Acrosterigma capricorne, Vidal & Kirkendale, 2007

Vidal, Jacques & Kirkendale, Lisa, 2007, Ten new species of Cardiidae (Mollusca, Bivalvia) from New Caledonia and the tropical western Pacific, Zoosystema 29 (1), pp. 83-107 : 96-97

Acrosterigma capricorne n. sp.

( Fig. 4 G-J; Table 5)

TYPE MATERIAL. — Holotype: 1 rv, 40.5 × 30.6 × (26.8) mm. Paratypes (4): 1 lv, 39.2 × 30.2 × (27.4) mm; 1 rv, 30.4 × 25.0 × (22.0) mm; 1 rv, 26.1 × 21.2 × (18.4) mm; 1 rv, 58.1 × 47.6 × (35.0) mm.

TYPE LOCALITY. — BENTHAUS 2002, Austral Ridge, Banc Arago, stn DW 1985, 23°26.3’S, 150°44.2’W, 100- 107 m (first two paratypes come from type locality).

ETYMOLOGY. — Found just south of the Tropic of Capricorn.

MATERIAL EXAMINED. — Austral Islands. BENTHAUS 2002, Austral Ridge, Tubuai Is., stn DW 1961, 23°20.9’S,

149°23.5’W, 470-800 m, 1 lv. — Récif Neilson, stn DW 1925, 27°00.3’S, 146°05.1’W, 560-790 m, 1 rv (fourth paratype). — Marotiri Is., stn DW 1885, 27°51.8’S, 143°32.6’W, 700-800 m, 1 rv. — Marotiri Is., stn DW 1887, 27°52.00’S, 143°32.7’W, 750-1000 m, 1 rv. — Tubuai Is., stn DW 1958, 23°19.6’S, 149°30.3’W, 80-150 m, 1 lv. — E of Rapa, stn DW 1891, 27°37.1’S, 144°15.4’W, 800-850 m, 1 rv. — E of Rapa, stn CP 1892, 27°38.8’S, 144°15.6’W, 742-1000 m, 1 lv. — E of Rapa, stn CP 1910, 27°38.2’S, 144°15.4’W, 840-1200 m, 1 lv. — Récif Neilson, stn DW 1923, 27°01.3’S, 146°05.3’W, 360- 840 m, 1 rv and 1 lv. — Rurutu/Avera, stn DW 1996, 22°29.1’S, 151°21.9’W, 489-1050 m, 1 lv. — Stn DW 1997, 22°29.1’S, 151°22.3’W, 700-1350 m, 1 rv (third paratype). — E coast of Rurutu, stn DW 2003, 22°27.6’S, 151°18.9’W, 250-330 m, 1 lv.

DISTRIBUTION. — Found only on Austral Ridge, southwest Pacific.

DESCRIPTION

Shell of medium size, specimens on hand ranging from about 25 mm in length to 58.1 mm (paratype 4). Rather elongate ( L / H range 0.76-0.91, mean 0.83) and moderately inflated ( W / L range 0.65-0.90, mean 0.76). Shell never oblique, almost symmetrical ; posterior margin always curved, never truncated.

Lunular area narrow, slightly hollowed, with margins somewhat raised along entire extent.

External colour uniformly yellow; internal shell coloration white.

Hinge characteristic of the Acrosterigma uniornatum species group ( Vidal 1999: 271), with a rather symmetrical, moderately angled (A = 122-130°, see Fig. 2D ) hinge and cardinals connected in the right valve by a shallow saddle.

Ribs 42-47.

Posterior half of shell with a similar arrangement of oblique scales on top and posterior side of individual ribs. Ornamentation of the posterior slope may contrast with that of the medio-posterior slope, the axial furrow on the posterior ribs, generally present in Acrosterigma , but absent in this species group can be sporadically present, with sessile, oblique scales less numerous and more tubercular ( Fig. 4I ). The first ribs of the posterior zone, and the last ribs of the median zone can bear short scales on the anterior flank of the ribs, forming an asymmetrical chevron-like structure together with the longer top and posterior scales. These anterior scales progressively disappear towards the anterior and the posterior, where the scales shorten and become limited to the posterior flank of the ribs ( Fig. 4I, J ). On the medio-anterior slope the posterior scales appear on the anterior flank of the ribs, forming a herringbone structure. These angled scales become progressively curved, low-rounded ornaments on the anterior slope.

MICROHABITAT

Found in depths from 80-1350 m. Recently dead material found at the shallowest depth, with older shells found at deeper depth. This indicates that A. capricorne n. sp. may live at the shallow end of this depth range, and be transported down slope post mortem. However, this species, like other taxa in the Acrosterigma uniornatum species group ( Vidal 1999: 271), is really generally found at greater depth than most other Cardiidae (except Protocardiinae) in the western Pacific.

REMARKS

This species is allied to the A. uniornatum speciescomplex ( Vidal 1999). It differs from other members of the group mainly in patterns of ornamentation on the external shell surface. The ornamentation on the medio-posterior slope has more marked axial furrows, with oblique scales less numerous and more tubercular than on the adjacent posterior slope, compared with congeners ( Fig. 4I, J ).

Figures

FIG. 4. — A-C, Pseudofulvia caledonica n. gen., n. sp.; D-F, P. arago n. gen., n. sp.; G-J, Acrosterigma capricorne n. sp.; I, highlighting detail of posterior and medio-posterior zones of left valve; J, highlighting medio-zones, with no herringbone structures in central region. Scale bars: 5 mm.

FIG. 2.— Morphometrics used in this study to differentiate cardiid taxa:A, delineates height (H) and length (L) measurements;B, external shell regions PT1, PT2 (typical of Pseudofulvia n. gen.); C, width (W); D, angle A, the angle formed by the most dorsal cardinal and laterals;E, I, the lunular heart and II, the lunular area (typical of Fulvia); F, I, the lunule s.s. and II, the lunular area (typical of Ctenocardia). For A, B, D the left valve is figured. Refer to Materials and methods for further description of these terms, see also Vidal (1994).