Onchidoris expectata Martynov & Korshunova 2017, sp. nov.

Onchidoris expectata sp. nov.

(Figures 4; 5C)

http://zoobank.org/urn:lsid:zoobank.org:act:5305B972-7C4D-4903-AE5C-9CA75D30903F

Type Material. Holotype, ZMMU Op-593 (5 mm length), Kurile Islands, Matua Island, Cape Klyuv, 15 m depth, leg. N.P. Sanamyan, 26.08.2016. Paratypes, ZMMU Op-594 (6 mm length), 1 specimen, Kurile Islands, Matua Island, 15 m depth, leg. N.P. Sanamyan, 26.08.2016. ZMMU Op-595 (7 mm length), 1 specimen, NW Pacific near Kamchatka peninsula, Avachinskaya bay, 10 m depth, leg. N.P. Sanamyan, 16.09.2011.

Type locality. Matua Island, Middle Kurile Islands, NW Pacific.

Etymology. From the Latin expectatus which means both “expected” and “hoped for,” due to the expectation of the finding of new onchidoridid species in NW Pacific. The Russian word for “hope” is nadezhda which coincides with the first name of the collector of this species, Nadezhda Sanamyan.

Description. External morphology. The length of holotype is 5 mm, the width is 2.5 mm (Fig. 4 A, B). The lengths of two preserved paratypes are 6 and 7 mm, the widths 3 and 6.5 mm. The consistency of the living animals is hard. The notum is moderately broad, rounded in front and posteriorly. The rhinophores are long and retractable into sheaths with smooth edges, except for several tubercles of various sizes that are connected to the edges of the sheaths. The rhinophoral sheath margins are capable of some contraction in living specimens. There are 12–15 rhinophoral lamellae. The notum is densely covered with large club-shaped tubercles (Fig. 4 A). The larger tubercles dominate the notum, with some smaller tubercles scattered among large ones. The spicule rays that extend from the bases of tubercles form a dense network that shines through the notum surface. Each tubercle contains dense bundles of spicules, sometimes slightly protruding from the tubercle surface. The strongly calcified spicules are of various sizes. A gill cavity is absent. Inside the gill circlet are several narrow tubercles of differing heights. Seven to eleven uni- to bipinnate gills form a semicircle around the anus. The oral veil is semi-circular (Figs. 4 B). The foot is broad, anteriorly rounded and thickened, and posteriorly not projecting beyond the notum, forming a rounded tail (Fig. 4 B).

Colour. The living specimens are opaque whitish. A postbranchial gland is not evident, but typical for related species Onchidoris muricata.

Anatomy. Digestive system. The anterior part of the buccal bulb is modified to a prominent, broad, buccal pump that sits on a short, narrow stalk (Fig. 4 C, D). The buccal pump is fully banded by the broad peripheral muscle (Fig. 4 D). Lateral parts of the buccal pump are provided with thin muscular fibres. The rounded labial disk is covered by colourless cuticle without distinct labial elements (Fig. 4 F). The radular formula in two specimens (5–7 mm length) is 35– 38 x 1.1.1.1.1, radular teeth are almost colourless. The central tooth is distinct, relatively large, rectangular, and folded (Fig. 4 G, H). The first lateral tooth has a long, wide base and the strong, almost straight beak-shaped cusp bears 15–18 denticles (Fig. 4 H). The second lateral teeth are rectangular plates, with a downwardly directed cusp on its lower outside corner (Fig. 3 H). The stomach is relatively small and narrow (Fig. 4 E). The stomach caecum is absent.

Circulatory system. In the pericardial sac there is a wide triangular posterior auricle and a smaller sized and also triangular ventricle (Fig. 4 E). The massive blood gland forms a single piece that is located above the central nervous system and projects slightly anteriorly and posteriorly.

Central nervous system. The cerebral and pleural ganglia are well separated. The optic nerve is very short. The eyes had black pigment in all specimens studied. The pedal ganglia are smaller than the cerebrals, lay below them, and are connected to them by very short connectives. The rhinophoral ganglia are globular to elongate. The buccal ganglia are slightly oval. Gastro-oesophageal ganglia are present. Five pairs of cerebral nerves, two pleural, and three pedal ones are detected.

Reproductive system. (Figs. 4 E, 5C). The ampulla is narrow (Fig. 5 C, a). The prostatic part of the vas deferens is a relatively long loop adjacent to, but not encircling, the bursa copulatrix (Fig. 5 C, pr). The prostate is relatively broad, not granulated; it rapidly widens into a long swollen penial sheath and contains several folds of the ejaculatory duct (Figs. 4 E, 5C, psh). The elongate bursa copulatrix (Fig. 5 C, b) enters the proximal part of the vagina via a narrow, relatively long stalk. At its base, a duct with a similar diameter as the vagina exits the vagina and leads to the ovoid seminal receptacle (Fig. 5 C, rs). The vagina is narrow and long (Fig. 5 C, v).

Biology. Specimens were found on vertical rock and also on brown algae covered with several species of encrusting species of Bryozoa, at 6–15 m depth.

Distribution. Presently known from the Pacific Kamchatka and northern Kurile Islands.

Remarks. Onchidoris expectata sp. nov. differs externally from the sympatric O. muricata (Müller, 1776) and O. macropompa Martynov et al., 2009 by its distinct club-shaped dorsal tubercles. From O. macropompa O. expectata sp. nov. differs in having a completely smooth and non-denticulate first lateral tooth. The initial genetic data support the morphology-based results. Genetic p-distances also distinguish O. expectata sp. nov. from O. muricata despite a similar radula. Minimal uncorrected p-distances for the COI gene are very different between O. expectata sp. nov. and O. muricata, being 12.52 ± 1.3%. Intraspecific distances within O. expectata sp. nov. are within 0.91%. Intraspecific distances within O. muricata are up to 2.08% but interspecific distances between O. expectata sp. nov. and O. muricata reach 12.58%. Results obtained by PopART showed a network of haplotypes that clustered into two groups coincident with O. expectata sp. nov. and O.muricata (Fig. 6 B).