Myja hyotan, 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 hyotan sp. n. Figs 3, 4C, 5

Eubranchus sp. 7 Nakano, 2004: 244.

Type material.

Holotype, ZMMU Op-612, ca. 20 mm long alive, Japan, Osezaki, 10 Sept 2016, depth 7-15 m, stones, rocks, hydroids, collector Tatiana Korshunova, Alexander Martynov.

Type locality.

Japan, Osezaki.

Etymology.

After the Japanese name hyōtan (瓢箪, ヒョウタン) for the calabash Lagenaria siceraria , the fruits of which are very similar in shape to the peculiar cupola-shaped tip of cerata of this new Myja species.

Diagnosis.

Up to eight ceratal rows, ground colour translucent greyish, ceratal cores white to dark greyish, ceratal tops dull reddish, no apical white spot, anterior cerata with prominent dark brownish basal spot, sparse white spots in the first half of the dorsal part, white gonad spherules very dense, cerata considerably widened at top with smaller separate cupola-shaped tip, central tooth narrowly triangular with largely non-pitted top and only few denticles, up to ten small denticles, irregular in size; no accessory penial gland, penis unarmed.

Description.

Body very elongate, holotype ca. 20 mm (alive, Fig. 3 A–C). Rhinophores up to ca. two times longer than oral tentacles, smooth. Dorsal papillae cylindrical and then rapidly widened at the top, forming up to eight ceratal rows along dorsal edges. Apices of papillae considerably widened with smaller separate cupola-shaped tip appendage (Fig. 3D). 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, but because of presence of numerous, very densely placed gonad spherules, appears as rather opaque white. Oral tentacles and rhinophores with few scattered opaque white dots. On head after oral tentacles to in between of rhinophores shines a pinkish area, lateral sides of head without thin streaks of brown-orange pigment. Opaque white in anterior part of the body after rhinophores. Between rhinophores shines a large brownish area. Digestive gland in the cerata (ceratal cores) whitish to light creamy, digestive gland in upper part of cerata with dark brownish internal spot, apices mostly translucent, without small white band at very tip. Anterior cerata with prominent dark brownish basal spot. A spot similar in colour, but duller brown and smaller in size, may occur at basal part of other cerata. Central branches of digestive gland shining through dorsal part of body are dark brownish. Numerous, very dense small white gonads appear as white spherules that shine through dorsal surface and create a rather opaque white dorsal appearance. Jaws broadly triangular with prominent anterior wings, masticatory borders smooth (Fig. 3F, G). Radula uniserial, very small compared to internal volume of the pharynx (Fig. 3H). Radular formula 15 × 0.1.0. Central tooth narrowly triangular with sharp or rarely pitted top and up to ca. ten (often no more than five) relatively distinct small denticles in anterior part of radula (Fig. 3 I–K) to completely smooth or with very indistinct denticles in posterior part of radula (Fig. 3K). Few teeth in posterior part of radula may have pitted top (Fig. 3K)

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

Biology.

Subtidal, on stony and rocky area with hydroids Pennaria sp. (Fig. 3E). No data on egg mass so far.

Distribution.

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

Remarks.

The type species of the genus Myja , M. longicornis is somewhat similar externally to Myja hyotan sp. n. by presence of brown anterior basal ceratal spot, but it is readily distinguished by dark brown and not green main branches of digestive gland, and also by the white rather than green ceratal cores. Another notable difference between the type species and all other species described here from M. hyotan sp. n. is the very densely placed white spherules of the gonad that shine through the dorsal body and appear as opaque white in M. hyotan . The shape of the cerata in M. hyotan sp. n. also readily differentiates it from M. longicornis , M. cf. longicornis , and M. karin sp. n. with the presence of an additional, separate, cupola-shaped top chamber in the ceratal apices (Fig. 3D). Furthermore, the radula of M. longicornis as depicted in Bergh (1896; reproduced here Fig. 1J) has a sharp apical part, somewhat similar to that of M. hyotan sp. n., but the denticles in M. longicornis are much more distinct, compared to M. hyotan sp. n., in which in most of the radula (except few anterior most teeth) has lateral denticles either absent or very indistinct (Fig. 3I, K). Myja cf. longicornis differs from M. hyotan sp. n. by the reddish and not brownish basal anterior ceratal spot and very considerably by the morphology of radula (compare Fig. 1 F–H and Fig. 3 I–K). Myja hyotan sp. n. differs from the other new species of the genus Myja from Japan, M. karin sp. n., by the shape of the cerata (including cupola-shaped separate tip), very dense white spherules of gonads, presence of white spots on the dorsal part behind the rhinophores, by radular characteristics ( M. hyotan sp. n. fully devoid of peculiar furrows and ridges on the teeth as present in M. karin sp. n., and many teeth of M. hyotan sp. n. almost smooth, without denticles) (see Table 2 for details) and according to the molecular phylogenetic data (Fig. 5). Minimum uncorrected p-distances of the 16S marker which separate the M. hyotan sp. n. from M. cf. longicornis is 2.55% and from M. karin sp. n. is 4.41%. Minimum uncorrected p-distances of the H3 marker which separate M. hyotan sp. n. from M. cf. longicornis is 3.36% and from M. karin sp. n. is 3.98%.