Leptochiton longibranchiae Sirenko, 2015, n. sp.

Leptochiton longibranchiae n. sp.

(Figs 8 D, 23–26)

Leptochiton sp. Schwabe & Sellanes 2010: 36 –37, Figs E–I.

Types. Holotype (ZISP 2212), now disarticulated consisting of SEM stub of valves I, V, VI, VIII, part of perinotum and radula, mount of part of perinotum and radula and vial with other valves, and 15 paratypes (ZISP 2213).

Type locality. Peru-Chile Trench, 12°29’8’’S, 77°32’1’’W, 560 m, (RV Akademik Kurchatov, cruise 4, st. 274).

Etymology. The name is Latin for long line of gills. This species has very extensive row of ctenidia which is unusual for the genus Leptochiton.

Material examined. Peru-Chile Trench, RV Akademik Kurchatov, cruise 4, st. 274, 12°29’8’’S, 77°32’1’’W, 560 m, holotype, BL— 8.5 mm and 15 paratypes BL— 8–15 mm; Peru, W of Hucho Lima dept. 11°05’S, 78°19.9’W, 400 m, 1 spm (BL— 25 mm), 15.05.971, examined by author in LACM.

Distribution. Peru and Chile from 11°05’S to 22°51’S, 294–560 m

Diagnosis. Animal of small size. Valves lowly elevated, carinated. Tail valve wider than head valve, mucro subcentral. Tegmentum with distinct oval granules arranged in longitudinal rows on central area of intermediate valves and antemucronal area of tail valve and quincuncially in other areas. Each granule with one megalaesthete and eight surrounding micraesthetes. Dorsal scales elongate, sharply pointed, with one rib. Central teeth of radula rectangular, major lateral teeth with short unidentate distally narrowed cusp. Eighteen gills per side from valve III to anus.

Description. Holotype BL— 8.5 mm. Shell widening to tail (animal ovoid in outline). Valves carinated, low (elevation ratio 0.34 in valve V), not beaked. Color of tegmentum white. Head valve semicircular, noticeably narrower than tail valve (ratio of width of tail valve to width of head valve 1.2). Intermediate valves rectangular, short and wide, anterior and posterior margins nearly straight, lateral margins rounded, not beaked, lateral areas not raised. Tail valve large with subcentral mucro, antemucronal slope straight, postmucronal slope convex.

Tegmentum sculptured with distinct oval granules arranged in longitudinal chains in central areas of intermediate valves and in quincuncial pattern in other areas. Each granule with one megalaesthete and eight micraesthetes around megalaesthete, aesthete pores of megalaesthete (10 µm) a little larger than pores of micraesthetes (7–8 µm).

Articulamentum moderately developed, apophyses short and wide (width of apophyses more than width of jugal sinus).

Girdle relatively narrow (about 0.5 mm, near valve V, 9.3% of total width), dorsally covered with elongate sharply pointed scales (77– 78 x 18–20 µm) with one rib. Intersegmental area with long needles (140 x 12 µm). Marginal needles shorter than sutural ones (114 x 8 µm). Girdle covered ventrally with elongate, smooth, sharply pointed scales (60 x 16 µm).

Radula of holotype 4.4 mm long with 33 transverse rows of mature teeth. Central teeth rectangular, major lateral teeth with short unidentate, distally narrowed cusp.

Eighteen gills per side arranged from valve III to anus.

Remarks. The paratypes have different number of gills depending on body size: smallest paratype (BL— 8 mm) has 16 gills whereas the largest paratype (BL— 15mm) has 22 gills per side. Two more specimens: LASM, Peru, W of Hucho Lima dept., 11°05’S, 78°19.9’W, 400 m, 1 spm, (BL— 25 mm) (studied by author) and ZSM Mol 20070757, Chile off Mejillones, 22°51.99’S, 70°29.40’W, 294 m, 1 spm, (BL— 11 mm) (Schwabe & Sellanes, 2010) have 23 and 21 gills per side respectively, arranged from valve III to anus.

This new species is easily distinguishable from many other species of the genus Leptochiton by the long line of its gills which reach to valve III, the large tail valve and the unidentate cusp of major lateral tooth. It is most similar to L. halistreptus which has an extended row of gills (24 gills arranged from valve V to anus) and a wide tail valve and it differs in having longitudinal rows on central areas of intermediate valves (quincuncially in L. halistreptus), the number of aesthetes pores (9 in L. longibranchiae and 5 in L. halistreptus), the distally narrowed cusp of major lateral teeth and in having one rib on dorsal scales (2 in L. halistreptus). From Leptochiton tenuidontus Saito & Okutani, 1990 which also has an extended row of gills L. longibranchiae differs in the number of ribs on the dorsal scales (1 in L. longibranchiae and 7–8 in L. tenuidontus), in having carinated valves (rounded in L. tenuidontus) and a distally narrowed cusp of the major lateral teeth of the radula.