Thecodonta rainesi Raines & Huber 2012, sp. nov.

Thecodonta rainesi Huber in Raines & Huber sp. nov.

Figures 31 A–G

Type Material. Holotype: LACM 3201, single LV, 3.7 mm. Paratype 1: LACM 3202, single RV, 3.2 mm, from type locality. Paratype 2: LACM 3203, single LV, 3.6 mm, from EI, Mataveri Bay, 50 m. Paratypes 3–4: Coll. MHU, 2 single valves, from EI, Hanga Roa, 50 m. Paratypes 5–6: Coll. BR, 2 single valves, from type locality.

Type Locality. Dredged at 30–50 m in fine sand, off the western coastline near Tahai, Easter Island, 27°07’20” S, 109°26’30” W.

Description. A small, whitish Thecodonta species, ovate, smooth, with low and strongly prosogyrate umbones. Shell small, not exceeding 3.7 mm; ovate, gently rounded posteriorly and anteriorly; equivalve, strongly inequilateral; rather compressed; thin; white, showing spots and blotches in the substance of shell which lend to it a watered silk effect. The umbones are low and strongly prosogyrate and are situated near the anterior end. Prodissoconch, P1 typically eroded, possibly D-shaped, ca. 130 µm length by 85 µm height, P2 ovate with slender growth lines, white to tan in color, ca. 430 µm length by 369 µm height. In adult valves the dorsal line curves gently from the umbo to the posterior end. The surface is nearly smooth, marked by weak commarginal growth stops only. The elongate lateral dorsal tooth, stronger expressed in the receiving LV is marginally placed. The shorter anterior laterals are equally marginally placed. The single cardinal tubercles in either valve rest at the upper margin of the anterior lateral. The laterals are smooth not serrated. The posterior scar is narrow, elongate, rather small and marginally placed; the posterior scar is suborbicular, also small and marginally placed. The connecting, continuous pallial line is very weakly impressed, but comparatively broad and patchily interrupted. The margins are smooth.

Comparative diagnosis. The very inequilateral-ovate shape and thin structure, together with a continuous pallial line, characteristic dentition, and minute size closely resembles the small Thecodonta -group.

In umbonal position Bernard’s (1983: 68) prosogyrate view for thecodontinids is still applied. Prosogyrate beaks were also diagnosed by Chavan in Moore (1969: N532) for Thecodonta. However, Dall, Bartsch and Rehder (1938: 148) noted the reverse montacutinid opisthogyrate position. The as yet unknown anatomy of Thecodonta will be needed to decide once and for all.

Japanese authors synonymized Dall, Bartsch & Rehder, 1938 ’s genus Kona with Thecodonta and recognized only the type species as valid from Japan (e.g. Higo et al., 1999: N732). Possible syntypes of smaller and larger Japanese specimens are illustrated in Higo et al. (2001 B732 and B732s). Severns (2011: 470, pl. 215 fig. 6) recognized only Thecodonta symmetrica Pilsbry, 1921, as valid from the Hawaiian Islands, and consequently Kona bucki Dall, Bartsch & Rehder, 1938 as a synonym. Both views are shared.

Moreover, due to a similar morphology, the nearly identical shape changes during growth and similar maximum sizes 4.7 mm (T. sieboldi) and 4 mm (T. symmetrica) synonymy cannot be completely excluded, because genetic data may one day reveal these two as conspecific. Most distinct at present are their habitats, i.e. subtidal 2– 10 m (T. symmetrica) and sublittoral 50–100 m (T. sieboldi).

The respective data of T. rainesi is within 30– 50 m. There is no commensal mode of life data recorded for either the Japanese or Hawaiian species of Thecodonta. Severns (2011) noted the habitat as silty sand, inshore of the reef, Japanese authors recorded sandy and muddy bottoms, while T. rainesi is found on fine sandy bottoms.

Morphologically, T. rainesi differs significantly in shape from its two congeners. Equal sized T. sieboldi and T. symmetrica are shorter and oblique in shape. Larger congeners approach T. rainesi somewhat more, but still have a stronger curved ventral margin and stronger developed teeth. The nearly smooth sculpture with watery blotches is similar to Japanese specimens, whereas all Hawaiian specimens seen are rather dull.

Remarks. At first this species was misidentified by the junior author. However, the senior author observed differences which finally led to a new description by the junior author.

Habitat. Occasionally found at several locations around EI, dead in sand, from 30–50 m.

Distribution. At present Thecodonta rainesi is only known from Easter Island— E1.

Etymology. This new species honors Bret Raines’ relentless efforts during many years of his life for a better understanding of the bivalve fauna of Rapa Nui.