Ziminella japonica (Volodchenko, 1941)

Ziminella japonica (Volodchenko, 1941) View in CoL

( Figs 3 F, G View FIG , 6 View FIG , 9 A–I, L, M View FIG )

Coryphella japonica Volodchenko, 1941b: 57 , pl. 3.3, pl. 4.1; Martynov, 1998: 207; Martynov, 2006: 284–285, pl. 13: 6 A, B; Martynov, 2013: 112-114 ( partim), pl. 1.11, 1.12, pl. 2.1–2.8, pl. 5.1–8.

Coryphella stimpsoni – Roginskaya, 1878: 169–177, partim (not of Verrill, 1879).

Type material. Lectotype (dissected): ZIN24615 View Materials , Sea of Japan, off Askold Is., 120 m depth, coll. Derzhavin, 25.06.1928 (designated by Martynov [2013]) . Paralectotype ZIN63758 View Materials , dissected, same locality, depth and collector as in lectotype .

Additional material studied: Sea of Japan, R / V “ Akademic Oparin ”, st. 80, 43°18.7’N 135°10.0’E, 219 m depth, coll. O. V. Chichvarkhina, 09.07.2021, MIMB50759 View Materials (2 spm). Sea of Japan, R / V “ Akademic Oparin ”, st. 56, 44°58.3’N 136°53.1’E, 120 m depth, coll. O. V. Chichvarkhina, 04.07.2021, MIMB50760 View Materials (1 spm). Sea of Japan, R / V “ Krasny Yakut ”, st. 119/4, 45°05.3’N 136°56’E, 115 m depth, coll. P. V. Ushakov, 10.07.1930, ZIN63756 View Materials (1 spm). Sea of Japan, R / V “ Leitenant Dydymov ”, st. 431, 48°08’30.0”N 140°08’30.0”E, 122 m depth, coll. Soldatov, 15.09.1913, ZIN63759 View Materials (1 spm) GoogleMaps .

Type locality. Sea of Japan, off Askold Is., 120 m depth .

Description (based on studied specimens). External morphology: Body length up to 14 mm (preserved). Body wide. Foot wide, anterior corners short, rounded. Notal edge well-developed, continuous, elevations not extended above lateral body sides. Oral tentacles short, wide, conical. Rhinophores rugose, same size as oral tentacles, thinner than oral tentacles. Cerata in distinct rows, attached directly to well-defined notal edge. Cerata fusiform to cylindrical, lateral cerata much smaller than dorsal. Digestive gland diverticula filling from 1/3 to 3/4 of ceratal volume. Anal opening on right side right below notal edge, at beginning of posterior half of body. Reproductive opening on right side on anterior part of body.

Coloration: Background body color pinkishwhite. Dorsal side of body lacking cerata, oral tentacles and rhinophores covered with extensive orange to reddish pigment, which becoming paler to peachy on tips of cerata and oral tentacles. Cerata orange-tan to deep red-brown, distal tip with white ring.

Internal morphology (based on studied specimens): Jaws massive, oval-triangular with well-developed masticatory border bearing several rows of low blunt denticles or elevations. Radular formula: 17–28 × 1.1.1. Rachidian teeth massive, triangular. Central cusp strong, conical, elongated, non-compressed by adjacent denticles. From 6 to 11 small sharp and slightly curved denticles on each side of cusp, commonly from 7 to 9, denticles of about same size. Denticles may be ramified to 2–4 smaller denticles. Lateral teeth triangular plates, denticulation always present on first half. From 7 to 19 denticles on inner side. Reproductive system diaulic.Ampulla long, narrow and convoluted. Vas deferens extremely long with slightly expanded prostatic part. Penis conical or slightly folded. Seminal receptacle absent.

Distribution. The Sea of Japan from 115 to 528 m in depth [ Martynov, 2013; Korshunova et al., 2017; present study]. Specimens with similar morphology are also known from the Sea of Okhotsk, the Southern Kuril Islands and from the British Columbia from depth of 20 m to 560 m (this study), but molecular data are needed for confirmation of their species identity (see below).

