Ischnochiton sp.

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

Ischnochiton sp.

( Figures 3A–J )

Material examined: MLD03A: 1 spm (slightly curled, estimated length 4.4 mm).

Description: Animal of small size, apices not beaked.

Head valve semicircular, posterior margin not notched in the middle. Intermediate valves rectangular, hind margin straight, apex inconspicuous, lateral area hardly raised, clearly defined. Tail valve with mucro not prominent, anteriorly or subcentrally situated, hind slope slightly concave directly posterior to the mucro.

Tegmentum smooth, with some faint undulations visible in lateral areas ( Fig. 3B ). Apical caps visible on the megalaesthetes and micraesthetes ( Figs 3H, J ), uniformly longitudinally distributed in the central and antemucronal areas, radially in the head valve, and in lateral and postmucronal areas, and patterned with a semi-spherical layout, at least from the outside resembling small ocelli.

Articulamentum only partly investigated, the only half intermediate valve dismounted confirm the assignment to Ischnochiton because there is one slit and the insertion teeth are not pectinated ( Figs 3E, F ).

Girdle dorsally covered with oval, bent, imbricating scales, the base lozenge-shaped, ca. 160–180 µm wide, with ca. 12–14 narrow, shallow grooves, not covering the whole surface of the scales ( Fig. 3I ). Ventrally the girdle is paved with radiating rows of elongate, rectangular, smooth scales ( Fig. 3G ).

Remarks: The single specimen was preserved in alcohol and appears to have assumed a reddish colour, so it is not possible to study the original colouration. We consider it an Ischnochiton on the basis of its general appearance, the lack of pectinated insertion plates, and the characteristics of the girdle.

Ischnochiton feliduensis E.A. Smith, 1903 , only known from the holotype, appears, at a first glance, to be similar to the undescribed species in general appearance, and also in girdle characteristics (compare Fig. 54/ 3 in Kaas & Van Belle 1990 with our Fig. 3H ), but I. feliduensis has a rough tegmentum with quincuncially arranged granules on the head valve, and on lateral and postmucronal areas, and also has longitudinal riblets on central and antemucronal areas.

Distribution: Ischnochiton sp. is known only from the Maldive Islands.

Figures

FIGURE 3A–J, Ischnochiton sp., Maldive, living specimen, MLD03A. A, complete specimen, lateral view. B, detail of valves vii viii, lateral view. C, close-up of head valve. D, detail of the tegmentum covered by very small apical caps patterned with a semi-spherical layout. E–F, half detached intermediate valve, ventral views. G, ventral (left) and dorsal (right) girdle elements in situ. H, detail of perinotum elements in situ. I, J, dorsal and lateral views (respectively) of the apical caps patterned with a semi-spherical layout. K–M, Ischnochiton feliduensis E.A. Smith, 1903, Maldive, Felidhoo Atoll, Holotype, BMNH 1903.9.17.25, an intermediate valve detached (not coated). K, dorsal view. L, close-up of K. M, granules of the central area. Scale bars 1 mm: A; 500 µm: C, K; 100 µm: B, E, F, L; 50 µm: D, G, I, M; 10 µm: H, J.