Belknapchiton australis Sirenko, 2023, n. sp.

Belknapchiton australis n. sp.

(Figures 7C, D, 19–22)

Leptochiton cf. opiparus; Sirenko 2020: 426, figs. 32–35, 44A, B.

Belknapchiton sp.; Sirenko et al. 2022: 104.

Type material. Holotype (TMAG, E 45530), now disarticulated consisting of SEM stub of valves I, II, IV, V, VIII, part of perinotum and radula, mount of part of perinotum and vial with other valves.

Type locality. Australia, S Tasmania slope, 44.2012°S, 146.1992°E, 1050–1230 m.

Material examined. New Zealand, Kermadec Islands, NNE of Herald Islets, Raoul Island, 29°12.00’S, 177°49.30’W, R/ V Acheron, stn 1973312, 1188–1225 m, 1 spm (NMNZ M 222132) BL 16.0 mm, 05.04.1973; Australia, S Tasmania slope, 44.2012°S, 146.1992°E, 1050–1230 m Southern Surveyor stn 50 Lot 50, Sherman sled, CSIRO, holotype (TMAG, E 45527) BL 13.0 mm, 06.04.2007.

Etymology. From the Latin australis, south.

Distribution. Southern Ocean, Off Tasmania and Kermadec Islands, 1188–1225 m.

Diagnosis. Animal rather small, elevated, valves rather long, arched, not beaked, tail valve triangular, mucro postmedian, postmucronal slope straight; tegmentum sculptured by small triangular granules arranged in quincunx; each granule with one megalaesthete and 2–4 micraesthetes, surface between granules noticeably striated; dorsal scales sculptured with three to six fine, barely visible riblets; central teeth of radula wide, major lateral teeth with sharply pointed, unidentate head with small denticle-like appendix at inner edge; twelve gills per side.

Description. Holotype (BL 13.0 mm) elongate oval in outline. Valves arched, moderately elevated (elevation ratio 0.45 in valve V), not beaked. Tegmentum white.

Head valve semicircular, wider than tail valve, posterior margin widely V-shaped. Second valve oval, anterior and posterior margins convex. Other intermediate valves almost rectangular, lateral areas not raised, anterior margin slightly concave in central part, posterior margin weakly convex, lateral areas not raised. Tail valve triangular, mucro postmedian, antemucronal slope slightly convex, postmucronal slope straight.

Tegmentum uniformly sculptured with triangular granules arranged quincuncially, each granule with one megalaesthete and two–four micraesthetes. Surface between granules noticeably striated.

Articulamentum well developed for this genus, apophyses small, widely separated, subtriangular in valves II–VII, more or less trapezoidal in tail valve, jugal sinus twice as wide as apophyses.

Girdle narrow (0.5 mm near valve V), dorsally covered in elongate, slightly curved, pointed scales, sculptured with three to six fine, barely visible riblets (85–90 x 35–36 μm), marginal needles smooth, not numerous (160 x 15 μm). Ventral scales lanceolate, smooth (70 x 15 μm).

Radula 5.6 mm long with 29 transverse rows of mature teeth, somewhat assymmetrical. Central teeth wide with round, narrow blade, major lateral teeth with sharply pointed, unidentate head with small denticle-like apendix at inner edge.

Twelve gills per side extending from valve VII to anus.

Gut containing foraminifera and detritus.

Remarks. The second specimen of this species (Leptochiton cf. opiparus) collected off Kermadek Islands (identified as Leptochiton cf. opiparus by Sirenko 2020) closely resembles the holotype in the shape of the valves, the sculpture of the tegmentum, the weak development of the ribs on the dorsal scales and the general shape of the teeth of the radula, However, it differs in some minor details: in the shape of the valves, the sculpture of the tegmentum, the weak development of the ribs on the dorsal scales and the general shape of the radular teeth, the blade of the central radular teeth is straight, not curved, and the radula is symmetrical. However, I consider these differences as intraspecific variability. This is the second species of the genus Belknapchiton in the Southern Ocean.

The new species differs from B. opiparus in having weakly developed, thin riblets on the dorsal scales (vs 6–8 distinct ribs in B. opiparus), valve II is narrower than valve III (vs valve II widest in B. opiparus), smooth marginal needles (vs. part of the marginal needles with ribs in B. opiparus).