Belknapchiton giganteus ( Nierstrasz, 1905 )

Sirenko, B. I., 2023, Indonesian chitons of the genera Belknapchiton Sirenko, Saito et Schwabe, 2022 and Leptochiton Gray, 1847 (Mollusca: Polyplacophora: Leptochitonidae), Ruthenica, Russian Malacological Journal 33 (2), pp. 79-94 : 80-81

Belknapchiton giganteus ( Nierstrasz, 1905)

( Figs 1–9 )

Lepidopleurus giganteus Nierstrasz, 1905: 3 , pl. 1, figs 1, 2, pl. 2, figs 39–43.

Leptochiton alveolus . – Ferreira, 1979: 157, figs 15, 16, not Chiton alveolus Lovén, 1846 .

Leptochiton (Leptochiton) alveolus . – Kaas, Van Belle, 1985: 36, not C. alveolus Lovén, 1846 .

Leptochiton (Leptochiton) belknapi . – Kaas, Van Belle, 1987: 24, not C. belknapi Dall, 1878 .

Leptochiton giganteus . – Sirenko, 2015: 154.

Belknapchiton giganteus . – Sirenko et al., 2022: 103.

Type material. Lectotype ( ZMA.MOLL.138622), and 2 paralectotypes ( ZMA.MOLL.138611), desig - nated by Ferreira [1979].

Type locality. Indonesia, the Banda Sea , 6°24’S, 124°39’E, 2798 m .

Material examined. Lectotype ( BL 42 mm) and 2 paralectotypes ( BL 25 and 33 mm). Indonesia, Banda Sea, R / V Vitjaz, cruise 54, stn. 6778,

5°15’S, 128°25’E, 3200 m, 4 spms, BL 5.0–26.0 mm, 11.04.1973 .

Distribution. The species is only known from two stations in the Banda Sea, Indonesia, 2798–3200 m.

Revised diagnosis. Animal elevated, intermediate valves rather wide, carinated, not beaked, tail valve triangular, mucro postmedian, postmucronal slope convex; tegmentum sculptured by small elon- gate granules arranged in quincunx; each granule with one megalaesthete and 8 micraesthetes; dorsal scales without ribs or rarely with 1–3 obsolete longitudinal riblets; central tooth of radula wide, major lateral teeth with sharply pointed, unidentate head, 13–14 gills per side.

Amended description. Chiton large ( BL of the lectotype 42 mm) elongate oval in outline. Valves carinated, moderately elevated (elevation ratio 0.34–0.40 in valve V), not beaked. Tegmentum white in color.

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 concave.

Tegmentum uniformly sculptured with elongateoval granules in central areas, or rounded ones in head valve and lateral areas of intermediate valves, arranged quincuncially, each granule with one megalaesthete and 8 micraesthetes. Surface of tegmentum in central areas between granules smooth.

Articulamentum well developed, apophyses small, widely separated, subtriangular in valves II–VII, more or less trapezoidal in tail valve, jugal sinus wider than apophyses.

Girdle narrow (0.7 mm near valve V), dorsally covered with elongate, pointed scales, without ribs or rarely with 1–3 obsolete longitudinal riblets (60 x 32 μm), Sutural and marginal needles smooth, not numerous, flattened (140 x 30 μm). Ventral scales lanceolate, smooth (80 x 30 μm).

Radula of specimen with BL 21 mm is 6.6 mm long with 32 transverse rows of mature teeth. Central tooth wide with round, narrow blade, major lateral teeth with sharply pointed, unidentate head.

Thirteen gills per side extending from valve VII to anus in specimen with BL 21 mm. Nephridiopore situated between twelve and thirteen gills, gonopore situated between thirteen and fourteen from tail gills in lectotype.

Remarks. This species is most similar to Belknapchiton bergenhayni (Saito, 2011) which also has 8 micraesthetes per granule, but differs from it by the absence or very absolete riblets in dorsal scales (vs 10–12 well developed riblets in B. bergenhayni ), and the absence of small longitudinal grooves between the granules (vs small longitudinal grooves between the granules in the central area of the intermediate valves in B. bergenhayni ).

Six specimens of B. giganteus of different sizes with BL 5.0–42.0 mm allowed the study of the age variability of some features of this species ( Table 1).

With increasing body size, the number of gills naturally increases from 5 in chitons with a body length of 5.0 mm to 16 in the 42.0 mm long lectotype. With the growth of the body, the apophyses gradually expand, while the ratio of the width of the jugal sinus to the width of the apophysis decreases. Also, with an increase in body size, the postmucronal area lengthens more quickly compared to the antemucronal area and the number of micraesthetes on granules increases from 2–4 in chitons with BL 5.0 mm up to 8 in chitons with BL 21.0 mm. In addition, the granule size in the central areas increases with size from 50 µm in chitons with a body length of 5.0 mm to 75 µm in individuals with a body length of 21.0 mm and also the distance between neighboring grains increases with size. Small chitons with a body length of 5.0 mm have rounded shell valves, which become carinated with larger sizes.

Figures

FIG. 1. Belknapchiton giganteus, Banda Sea, Indonesia, 3200 m, BL–21.0 mm. A. Head valve, dorsal view. B. Valve V, dorsal view. C. Valve VIII, dorsal view. D. Part of valve IV, ventral view. E. Valve V, tegmentum sculpture in central area. F. Tail valve, lateral view. G. Valve V, frontal view.