, below first anterior and second posterior rows of cerata. Ground colour translucent greyish. Oral tentacles and rhinophores with scattered opaque white dots. On head after oral tentacles shines a small pinkish area, lateral sides of head with thin streaks of brown-orange pigment. Opaque white spots in anterior part of the body behind rhinophores absent. Between rhinophores shines a large brownish area. Digestive gland in the cerata (ceratal cores) whitish to light creamy and light greyish (basal parts can be very pale greenish), digestive gland in upper part of cerata with dull pinkish-brownish internal spot, apices mostly translucent with small white band at very tip. Anterior cerata with prominent brownish basal spot. A spot similar in colour, but duller brownish and smaller in size, may occur at basal part of other cerata. Central branches of digestive gland shine through dorsal part of body and are brownish with few greyish parts. Numerous, moderately dense, small, white gonads appeared as white spherules that shine through dorsal surface. Jaws broadly triangular with prominent anterior wings, masticatory borders smooth (Fig. 2I, J). Radula uniserial, very small compared to the pharynx internal volume (Fig. 2K). Radular formula 17 × 0.1.0. Central tooth narrowly triangular with very sharp top and up to ca. 20-30 (and probably more) small denticles, irregular in size (Fig. 2 L–N), often hard to delineate with very distinct dorsal denticle furrows and fine rib-like structures (Fig. 2M, N).

Reproductive system diaulic (Fig. 4B). Ampulla moderate in size, slightly widened in the middle (Fig. 4B, am). Vas deferens short, without distinct prostatic portion (Fig. 4B, vd), penial sheath widened (Fig. 4B, psh), penis unarmed, with at least two unequal elevations (Fig. 4B, p). Single proximal receptaculum seminis very large, elongated (Fig. 4B, rsp).

Biology.

Subtidal, on stony and rocky area with the hydroids Pennaria sp. (Fig. 2F). Egg mass is a long, convoluted ribbon (Fig. 2D). Veligers are planktonic, with turbospiral shell (Fig. 2G, H).

Distribution.

Central parts of the Pacific coast of the main Japanese island of Honshu; potentially can occur at least at the southern parts of Honshu and Kyushu.

Remarks.

The type species of the genus Myja , M. longicornis , is similar externally to Myja karin sp. n. by presence of brown anterior basal ceratal spots, bur readily distinguished by predominantly brownish-pinkish, and not green, main branches of digestive gland, and also by white to greyish rather than green ceratal cores (Fig. 2). Bergh (1896; see Fig. 1) also reported seven pairs of cerata for three large specimens (up to 15 mm alive, 9.5-10 mm fixed), whereas M. karin sp. n. of ca. 12 mm length alive has up to ten cerata (Fig. 2 A–C). Furthermore the radula of M. longicornis as depicted in Bergh (1896) has a sharp apical part (Fig. 1J), somewhat like in M. karin sp. n., but there are considerably fewer lateral denticles [6-7 on the figure in Bergh (1896), up to ten in the description in Bergh (1896)], compared to M. karin sp. n. with up to 20-30 lateral denticles at least (Fig. 2M, N). Myja cf. longicornis from Thailand differs from Myja karin sp. n. by its reddish and not brownish basal anterior ceratal spots and very considerably by the morphology of its radula (compare Fig. 1 F–H with Fig. 2 L–N). One more new species of the genus Myja , Myja hyotan sp. n. described below from Japanese waters, differs from Myja karin sp. n. by details of body colour, radular characteristics (see detailed remarks below and Table 2 for details), and according to molecular phylogenetic data (Fig. 5). Minimum uncorrected p-distances of the COI marker which separate M. karin sp. n. from M. cf. longicornis are 11.9%. Minimum uncorrected p-distances of the 16S marker which separate the M. karin sp. n. from M. cf. longicornis are 3.71% and from M. hyotan sp. n. are 4.41%. Minimum uncorrected p-distances of the H3 marker which separate M. karin sp. n. from M. cf. longicornis is 4.28% and from M. hyotan sp. n. is 3.98%. P-distances between the two specimens of M. karin sp. n. for the COI, 16S, and H3 markers are 0.5%, 0.7%, and 0% respectively.