Remarks. The type material ( lectotype ZIN24615) of Z. japonica was dissected and no slides of the radula and jaws of these specimens was available for study. At the same time, the original description provides the illustrations of the radular morphology [ Volodchenko, 1941b: pl. 4-1], which corroborates well with our morphological investigations. The external morphology of specimens collected from adjacent areas to the type locality and at the same depths (minimum distance ~ 250 km) corroborates with the initial description and the external morphology of the type material (see Volodchenko [1941b]: pl. 3.3, and Martynov et al. [2013]: pl.1.11, 1.12). Ziminella japonica shows several consistent differences with Z. abyssa and Z. salmonacea . Accordingly, Z. abyssa has more denticles on the rachidian teeth and shorter central cusps, and its lateral teeth are smooth and narrow ( Fig. 7), while in Z. japonica laterals are triangular and bear small sharp denticles at first half of teeth ( Fig. 9 D, G–I View FIG ). Ziminella japonica differs from the Arctic-Atlantic species Z. salmonacea by the shape of the rachidian teeth (elongate-triangular in Z. salmonacea , while in Z. japonica they are clearly wider). Also, they show differences in the shape of lateral teeth (with narrow bases in Z. salmonacea , wide base in Z. japonica ; in the former species lateral teeth are also slightly curved). The molecular analyses showed that Z. japonica from the Sea of Japan forms a separate clade, which is distinct from all other species of the genus ( Fig. 2B View FIG ). Minimal interspecific differences were identified between Z. japonica and the North- Pacific bathyal species Z. vrijenhoeki ( Table 1). Both these species are very similar morphologically. It was suggested that they differ in coloration [ Valdés et al., 2018]. Although Valdés et al. [2018] compared Z. vrijenhoeki with specimens from British Columbia, which possess cream to pinkish coloration with white or beige cerata [ Behrens, Hermosillo, 2005; Calder et al., 2015], true Ziminella japonica from the Sea of Japan has orange to reddish dorsum and dark rhinophores and cerata ( Fig. 3 F, G View FIG ). This coloration is different from both Z. vrijenhoeki and specimens of “ Z. japonica ” from British Columbia [ Calder et al., 2015; Valdés et al., 2018]. There were also reports on different penial morphology of these species, as Z. vrijenhoeki has a folded penis, while it is conical in Z. japonica [ Valdés et al., 2018]. Our results do not confirm this, since specimens from British Columbia have a folded penis ( Fig. 3H View FIG ), and in the North-West Pacific Z. japonica the penis may have a different shape, and we suggest this is likely a result of tissue contraction during fixation. Although slight differences may be identified between radulae of these species, Z. japonica shows considerable variation in radular characters across the putative geographic range ( Fig. 9 View FIG ). For instance, we have found a specimen (ZIN63759, the Sea of Japan) in which denticles on the rachidian teeth were highly branched, making the denticles to be brush-like ( Figs 9 E–H View FIG ).

We have also studied several specimens from the Sea of Okhotsk and the Kuril Islands (ZIN63757, ZIN63763, ZIN63764, MIMB15111, see Table S1), which we initially identified as Z. japonica .Although most of them fit the diagnostic features of Z. japonica , a slight variation may be observed in external morphology (body length and shape, the extension of the notal edge). Also, some of these specimens show differences in radular morphology from true Z. japonica . For example, in specimen ZIN63764 the rachidian teeth closely resembles those of Z. abyssa (more than 15 denticles and short cusp), but the lateral teeth were similar to those of Z. salmonacea ( Fig. 9J View FIG ). Externally, the specimen ZIN63764 has short extensions of notal edge, typical of Z. japonica .

To sum up, although minor differences were found between Z. japonica and Z. vrijenhoeki based on the material from the type locality, the species status of Z. vrijenhoeki needs additional verification. We also cannot precisely identify several specimens collected outside the type localities, i.e. specimens from the Sea of Okhotsk, the Kuril Islands, and British Columbia, because no molecular data are available and the morphology and coloration of the living animals is unknown in most cases. Considering the slight variation of radular characters and coloration (e.g., creamy white specimens from British Columbia), we suggest Z. japonica could be either a species complex, or there is a single species with considerable intraspecific morphological and genetic variability as a result of wide geographical and bathymetrical distribution.