Parvamussium whissoni, 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 : 146-148

Parvamussium whissoni sp. nov.

Figs 11, 15A–D,F

Holotype (lv) (H 5.3 mm, L 5.2 mm) WAM S77663 : southern Western Australia, off Albany, 35°23'33"– 35°24'04"S 118°18'32"– 118°18'22"E, dead, 748–776 m, hard substrate, leg. C. S. Whisson & J. Fromont, 23 Nov 2005 (CSIRO RV Southern Surveyor stn SS 1005/023) . Paratypes (19: 13 lv + 6 rv): 12 lv + 5 rv, WAM S77664 ; 1 lv + 1 rv, ZMA Moll.4.12.001. Paratypes from the same locality. Largest paratype (rv): H 6.0 mm, L 6.1 mm.

Additional material examined. — AUSTRALIA: WESTERN AUSTRALIA: off Albany, 35°25'03"– 35°25' 58"S 118°22'26"– 118°21'58"E, dead, 1019–1031 m, soft substrate, leg. C. S. Whisson & J. Fromont, 26 Nov 2005, CSIRO RV Southern Survey stn SS 1005/043 (3 v, WAM S77659 ) .

Figure 15. A–D, F, Parvamussium whissoni sp. nov., paratypes, WAM S77664 , CSIRO RV Southern Surveyor stn SS1005/023, off Albany, southern WA, 35°23'33"– 35°24'04"S 118°18'32"– 118°18'22"E, 748–776 m; rv anterior auricle exterior (A), lv interior (B), lv exterior (C), rv exterior (D), lv anterior auricle exterior (F). E, G, J, Cyclopecten horridus Dijkstra, AM C. 165232, pair, HMAS Kimbla stn 4, Capricorn Channel, QLD, 23°11.5'S 152°14.5'E, 188 m; rv exterior (E), lv exterior (G), rv auricles exterior (J). H, I, K, Cyclopecten cancellus Dijkstra , lv only, AM C.374666, beach, N side of Margaret River, Kilcarnup, WA, 33°57'S 114°59'E; lv exterior (H), lv umbonal sculpture (I), lv anterior auricle exterior (K). Scale bars represent 200 µm (A, F, I, K), 3 mm (B, D), 4 mm (C), 2 mm (E, G, H), 1 mm (J).

Description. Shell small, up to c. 6 mm high, fragile, semitranslucent to opaque, left slightly more convex than right valve, almost circular, inequivalve, equilateral, slightly anteriorly elongate, auricles dissimilar in shape, anterior auricle somewhat larger in size than posterior one, umbonal angle c. 95°. Left valve with small white longitudinal maculations, umbonal top whitish opaque, right valve whitish translucent.

Disc and auricles of left valve completely smooth and glossy, with traces of commarginal growth lines. Right valve with widely spaced commarginal lirae (7–8 per mm on central part of disc). Anterior auricle of right valve with weak commarginal lamellae and one radial riblet near pseudofasciole, posterior auricle smooth and continuous with disc. Dorsal margin straight. Byssal notch shallow. Left valve with 11 interior riblets (plus one rudimentary in holotype), and one auricular on each side. Right valve with 11–13 riblets and one auricular posteriorly. Riblets commence after c. 2 mm and extend almost to ventral margin.

Discussion. Parvamussium multiliratum Dijkstra, 1995 is a closely similar bathyal and abyssal species, known from New Caledonia (Dijkstra, 1995b: 26), Wallis and Futuna and Vanuatu (Dijkstra, 2001: 83), Fiji and Tonga (Dijkstra & Maestrati, 2008: 91), and herein from the Solomon Islands , Coral Sea, Queensland, and Tasman Sea. Parvamussium whissoni and P. multiliratum are similar in shape (almost circular), but differ somewhat in size ( P. whissoni up to c. 6 mm high, P. multiliratum up to c. 9 mm high), and can be further differentiated by the following characters: P. whissoni lacks sculpture on disc and auricles, whereas P. multiliratum has commarginal lamellae and some specimens have very weak radial threads interstitially; P. whissoni lacks sculpture on the posterior auricle of the right valve, whereas P. multiliratum has delicate, closely spaced commarginal lamellae. The internal riblets of the two species are equally developed, but differ in number ( P. whissoni has 11–13, P. multiliratum 14+).

A somewhat similar-looking smooth new congener from Western Australia is Parvamussium slacksmithae , which is similar in size, but differs in shape, having larger auricles and the posterior auricle demarcated from the disc (continuous with disc in P. whissoni ), by lacking commarginal sculpture on the right valve (present in P. whissoni ), and by rudimentary internal riblets, two anteriorly and two posteriorly (11–13 well-developed in P. whissoni ).

Habitat. The present specimens are so far only dead-taken in the bathyal zone on hard and soft sediment.

Distribution. Southern Western Australia, so far from 35°S and 118°E at 748–1031 m (single valves only).

Etymology. Named after Corey S. Whisson, technical officer of the Mollusca Section, Department of Aquatic Zoology (WAM).