Boreochiton jakovlevae Sirenko sp. nov.

Figs 1–5

urn:lsid:zoobank.org:act: F2774A7F-6B10-4F31-83BB-1FC1545DDC56

Tonicella sp.: Sirenko et al., 1987, p. 216.

Boreochiton sp.: Sirenko, 2000, p. 72; Sirenko 2013, p. 149.

Type material. The holotype (ZISP no. 1916), now disarticulated, consisting of mount of part of girdle and radula, vail with valves, part of radula and part of girdle, and 13 paratypes (ZISP nos. 2268, 2269, 2270, 2271).

Type locality. The Sea of Okhotsk, Iona Isl., 56°24′ N, 143°23′ E, depth 20 m, on rocks.

Etymology. After A.M. Jakovleva, who prepared the first guidebook for identification of chitons from Russian seas.

Material examined. The Sea of Okhotsk, 0.5 nautical mile from Iona Isl., depth 21.7 m, trawl, stones, 3 paratypes (ZISP no. 2271), BL 15.0–18.0 mm, 09.06.1914, leg. G. R. Meder; off Iona Isl., depth 13 m, SCUBA, st. 94, stones, rocks, on rhizoids of Laminaria sp., 1 specimen, BL 32 mm, 21.06.1978, leg. A. Sheremetevsky; depth 15 m, SCUBA, st. 95, stones, rocks, 6 paratypes (ZISP no. 2270), BL 14–33 mm, 21.06.1978, leg. A. Sheremetevsky; depth 20 m, SCUBA, st. 96, stones, rocks, holotype (ZISP no. 1916), 3 paratypes (ZISP no. 2268), BL 4.5–34 mm, 21.06.1978, leg. A. Sheremetevsky; depth 20 m, SCUBA, st. 97, stones, rocks, 1 specimen, BL 26 mm, 21.06.1978, leg. A. Sheremetevsky; depth 43 m, Okean bottom sampler, st. 85, stones, 1 paratype (ZISP no. 2269), BL 3.5 mm, 21.06.1978, leg. V. Koblikov; depth 50 m, Okean bottom sampler, st. 148, stones, 1 specimen, BL 15 mm, 31.07.1978, leg. V. Koblikov.

Distribution. Near Iona Isl., the Sea of Okhotsk, in a depth range of depth 13–50 m, on stones, rocks and rarely on rhizoids of Laminaria sp.

Diagnosis. Animal of moderate size, attaining a length of 34 mm, oval. Shell very low, round-backed; tegmentum smooth with growth lines; jugal sinus very wide. Girdle wide about half of valve V, looking smooth, leathery. Dorsal spicules small partly embedded into cuticle, ventral spicules with 2–3 ribs in their upper half. Height of head of major lateral tooth of radula equal or larger than its width.

Description. The holotype, 27.0x 21.4 mm, oval, low elevated (dorsal elevation: 0.24), shell rounded, side slopes slightly convex, valves somewhat beak. Tegmentum fulvous, marmorated.

Head valve semicircular, front slope slightly convex. Intermediate valves rectangular. Valve II appreciably narrower than valve V, with convex front margin. Valve V with nearly straight front margin; side margin little rounded; apices neatly indicated; lateral areas not raised. Tail valve narrower than head valve, elliptical in outline; mucro a little behind the center; posterior slope convex.

Tegmentum smooth to the naked eye, several growth lines clearly indicated.

Articulamentum strongly developed, rose color in most the surface of valves except white insertion plates; apophyses wide, broadly triangular with rounded top, more or less trapezoid in the tail valve, separated by a rather wide jugal sinus (ratio of width of apophyses to width of jugal sinus 1.1). Slit formula 9/1/10; teeth sharp, smooth; slit rays slightly indicated.

Girdle of the fixed holotype looking smooth and leathery, rather wide (width of girdle near valve V about half of width of the valve), dorsally not densely covered with small fulvous elongate oval spicules (51–60x17–20 µm) partly embedded into cuticle, short chitinous bristles (55 µm) with small spicule on its top scattered among spicules. Short marginal needles (cc. 100 µm) with 3–5 shallow grooves arranged diagonally on both their sides. Ventrally small sharply pointed spicules (50–52x18–21 µm) with 2–3 ribs on their upper half. There is one row of large, flattened smooth spicules (80x30 µm) near outer margin.

Radula of the holotype 9.0 mm long, with 49 transverse rows of mature teeth, central tooth long, major lateral tooth with high tricuspid head, outer denticle noticeably broader than others (ratio of height of head to its width 1.16), major uncinal tooth with wide broom-like top.

The holotype with 24 gills on both sides, extending from valve III to valve VII.

Gut contents containing red crustose calcareous algae (90–95%), diatoms, detritus and sand.

Remarks. The examination of the paratypes showed some variability. Most paratypes have similar marmoreous color of tegmentum, but one paratype has monochromatic fulvous color. Size of denticles of head of major lateral tooth of radula undergo age variability: outer denticle is noticeably enlarged and ratio of height of head to its width is increased with age. The ratio of length of antemucronal area to length of postmucronal area in holotype is 1.1, whereas the ratio in four paratypes (BL 14, 18, 18, 32 mm) varied from 0.7 to 1.0.

B. jakovlevae differs from other species of genus in having higher head of major lateral tooth of radula, small and scattered dorsal spicules, wider jugal sinus and marmorated color of tegmentum. New species differs from B. granulatus also in having smooth tegmentum (vs. granulated in B. granulatus). The marmorated color of tegmentum, low valves and wide girdle of B. jakovlevae are similar to those of Tonicella submarmorea Middendorff, 1847, but the new species easily differs from the latter one by having fenestral gland.