Propeamussium alcocki (E. A. Smith, 1894 )

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 : 122-124

Propeamussium alcocki (E. A. Smith, 1894)

Figs 1A,D, 2

Amussium alcocki Smith, 1894: 172 , pl. 5, figs 15–16; Alcock & Anderson, 1897: pl. 2, figs 3–3a; Alcock, 1902: 282, fig. 79; Smith, 1906: 255; Thiele & Jaeckel, 1931: 8; Winckworth, 1940: 26.

Propeamussium alcocki (Smith) .–Abbott & Dance, 1982: 303; Dijkstra, 1995b: 13, figs 1–4, 133–137; Dijkstra & Kastoro, 1997: 247, figs 1–4; Dijkstra & Marshall, 1997: 4, pl. 1, figs 1–6; Dijkstra, 2001: 74; Dijkstra & Maestrati, 2008: 78; Dijkstra & Janssen, 2013: 183, figs 4–6.

Type data. Lectotype (pr) ZSI 6154 /9, designated by Dijkstra (1995b: 13), 2 paralectotypes (pr): AM C.003278, NHMUK 1894.9.11.1, NMW 1955.158.785. Type locality: Laccadive Sea, 15°02'N 72°34'E, alive, 1353 m ( Investigator stn 105).

Comments on type data. The type specimens are somewhat damaged near the margins, and the right valves of three of them ( ZSI, NHMUK, NMW) have been restored .

Additional material examined. — AUSTRALIA: QUEENSLAND: Swain Reefs, 23°15'S 153°18' E, alive, 424 m (6 pr, QM MO33657 ). WESTERN AUSTRALIA: Exmouth, North West Cape,off Yardie Creek , 400–450 m, trawled alive on coral & rubble bottom (1 pr, HMP0234 ) .

For records from the Arafura Sea (Tanimbar Islands ), Coral Sea (Chesterfield Islands ), northern Tasman Sea ( Norfolk Island), New Caledonia, Wallis and Futuna Islands , Vanuatu, Solomon Islands , Fiji and Tonga, see Dijkstra (1995b: 13), Dijkstra & Kastoro (1997: 247), Dijkstra & Marshall (1997: 74), Dijkstra (2001: 74) and Dijkstra & Maestrati (2008: 79).

Description. Shell fragile, hyaline, up to c. 50 mm high, semicircular, inequivalve, inequilateral, left valve slightly more convex than right, prodissoconch c. 240 µm in height.

Left valve with a few commarginal growth lines, and 1–3 delicate radial lirae near posterior margin. Anterior margin more convex than posterior. Auricles equal, smooth, anterior margin somewhat raised. Eight to 10 internal riblets entering from near resilifer to two-thirds length of adult disc, somewhat finer than on right valve; one auricular ridge on each side.

Right valve with wide-set commarginal lirae, commencing at c. 3 mm shell length, with granulate interstitial microsculpture.Auricles with narrow commarginal striations, prominent scales on posterior and anterior dorsal margins. Resilifer triangular. No byssal notch.

Dimensions. Illustrated specimen, WA, Exmouth, off Yardie Creek, 400–450 m (HMP0234): rv: H 26.1 mm (ventral skirt broken off), L 26.1 mm; lv: H 33.4, L 31.2 mm; D 4.3 mm.

Habitat. Living in the bathyal zone free on soft sediment, muddy sand or sandy mud.

Distribution. Gulf of Aden, 1469 m, Laccadive Sea: 1337–1410 m, Bay of Bengal, 1026 m ( Knudsen, 1967); Arafura Sea, Indonesia, 884–1266 m (Dijkstra & Kastoro, 1997; Chesterfield Islands , Coral Sea, 650 m, dead; Loyalty Islands , 780–800 m (Dijkstra, 1995b); New Caledonia, 708–860 m (Dijkstra, 1995b; Dijkstra, 2001); Wallis and Futuna Islands, 1015 –1300 m; Vanuatu, 690–750 m (Dijkstra, 2001); Norfolk Island, 949–952 m (Dijkstra & Marshall, 1997), Solomon Islands , 1001–1012 m; Fiji, 427–787 m; Tonga, 824 m (Dijkstra & Maestrati, 2008). Maximum depth range of live-taken specimens is 424–1469 m. Present specimens from Australia alive at 424 m.

Remarks. Propeamussium alcocki is morphologically close to Propeamussium watsoni (E. A. Smith, 1885) , which is known from similar localities (Dijkstra, 1995b), but differs somewhat in transparancy ( P. alcocki hyaline, P. watsoni opaque and whitish), in shape ( P. alcocki slightly more elongate, P. watsoni circular), in texture of the left valve ( P. alcocki smooth, P. watsoni with delicate radial sculpture in

Figure 1. A, D, Propeamussium alcocki (Smith) , pair, HMP0234, off Yardie Creek, Exmouth,WA, 400–450 m; lv exterior (A), rv exterior (D). B, C, I, Propeamussium caducum (Smith) , rv only, AM C.165479, FRV Kapala stn K 77-03-09, off Ulladulla, NSW, 35°30'– 35°32'S 150°48'– 150°47'E, 549 m; anterior auricle exterior (B), exterior (C), interior (I). E, K–M, Propeamussium investigatoris (Smith) , separate valves, QM MO18021d, E of GBR, QLD, 17°35'S 146°53'E, 458–500 m; rv exterior and interior (E, M), lv exterior and interior (K–L).F–H, J, N, Propeamussium siratama ( Oyama, 1951) , separate valves, AM C.165218, E of GBR, QLD, 18°11.01'S 147°25.5'E, 472–490 m; lv anterior auricle exterior (F), rv exterior (G), lv exterior (H), lv interior (J), rv anterior auricle exterior (N). Scale bars represent 10 mm (A, D–E, K–M), 0.5 mm (B, F, N), 5 mm (C, G–J).

early ontogeny), and in internal riblets ( P. alcocki delicate and weak, P. watsoni more solid).

Knudsen (1967: 281) considered the two names to be synonymous, with variations in shape (circular to almost oblong), commarginal sculpture of the right valve (varies in position), and radial sculpture of the left valve (varies in development) of examined material from the Indian Ocean. These variations are not observed in material from the Southwest Pacific (Dijkstra, 1995b: 13), and we consider these to be two separate species.