Myja cf. longicornis Bergh, 1896

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 cf. longicornis Bergh, 1896 Figs 1, 4

Material.

1 specimen, ZMMU Op-667, 6 mm long (fixed), Thailand, Koh Samaesan, 21 June 2018, depth 8 -16 m, soft sediment habitats, hydroids, collectors Rahul Mehrotra, Suchana Chavanich. 2 specimens, ZMMU Op-668, ca. 3 and 2 mm (fixed) same locality and collectors.

Locality.

Thailand, Chonburi, Koh Samaesan.

Diagnosis.

Up to eight ceratal rows, ground colour translucent greyish, ceratal cores light to dark greyish, ceratal tops dull reddish, apices with white spot, anterior cerata with prominent reddish basal spot (distributed over the whole surface in some cerata), white gonad spherules moderately dense, sparse white spots in the first half of the dorsal part, cerata moderately widened at top without smaller separate cupola-shaped tip, central tooth with sharp to pitted top and numerous lateral denticles, up to 23 small denticles, irregular in size, no distinct furrows and ridges on the teeth surfaces, no accessory penial gland, penis unarmed.

Description.

Body very elongate, up to 6 mm in preserved length (up to 10 mm alive) (Fig. 1A, B). Rhinophores similar in size to oral tentacles, smooth. Dorsal papillae cylindrical, forming eight ceratal rows along dorsal edges. Apices of papillae moderately to slightly widened, without cupola-shaped appendage (Fig. 1C). 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. Digestive gland in the cerata (ceratal cores) light to dark greyish, digestive gland in upper part of cerata with reddish internal spot, apices with white spot. Anterior cerata with prominent reddish basal spot in some cerata distributed over its whole surface. Central branches of digestive gland visible through dorsal part of body greyish. Numerous small, moderately dense white gonads appear as white spherules that shine through dorsal surface. Jaws broadly triangular with prominent anterior wings, masticatory borders smooth (Fig. 1D). Radula uniserial, very small compared to pharynx internal volume (Fig. 1E, indicated by an arrow). Radular formula 13 × 0.1.0. Central tooth narrowly triangular, with sharp to pitted top (Fig. 1F, G, H) and up to 23 denticles that are small and irregular in size, without distinct dorsal denticle furrows, only sometimes with fine rib-like structures (Fig. 1H).

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

Biology.

Subtidal, highly cryptic on Pennaria disticha hydroids in soft sediment habitats beyond the coral reef or on the same hydroids at the reef edge (Fig. 1A, B). Egg mass is a long narrow ribbon, white, laid directly onto host hydroids (Fig. 1B).

Distribution.

Presently found only at Koh Samaesan, Thailand, but expected to be found in neighbouring regions of the Indo-West Pacific.

Remarks.

Thai specimens show closeness to the type species of the genus Myja longicornis from Ambon (Indonesia) in such features as the apically widened cerata, only a single pair of anterior cerata, acleioproctic anus, winged jaws, and small uniserial radula. Therefore, studying these specimens allows us to reveal the phylogenetic and taxonomic position of the genus Myja via both morphological and molecular means. However, while M. cf. longicornis from Thailand is similar to the type species of the genus Myja , M. longicornis from the type locality in Ambon as described in the original description by Bergh (1896), there are differences in several external and internal characters which do not allow us to identify the Thai material as M. longicornis and Bergh’s figures are reproduced here (Fig. 1I, H). We therefore record here the specimens from Thailand as M. cf. longicornis . The distinguishing features of M. cf. longicornis are predominantly greyish without the green digestive gland branches both in the body and in the cerata, as was clearly indicated for M. longicornis in the original description (Bergh, 1896: 389, 390). It has a reddish and not brown-chocolate basal spot at anterior pair of cerata, and similar reddish (and not brown) pigment at ceratal apices. Furthermore, the radula of M. longicornis as depicted in Bergh (1896; reproduced here Fig. 1J) has more distinct lateral denticles, which are lower in number (10), compared to M. cf. longicornis (at least 23) (see Fig. 1H). We suspect that there is hidden species diversity in the genus Myja of the Indo-West Pacific. Specimens collected in 2016 reveal the presence of at least two more species of the genus Myja , which differ from M. cf. longicornis based on morphological and molecular data and from M. longicornis according to the morphological data, are described as new to science, Myja karin sp. n. (see Fig. 2) and Myja hyotan sp. n. (see Fig. 3). Minimum uncorrected p-distances of the COI marker which separate M. cf. longicornis from M. karin sp. n. are 11.9%. Minimum uncorrected p-distances of the 16S marker which separate M. cf. longicornis from M. karin sp. n. are 3.71% and from M. hyotan sp. n. are 2.55%. Minimum uncorrected p-distances of the H3 marker which separate M. cf. longicornis from M. karin sp. n. are 4.28% and from M. hyotan sp. n. are 3.36%, whereas p-distances between the two specimens of M. cf. longicornis for COI, 16S, and H3 markers are 0.2%, 0%, and 0% respectively.