Kaasichiton kamchatkensis Sirenko, 2024, n. sp.

Kaasichiton kamchatkensis n. sp.

(Figures 16–22)

Leptochiton lukini Sirenko, 1990: 97; 2013: 148. (in part, non L. lukini Sirenko, 1990)

Leptochiton (Leptochiton) lukini; Kaas & Van Belle 1994 (in part, non L. lukini Sirenko, 1990)

Type material. Holotype (ZIN 2464) and 3 paratypes (ZIN 2465, 2466, 2467) now disarticulated consisting of SEM stub of valves I, II, V, VIII, part of perinotum and radula, mount of part of perinotum and radula and vial with other valves. Some of the material turned out to have severely damaged shell valves and girdle scales due to primary fixation with formalin, which did not allow us to obtain high-quality photographs on a SEM.

Type locality. Bering Sea, Kamchatka Peninsula, Olutorskiy Bay, Yuznaya Glubokaya Bight, 116 m.

Etymology. Named after the Kamchatka Peninsula.

Material examined. Four specimens in total. R/ V Raduga, Kamchatka Peninsula, Olutorskiy Bay, Yuznaya Glubokaya Bight, 116 m, Sigsby trawl, holotype (ZIN 2464), BL 4.0 mm, paratype (ZIN 2465), BL 5.5 mm, 25.07.1975; Karaginskiy Island, eastern coast, south of the Promezutochniy Cape, 140 m, sandy silt with stones, grab, paratype (ZIN 2466), BL 3.7 mm. 02.08.1975; R/ V Academik Shirshov, 27 th cruise, st. 11, south of the Providenia Bay, 63°59′6’’N, 172°29′0’’W, 59 m, mud with stones, paratype (ZIN 2467), BL 4.5 mm, 16.06.1981.

Distribution. Bering Sea, Asian coast, 59–140 m

Diagnosis. Animal small. Shell lowly elevated, subcarinated, intermediate valves almost rectangular. Tail valve little narrower than the head valve, mucro wide, posterior, antemucronal slope convex. Tegmentum smooth to the naked eye, without granules, each element of microsculpture with one aesthete pore. Girdle very narrow, dorsal scales elongate, bluntly pointed with vague riblets in upper part, ventral scales elongate oval, smooth. Central teeth of radula narrow with slightly expanded blade in upper part and sharply pointed in lower part, first lateral teeth with same length as central teeth, wing-shaped with very short blade at top, major lateral teeth with narrow bidentate cusp, inner denticle long, obtusely pointed, outer denticle very small but framed.

Description. Holotype 4.0 x 2.1 mm. Shell elongate oval, lowly elevated (dorsal elevation ratio of valve V 0.39). Valves thin, subcarinated, not beaked, lateral slopes convex, lateral areas not raised, color of tegmentum white, most of the surface of tegmentum and girdle except narrow band in the anterior edge of intermediate and tail valves covered with dirty grey sedimentary material.

Head valve semicircular, hind margin V-shaped. Intermediate valves almost rectangular, anterior margin slightly convex, posterior margins nearly straight, lateral margins rounded. Tail valve little wider than head valve, with posterior, slightly damaged mucro, ratio of width of head valve to width of tail valve 1.09, antemucronal area convex, postmucronal slope straight, ratio of length of antemucronal area to length of postmucronal area 1.19.

Tegmentum smooth to the naked eye, no granules visible; under high magnification, microscopic sculpture of tegmenum in form of thin irregular longitudinal grooves interrupted by similarly thin growth grooves visible on narrow strips of anterior margin of intermediate and tail valves, without deposit cover; single aesthete only.

Articulamentum weekly developed, apophyses short, narrow, width of apophyses about 1.6 times less than width of jugal sinus. Pores arranged in random manner. Girdle narrow of articulamntum about 9 times wider than girdle of valve V, dorsally covered with elongate, bluntly pointed scales (66–118 × 20 μm) with vague riblets in upper part. Scattered among them strong, smooth or ribbed needles (100–119 × 17 μm) are found, those near sutures similar. Marginal needles of same shape as sutural needles but little shorter (75 × 17 μm). Ventrally girdle covered with elongate sharply pointed scales (50–62 × 20 μm).

Holotype has 5 gills per side, arranged from valve VI to the anus.

Radula of holotype 1.7 mm long with 84 transverse rows of mature teeth. Central teeth of radula, narrow with slightly expanded blade in upper part and sharply pointed in lower part, first lateral teeth of same length as central teeth, wing-shaped with very short blade at top, major lateral teeth with narrow bidentate cusp, inner denticle long, obtuse pointed, outer denticle very small but framed.

Remarks. On all four available specimens, almost the entire surface of the tegmentum of valves is covered by a thin layer of deposit, which is not destroyed even by boiling in KOH. Only on the narrow strips at the front edge of the valves II–VIII, the sculpture of tegmentum can be seen.

The new species differs from other members of the genus by having a smooth tegmentum without any granules visible to the naked eye (vs granules present in K. lukini, K. incubatus and K. ionensis n. sp.) and its central and first lateral teeth of radula have the same length (vs first lateral tooth longer than central one in the three other species)

A specimen of this species (ZIN 2467) with a body length of 4.0 mm, collected in June in the Bering Sea and previously identified as K. lukini (Sirenko 1990) had a 700 µm long juvenile chiton in the pallial groove with 8 formed valves.