Ischnochiton albinus Thiele, 1911

Dell’Angelo, Bruno, Gori, Sandro, Baschieri, Leonardo & Bonfitto, Antonio, 2010, Chitons (Mollusca, Polyplacophora) from the Maldive islands, Zootaxa 2673 (1), pp. 1-38 : 7-9

Ischnochiton albinus Thiele, 1911

( Figures 2A–Q , 12B )

Ischnochiton albinus Thiele 1911: 400 , pl. 6, fig. 4; Iredale & Hull 1927: 21; Saito 1998: 148, figs 2C, 3; 2000: 11, pl. 6, fig. 13; 2005: 107; 2006: 120, fig. 5B; Slieker 2000: 52, pl. 14, fig. 4; Strack 2003: 13; Schwabe 2005: 52, pl. 1, fig. 7; 2007: 146, figs 2A, B; Thach 2005: 22; Burghardt et al. 2006: 30; Anseeuw et al. 2009: 152.

Ischnochiton (I.) albinus ; Kaas & Van Belle 1990: 218, fig. 98 (bibliography and synonymy).

Ischnochiton (I.) baliensis Kaas & Van Belle 1990: 142 , fig. 62 (fide Schwabe 2007).

Type: Holotype ZMB 101.999 .

Type locality: SW Australia, Sharks Bay , Surf Point, Outer Bar (St. 25), 0.5–3.5 m .

Material examined: M13: 1 spm (estimated length 5.2 mm, slightly curled, Fig. 12B ), 1 intermediate valve; LB05: 1 intermediate valve.

Remarks: Detailed descriptions of this species are given by Kaas & Van Belle (1990) and Saito (1998).

The species is characterized by a finely quincuncially granulated tegmentum – on central and antemucronal areas the granules are connected resulting in a pitted or net-like appearance ( Figs 2F, I ) – and by the squarish imbricating scales of the perinotum, which are sculptured with longitudinal ribs and 8–9 spherules along the distal margin ( Fig. 2M ).

We found only two intermediate valves and a living specimen, well lying within the size-range for the species ( Saito 2006 reported seven specimens with body lengths between 4.7 and 7.8 mm).

The present material of this species provides some additional information and points out some slight differences in comparison with previous descriptions. The net-like arranged squarish granules of the central and antemucronal areas bear a cluster of 8–9 aesthetes of the same size, characteristically formed by a central one and a series of 7–8 irregularly arranged along the margin of the granules ( Fig. 2K ). The elongate granules of the head valve ( Fig. 2C ), lateral areas ( Fig. 2J ) and postmucronal area ( Fig. 2E ) are arranged along the concentric growth lines, and bear 6–7 equal aesthetes along the margin ( Fig. 2J ), leaving the centre free.

The perinotum shows imbricating scales with a width of 90–110 µm (differing from the 55–70 µm reported by Saito 1998, and 68 µm by Kaas & Van Belle 1990), have an angled basal margin ( Fig. 2L ) and a sculpture of 20–22 longitudinal riblets (differing from the ca. 30 reported by Saito 1998, and 16 by Kaas & Van Belle 1990), and 7–9 spherules along the distal margin ( Fig. 2M ), the central one largest (diameter ca. 16 µm), diminishing in size towards the sides.

Distribution: Ischnochiton albinus seems to be a rather rare species, probably widespread in the Indo- West Pacific ( Saito 2006), but until now has only been reported from disjunct localities: Shark Bay, SW Australia (type locality), the Vietnamese coast ( Leloup 1952; Strack 2003; Thach 2005), the Goto Islands and off the Amami-Oshima Islands, Japan ( Saito 1998, 2005), Mindoro Island, the Philippines ( Saito 2006), and Indonesia (Bali Island, Kaas & Van Belle 1990; Sulawesi, Burghardt et al. 2006). The bathymetrical range varies from 0.5–3.5 m (the holotype) up to 202 m ( Saito 2005). This find in the Maldive Islands considerably extends the distribution of this species.

Figures

FIGURE 2A–Q, Ischnochiton albinus Thiele, 1911, Maldive, M13. A–E, L–Q, living specimen. A–B, dorsal and lateral view (respectively) of a complete specimen. C, close-up of the head valve. D, lateral view of valves vii-viii. E, close-up of the tail valve. L, perinotum elements in situ. M, detail of the perinotum elements, showing the sphaerules along the distal margin in situ. N, hyponotum elements in situ. O, radula. P, an isolated blade of the second lateral tooth. Q, detail of the central teeth. F–K, intermediate valve. F, dorsal view. G, frontal view. H, ventral view. I, detail of the sculpture, showing the contact between the right lateral and central areas. J, lateral view of elongate granules of the lateral area. K, the same, by a dorsal view. Scale bars 1 mm: A, B; 500 µm: C–H; 100 µm: I; 50 µm: J, K, N, O; 10 µm: L, M, P, Q.

FIGURE 12A, Parachiton hylkiae mauricejayi Schwabe, 2002, living specimen, Maldive, LB03B, length 12.8 mm. B, Ischnochiton albinus Thiele, 1911, living specimen, Maldive, M13, length 5.2 mm. C, Ischnochiton feliduensis E.A. Smith, 1903, Maldive, Felidhoo Atoll, Holotype, NHM 1903.9.17.25, length 6 mm. D, Tegulaplax hululensis (E.A. Smith, 1903), living specimen, Maldive, M28, length 10.5 mm. E–F, Callochiton sp., S. Nilandu Atoll, Maldive, NHM 1903.4.11.71 (living specimen collected by J.S. Gardiner), length 6.6 mm. G, Lucilina sp., living specimen, Maldive, M07, length 4 mm. H, Lucilina indica (Leloup, 1981), holotype (MNHN), Madagascar, Tuléar, length 8 mm. I, Acanthopleura miles (Carpenter in Pilsbry, 1893), living specimen, Japan, Okinawa, Sesoko Island, length 29.2 mm. J, Schizochiton incisus (Sowerby, 1841), living specimen, Indonesia: inside lagoon of Woka Island, off South Halmahera, 1°2.02’S, 128°13.97’E (ZSM Mol 20052075), length 40 mm. K, Acanthochitona penicillata (Deshayes, 1863), living specimen, Maldive, MLD04A, length 6.5 mm. L, Craspedochiton cf. laqueatus (Sowerby, 1842), living specimen, Maldive, M59, length 13.5 mm. M–Q, Craspedochiton laqueatus (Sowerby, 1842), NHM type material, Philippines, Mindoro, Calapan. M, lectotype (NHM 1988071/1), length 17.4 mm. N, paralectotype (NHM 1988071/2), length 17.5 mm. O–Q, paralectotypes (NHM 1988071/3-5), length respectively 16, 13.2 and 11.3 mm. R–S, Cryptoplax burrowi (E.A. Smith, 1884), living specimen, Maldive, M07, length ca. 11.7 mm. T, Cryptoplax burrowi (E.A. Smith, 1884), living specimen, Indonesia, ZSM Mol 20033081, length 25.4 mm. U–X, Cryptoplax burrowi (E.A. Smith, 1884), NHM 1992 053, syntypes, H. Cuming colln., Port Adelaide, S. Australia, length respectively 56.4, 38.5, 47 and 45.1 mm.