Hyphantosoma tenue, Raines & Huber, 2012

Raines, Bret & Huber, Markus, 2012, 3217, Zootaxa 3217, pp. 1-106 : 90-92

Hyphantosoma tenue sp. nov.

Figures 46 E–J

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

Pitar sp. — Trego, 1997: p. 199 (Lot M 1452 in part)

Type Material. Holotype: LACM 3183, 1 RV, 25.6 mm x 20.5 mm, ( Figs. 46 E–F). Paratype 1: SIO M1446 , 1 LV, 35.9 mm, from EI, La Perouse Bay , 40–100 m, 1957/58 Downwind Expedition (DWHD-76), ( Figs. 46 G–H) . Paratype 2: LACM 3184, 1 RV posterior part missing 5 mm, from EI, near Tahai, 50–80 m. Paratypes 3–4: LACM 3185, 1 LV 5 mm, and 1 RV 2.5 mm, from EI, Mataveri Bay , 50 m.

Type Locality. Dredged at 50 m in Mataveri Bay, EI, 27° 08’ 44” S, 109° 26’ 46” W .

Description. A medium-sized, ovate-elongate Hyphantosoma species , rather fragile and compressed. A large species for EI, reaching at least 35.9 mm, equivalve, subquadrate; rather fragile and thin shelled, as usual in this genus; white with irregular brown marks and deep red umbones; lunule not incised; umbones low, only slightly extending the dorsal line, prosogyrate, situated in the anterior third of the valve. Prodissoconch round, smooth, clearly defined, turned obliquely under the umbo and not readily visible in adults, P1 ca. 127 µm length by 93 µm height, P2 ca. 275 µm length by 227 µm height. Adult valves rather compressed. Sculpture of characteristic chevron marks, rather weakly impressed; periostracum whitish, dehiscent, as typically found in this group, still anteriorly and posteriorly preserved on holotype. Hinge line rather thin, broad, rather short, in LV with two dorsally connected and a separate elongate posterior cardinal, the central cardinal strongest and a marked, pointed anterior lateral tubercle; RV with dorsally connected anterior and posterior cardinal, central, unconnected cardinal strongest, two anterior lateral ridges for the reception of the strong lateral tubercle of the LV. Pallial sinus extending to about 40% of length, trigonal, broadly rounded, ventral leg ascending, dorsal leg weakly descending. External ligament brownish, narrowly extended and inset in a dorsal ridge. Margins smooth.

Comparative diagnosis. Of the ten described venerid Hyphantosoma species , this new EI species shares characteristics notably with H. festoui and H. healyi . The new species is best compared to H. festoui described from Tahiti. Its holotype [ Pitar (Hyphantosoma) festoui Harte, 1993 , UCMP 398606] ( Figs. 46 A–D), and a slightly larger specimen from Okinawa were studied for comparison. Both H. festoui and H. tenue share a comparable pallial sinus, a fragile structure, a rather narrow hinge, red umbones and a white background color with irregular brownish marks. However, they differ in shape, H. festoui is more ovate-rounded, comparatively higher, with stronger rounded, prominent umbones, which significantly protrude above the dorsal line. As with many Hyphantosoma species , H. festoui is a very inflated species, whereas H. tenue is rather compressed, thus the respective difference in depth measurements 0.64 vs. 0.54. The anterior lateral tubercle in the LV is more pronounced in H. tenue than in H. festoui . Hyphantosoma festoui has a clearly incised lunule, whereas none could be detected in the holotype of H. tenue . Hyphantosoma tenue reaches nearly 36 mm in height, a size which is unknown for H. festoui with a current maximum of 25 mm, or for the closely related H. healyi which only reaches 17.1 mm.

The newly described H. crassum , also from EI has thicker and more solid valves, is more inflated, and grows even larger. Furthermore, it has a weakly incised lunule, and a shorter pallial sinus. Both species, however, share similar colors, a white background with brown markings and reddish umbones. The other known Indo-Pacific Hyphantosoma species are very distinct from H. tenue by having a stronger inflation, shape, and/or colors ( Huber 2010: 402).

Distribution. Hyphantosoma tenue is occasionally found at several locations off Easter Island, sublittoral from ca. 40–100 m — E1.

Etymology. The new name reflects the rather fragile structure and the narrower hinge compared to its, also newly described Easter Island congener H. crassum .