Hyalopecten ponderi, Dijkstra & Beu, 2018

Dijkstra, Henk H. & Beu, Alan G., 2018, Living Scallops of Australia and Adjacent Waters (Mollusca: Bivalvia: Pectinoidea: Propeamussiidae, Cyclochlamydidae and Pectinidae), Records of the Australian Museum (Rec. Aust. Mus.) 70 (2), pp. 113-330 : 166

Hyalopecten ponderi sp. nov.

Figs 23B,E,G, 24

Holotype (pr), H 7.1 mm, W 7.0 mm, D 3.1 mm) AM C.209770, CORAL SEA: 11°40.98'S 145°36.85'E, alive, 2039–2052 m, benthic sled, leg. I. Loch, 23 Aug 1988 (ORV Franklin stn 15).

Description. Shell up to c. 7 mm high, fragile, opaque, whitish, left and right valve almost equally strongly convex, oblong, inequilateral, auricles unequal in shape and almost equal in size, umbonal angle c. 85°.

Left valve sculptured with strongly developed commarginal lamellae, widely spaced on central part of disc (4 per mm) but more closely spaced dorsally, ventrally and laterally (5–6 per mm), commencing at 0.5 mm shell height and enlarging near ventral margin, with unevenly spaced, interstitial radial riblets (5–7 per mm). Radial riblets weak, commarginal lamellae prominent, strongly projecting and curved towards ventral margin. Auricles with very closely spaced commarginal lamellae (10–12 per mm).

Right valve similar in sculpture to left valve, with somewhat weaker interstitial radial riblets on central part of disc, almost absent laterally. Dorsal margin straight. Byssal notch moderately deep, byssal fasciole rather broad, active ctenolium delicate, with 3 closely arranged teeth.

Discussion. Hyalopecten ponderi resembles H. mireilleae (Dijkstra, 1995) , recorded from New Caledonia, the Loyalty Islands and Vanuatu. Both species are strongly convex, with prominent commarginal and weak radial sculpture, and are not commarginally undulated. However, the present species is smaller (c. 7 mm high, H. mireilleae c. 13.5 mm high) and more circular in shape, H. mireilleae being more oblong and posteriorly oblique, with more prominent radial riblets than H. ponderi . The right valve of Hyalopecten ponderi also is less convex than that of H. mireilleae and the anterior auricle of the right valve is smaller (in H. mireilleae the right anterior auricle is broader with a much smaller byssal fasciole, which is almost absent).

Hyalopecten ponderi also resembles Hyalopecten tydemani (Dijkstra, 1990) from Indonesia, but they differ somewhat in size ( H. tydemani is c. 10 mm high) and in shape ( H. tydemani is more oblong). The radial sculpture of Hyalopecten tydemani is very weak and only developed at an early growth stage (more prominent throughout ontogeny in H. ponderi ). Hyalopecten hadalis ( Knudsen, 1970) from the Kermadec Trench is larger (up to c. 20 mm high), oblong, strongly commarginally undulated, and sculptured only with delicate radial striae. Hyalopecten sp. (anterior and posterior part of left valve) ( Knudsen, 1970: 102) from the Tasman Sea is much larger in size (up to c. 35 mm in height), also commarginally undulated, and sculptured with closely spaced radial riblets.

Kamenev (2016) described three new species of Hyalopecten from abyssal and hadal zones in the northwestern Pacific, and provided illustrations of and comparisons with all previously named living species. His illustrations ( Kamenev, 2016: fig. 5) show that the most nearly similar species in their weak commarginal plications, prominent commarginal lamellae and weak radial riblets are H. tydemani , which is taller and narrower than H. ponderi sp. nov., and H. bavayi Dijkstra, 1990 , which is shorter and wider than H. ponderi .

Habitat. Living in the upper abyssal zone on soft sediment.

Figure 24. Distribution of Hyalopecten ponderi sp. nov. (circle), Delectopecten alcocki (Smith) (stars), D. fosterianus (Powell) (triangles) and D. musorstomi (Poutiers) (squares).

Distribution. Only known from the type material at the type locality.

Etymology. Named after Dr Winston Ponder, former curator of the Malacology Section, AM, who stimulated this project.