Lasaea eastera Raines & Huber 2012, sp. nov.

Lasaea eastera sp. nov.

Figures 26 A–H

Type Material. Holotype: LACM 3150, 1 articulated specimen, 2.9 mm. Paratype: LACM 3151, 1 articulated specimen, 2 mm.

Type Locality. Dredged at 30–80 m in fine sand, off the western coastline near Tahai, Easter Island, 27°07’20” S, 109°26’30” W.

Description. A small, translucent white Lasaea species, with narrow, subcentral umbones. Shell small, below 3 mm, equivalve, subequilateral, oval with almost straight ventral margin; thin and translucent; uniformly whitishyellow outside, whitish inside; umbones narrow, pointed, subcentral, weakly prosogyrate. Prodissoconch, P1surface somewhat eroded, ovate and pronounced, ca. 75 µm length by 50 µm height, P2 smooth, with nearly obsolete growth striae, ca. 350 µm length by 340 µm height. Adult valves weakly inflated. Sculpture of weak commarginal striae, rather widely and somewhat irregularly spaced, radial elements absent. Hinge line thin, cardinals largely fused to lamellar posterior lateral and to shorter lamellar anterior lateral, laterals separated by a wide depression. Pallial line continuous. Ligament elongate, very thin and very narrow, marginally, embedded posterior to the umbones; resilifer subumbonal. Margins smooth.

Comparative diagnosis. Lasaea hawaiensis, originally described from the Hawaiian Islands, was recorded early by Rehder (1980) from EI and is present with numerous specimens. However, these are very uniform in shape, solidity and color, typically with reddish staining in the umbonal area. No other Lasaea species has been described from the Hawaiian Islands.

The new species has the same basic hinge configuration, though with a weaker, narrower hinge line and finer dentition. The structure is more fragile and translucent. The color is whitish and absent of any staining. The commarginal lines are wider and somewhat irregular. The umbones of adult L. eastera are most notably more pointed, narrower and stronger, exceeding the dorsal line and nearly centrally located.

Lasaea australis (Lamarck, 1818), from South Australia is more solid, with distinct color, stronger sculpture, and rather trigonal in shape with posteriorly placed umbones. It also grows to a much larger size, 6.5 mm. Lasaea colmani Ó Foighil & Thiriot-Quiévreux, 1999, from Sydney is less than half the size, broader with very low broad, posteriorly placed umbones.

Numerous species have been described from New Zealand. Thereof, L. hinemoa Finlay, 1928, (syn. L. rossiana Finlay, 1928), is similar in shape and color to L. hawaiensis, and shares similar distinctive markings as L. eastera, notably colors and stronger hinge. Lasaea hinemoa also grows larger (4.6 mm) than the EI species. Lasaea maoria (Powell 1933), shares its fragile structure, small size and white color with that of L. eastera, but the pronounced broader umbones and the lower and broader shape do not match. In addition, the hinge line of L. eastera is narrow and the dentition weak, whereas even the smaller L. maoria has a comparatively more solid and stronger dentition. Lasaea parengaensis Powell, 1935, is very distinct in oblique-trigonal shape.

Remarks. While L. hawaiensis is commonly encountered, L. eastera is very uncommon.

The deeper habitat of the two EI species is highly unusual compared to the otherwise shallow global Lasaea species. This change in habitat is probably influenced by the active downslope sediment transport noted by DiSalvo et al. (1988: p. 454).

Distribution. At present Lasaea eastera is only known from the type locality— E1.

Etymology. The name reflects the type locality.