Hyphantosoma crassum, Raines & Huber, 2012

Raines, Bret & Huber, Markus, 2012, 3217, Zootaxa 3217, pp. 1-106 : 88-90

Hyphantosoma crassum sp. nov.

Figures 45 A–G

Pitar cf. pellucida — Luke, 1995: p. 106. (Lot M 1446 in part)

Pitar sp. — Trego, 1997: p. 199. (Lot M 1443 in part)

Type Material. Holotype: SIO M1443 , 1 RV, 39.5 mm x 32.3 mm, ( Figs. 45 A & B) . Paratype 1: SIO M12339 , 1 LV, umbonal portion missing, ca. 45 mm x ca. 39 mm from the type locality, ex-M1443 . Paratype 2: SIO M12340, 1 RV, ventral portion missing, 22.9 mm, from EI, La Perouse Bay , 40–100 m, 1957/58 Downwind Expedition (DWHD-76), ex-M1446, ( Fig. 45 D) . Paratype 3: SIO M12341, 1 LV, dorsal portion missing, 38.2 mm, from EI, La Perouse Bay , 40–100 m, 1957/58 Downwind Expedition (DWHD-76), ex-M1446, ( Fig. 45 C) . Paratype 4: LACM 3181, 1 LV, ventral portion missing, ca. 22.8 mm, from EI, near Tahai, 50–80 m, ( Figs. 45 E & F). Paratype 5: LACM 3182, 1 RV, umbonal fragment, 5 mm, from EI, near Tahai, 50–80 m.

Type Locality. Dredged at 132–174 m off northeastern Poike Peninsula, EI, 27° 04’ S, 109° 16’ W .

Description. A medium-sized, solid Hyphantosoma species with a strongly marked chevron-sculpture and a solid and thick hinge plate. A very large species for EI, reaching at least 45 mm, equivalve, ovate; solid and comparatively thick-shelled for this genus; white with irregular brown marks and deep red umbones; lunule incised, heart shaped; umbones low, only slightly extending the dorsal line, prosogyrate, situated in the anterior third of the valve. Prodissoconch turned obliquely under the umbo and not readily visible, but assumed to be small, round and smooth. Adult valves moderately inflated. Sculpture of characteristic chevron marks, clearly and strongly expressed anteriorly and posteriorly, becoming somewhat irregular centrally; weak periostracum rubbed off. Hinge line solid, broad, rather short, in LV with two dorsally connected and a separate elongate posterior cardinal, the central cardinal knobby and prominent, additionally a pointed, strong anterior lateral tubercle; RV with dorsally connected anterior and posterior cardinal and a central, unconnected cardinal, which is the strongest, additionally two anterior lateral ridges for the reception of the lateral tubercle of the LV. Pallial sinus comparatively short, broadly-rounded, ventral leg ascending, dorsal leg almost horizontal. External ligament, brownish, strong, narrowly-extended and inset in a dorsal ridge. Margins smooth.

Comparative diagnosis. Of the nine described Hyphantosoma species , this new EI species shares characteristics with H. festoui ( Harte, 1993) , and H. healyi ( Lamprell & Stanisic, 1996) , known from French Polynesia and Melanesia. These two species are closely related to each other and are easily distinguished from H. crassum by the more fragile structure with rather delicate hinges. The chevron sculpture is not as pronounced and typically visible only with a lens. The umbones are more prominent in both, and neither is known to reach sizes approaching that of H. crassum . The pallial sinus in H. festoui is deeper and the upper leg closer to the dorsal line.

Hyphantosoma intricatum ( Dautzenberg, 1907) , described from Queensland is closer in size to the new species and is widely distributed within the Indo-Pacific. However, the shape of H. intricatum is very distinct, being more elongate, longer posteriorly, with a stronger descending anterior slope. The umbones are more prominent, more rounded and wider, protruding above the dorsal line. Reddish umbones are not present in any of the H. intricatum studied, but occur in all well preserved H. crassum . The chevron-sculpture in H. intricatum is more regular and finer. Both species, however, share similar hinge configuration and similar pallial sinus position.

The other known Indo-Pacific Hyphantosoma species are very distinct by their smaller size and/or weaker structure, usually with a visible chevron-sculpture only on the anterior ( Huber 2010: 402).

Remarks. There are no Hyphantosoma species known from the Hawaiian Islands. At present, with a size reaching at least 45 mm, H. crassum represents one of the largest bivalves ever found at EI, surpassed only by a worn Chama specimen. All specimens identified as “ Pitar sp. ” or even as “ Pitar cf. pellucida ” by previous authors have proven to represent the genus Hyphantosoma . True species of Pitar Römer, 1857 , appear absent from EI.

Distribution. Hyphantosoma crassum is occasionally found at several locations off Easter Island, sublittoral from ca. 50–150 m — E1.

Etymology. The name crassum reflects the solid structure and the stronger hinge compared to its, also newly described Easter Island congener H. tenue .