Parvamussium slacksmithae, 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 : 140-141

Parvamussium slacksmithae sp. nov.

Figs 10G–I, 11

Holotype (lv) AM C.157685, Western Australia, 72 n. ml NNW of Dampier , 19°28.9'– 19°29.0'S 116°29.4'– 116°29.0'E, dead, 110 m, coll. B. W. Jenkins, 26.X.1983 (FRV Soela stn 26-18), (H 5.1 mm, L 6.0 mm) . Paratypes (64, lv + rv): 50 AM C.209769, 14 ZMA Moll.409002; paratypes from the same locality. Largest paratype (lv): H 5.9 mm, L 6.5 mm .

Additional material examined. — AUSTRALIA: WESTERN AUSTRALIA: 85 n. ml NNW of Port Hedland , 19°00.4'– 19°03'S 118°01'E, dead, 116–120 m, sand & gravel, FRV Soela stn 29-26, coll. B.W. Jenkins, 29.X.1983 (1 v, C.157713) ; 94 n. ml NNE of Port Hedland , 18°48'S 119°0'E,dead, 92–94 m, FRV Soela stn 23-06, coll. B. W. Jenkins, 23.X.1983 (10 v, C.157658) . QUEENSLAND: SE of Swain Reefs , 22°20'09"– 22°26'16"S 153°17'05"E, dead, 187 m, HMAS Kimbla stn 7, coll. P. H. Colman, VI.2008 (5 v, C.462541).

Description. Shell small, up to c. 6 mm high, fragile, translucent, left and right valves equally convex, almost circular, inequivalve, equilateral, auricles almost equal in size, umbonal angle c. 90°. Left valve with small milky white maculations, right valve with milky white dot umbonally.

Both valves smooth and glossy, with traces of commarginal growth lines on left valve, some specimens with weak, closely spaced commarginal lamellae and rudimentary radial sculpture on anterior and posterior ends of disc. Anterior auricle of left valve smooth, some specimens with weak commarginal sculpture near periphery. Anterior auricle of right valve with one radial riblet near pseudo-fasciole, posterior auricle smooth. Dorsal margin straight. Byssal notch shallow. Interior surface with rudimentary short riblets, two anteriorly and two posteriorly.

Discussion. Parvamussium slacksmithae sp. nov. is somewhat similar to P. torresi (E. A. Smith, 1885) from northeastern Australia, but differs in size ( P. slacksmithae is c. 6 mm high, P. torresi is larger, up to c. 9 mm high), in external sculpture (right valve of P. slacksmithae is smooth, right valve of P. torresi has weak commarginal lamellae) and internal sculpture ( P. slacksmithae has a few rudimentary riblets, P. torresi has 10 prominent, well-developed radial riblets). Parvamussium vesiculatum Dijkstra, 1995 from New Caledonia also has rudimentary internal riblets, somewhat similar to the present species, but differs strongly externally ( P. slacksmithae is almost smooth, P. vesiculatum is prominently sculptured) (Dijkstra, 1995b: 39).

Habitat. The present specimens are so far only dead-taken on the continental shelf on soft sediment.

Distribution. Northwestern Australia near Port Hedland from 18°48'– 19°29'S, and 116°29'– 119°0'E, also from Queensland, taken dead at 92– 187 m.

Remarks. It is possible that the present species was collected alive at the type locality. Specimens are very fresh and soft parts are often lacking after sorting the material, as the valves become separated easily due to the weak external ligament. Specimens from Queensland have more prominent commarginal sculpture near the disc flank, but other characters are similar to the type material.

Etymology. Named after Mrs Shirley Slack-Smith, formerly curator of the Mollusca Section of the WAM, who generously supported this project and assisted in many ways.