Parvamussium torresi (E. A. Smith, 1885 )

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 : 144-146

Parvamussium torresi (E. A. Smith, 1885)

Figs 11, 14A–B,D–E,J

Amussium torresi Smith, 1885: 311 , pl. 23, figs 3–3b.

Glyptamusium torresi (Smith) .– Iredale, 1939: 370.

Propeamussium (Parvamussium) torresi (Smith) .–Dijkstra, 1990a: 3.

Parvamussium torresi (Smith) .–Dijkstra, 1995b: 36, figs 51–54, 125–128; Dijkstra & Kastoro, 1997: 262, figs 90–94; Dijkstra & Maestrati, 2008: 95; Dijkstra & Maestrati, 2015: 612, figs 5A–C.

Type data. Lectotype (pr) designated by Dijkstra (1995b: 36, figs 125–128) NHMUK 1887.2.9.3316, 5 paralectotypes (v) NHMUK 1887.2.9.3317/1–5. Type locality: Australia, QLD, E of Cape York, 11°38'15"S 143°59'38"E, alive, 285 m (Challenger stn 185B).

Figure 14. A, B, D, E, J, Parvamussium torresi (Smith) , separate valves, AM C.165565, MV Espiritu Santo stn E68/ 660, 200 miles [322 km] WNW of Port Hedland, WA, 19°29'S 115°38'E, 320 m; rv anterior auricle exterior (A), lv anterior auricle exterior (B), rv exterior (D), lv interior (E), lv exterior (J). C, F, G–I, Parvamussium vesiculatum Dijkstra , separate valves, AM C.165414, ORV Franklin stn 89-40, Gifford Guyot, Coral Sea, 26°45.27' S 159°30.59'E, 315–360 m; rv exterior (C), rv auricles exterior (F), rv interior (G), lv exterior (H), lv anterior auricle exterior (I). Scale bars represent 0.5 mm (A, B, I), 4 mm (C–E, H, J), 1 mm (F), 3 mm (G).

Comments on type data. The size of the lectotype (H 7.8 mm, L 7.9 mm) is somewhat different from Smith’s measurements, due to damage near the ventral margin of the left valve, and the marginal apron of the right valve is broken off .

Additional material examined. —AUSTRALIA: QUEENSLAND: ENE of Cape York Peninsula , 10°29'S 144°0'E, alive, 495–534 m (1 pr, C.166583) . WESTERN AUSTRALIA: Recherche Archipelago , 34°09'S 121°27'E, dead, 81 m (1 v, C.165557) ; SW of Cape Naturaliste , 33°44.5'S 114°26.1'E, dead, 183–238 m (22 v, C.165544; many v, C.165543) ; NW of Bunbury , 33°15'S 114°32'E, dead, 201–228 m (many v, WAM426.91 ) ; W of Bunbury , 33°03'S 114°44'E, dead, 156 m (18 v, C.165561) ; NW of Bunbury , 33°0'S 114°41'E, dead, 183 m (37 v, WAM431.91 ; 27 v, WAM432.91 ) ; W of Garden Island , 32°15'S 115°03'E, dead, 250–258 m (12 v, C.165549) ; SW of Rottnest Island , 32°05' S 115°04'E,alive, 583 m (1 pr, WAM456.91 ) ; W of Rottnest Island , 31°59.5'S 114°13'E, dead, 201–214 m (8 v, C.165552) ; W of Rottnest Island , 31°59'S 115°14' E, dead, 182 m (27 v, C.165541; 17 v,C.165542) ; NW of Rottnest Island, 31°45'S 115°02'E, dead, 265–276 m (5 v, C.165553); off Rottnest Island , 31°39.8'S 115°07.1' E, dead, 150 m (5 v, C.165556) ; N of Lancelin , 31°16'S 114°54'E, dead, 274 m (26 v, WAM441.91 ) ; NW of Rottnest Island , 31°44'S 115°03'E, dead, 183–192 m (4 v, C.165539) ; W of Rottnest Island , 30°59'S 114°51'E, dead, 125 m (5 v, WAM444.91 ) ; WNW of Lancelin , 30°58'S 114°51' E, dead, 183 m (many v, WAM428.91 ;28 v, WAM429.91 ) ; WNW of Lancelin , 30°58'S 114°49'E,dead, 212–215 m (many v, WAM430.91 ) ; off Cervantes Island , 30°32'S 114°41' E,dead, 238–247 m (8 v, C.165540) ; NW of Beagle Island , 29°43' S 114°17'E,dead, 174–283 m (4 v,C.165547); 200 ml WNW of Port Hedland , 19°29'S 115°38'E, dead, 320 m (12 v, C.165565); 100 n. ml NW of Port Hedland , 18°47'S 117°58'E, dead, 154 m (2 v, WAM460.91 ; 23 v, WAM461.91 ; 1 v, WAM462.91 ); 230 ml W of Roebuck Bay , 18°30' S 118°03'E, dead, 238 m (15 v, C.165567) ; S of Rowley Shoals , approx. 17°35'S 118°56'E, dead, 300 m, dead (9 v, WAM416.91 ; 1 v, WAM417.91 ) ; S of Rowley Shoals , approx. 17°35'S 118°56'E, dead, 266 m (1 v, WAM419.91 ; many v, WAM468.91 ; many v, WAM470.91 ; 1 v, WAM473.91 ) ; S of Rowley Shoals , approx. 17°35'S 118°56'E, dead, 260 m (many v, WAM423.91 ) .

Description. Shell fragile, small, up to c. 9 mm high, subcircular, inequivalve, left valve slightly more convex than right valve, auricles unequal in size, umbonal angle c. 100°, prodissoconch c. 200 µm in height. Internal riblets 10 in most specimens, some with 1 or 2 rudimentary interstitial riblets, plus 1 small auricular riblet on each side. Left valve translucent white, some specimens with white spots, right valve semi-translucent white.

Left valve glossy, very weakly sculptured with unevenly spaced commarginal growth lines; some specimens with minute, close-set commarginal lamellae near ventral margin. Auricles smooth; anterior auricle considerable larger than posterior. Dorsal margin straight.

Right valve with evenly spaced commarginal lirae, close-set near umbonal area, becoming more widely spaced towards ventral margin. Anterior auricle with 1–5 weak radial riblets, absent from posterior. Commarginal lamellae prominent near anterior margin, weaker near posterior margin. Dorsal margin somewhat raised towards ends due to prominent lamellae on antero- and postero-dorsal margins. Resilifer triangular. Byssal notch narrow.

Habitat. Living on the continental shelf and in the bathyal zone, free on soft sediment (sand, muddy sand or mud).

Distribution. Southern Philippine Islands , 350 m (Dijkstra, 1990a); eastern Indonesia, 170–212 m (Dijkstra & Kastoro, 1997); Chesterfield Islands , Coral Sea, 315–355 m; New Caledonia, 230–365 m, dead; Norfolk Ridge, 230 m, dead; Loyalty Islands , 100–600 m, dead; Vanuatu, 260–300 m; Wallis and Futuna Islands , 250–330 m, dead (Dijkstra, 1995b, 2001; Dijkstra & Maestrati, 2008); Fiji, 290–300 m; Tonga, 263–400 m; Solomon Islands , 225–281 m, dead (Dijkstra & Maestrati, 2008). Now also extended westwards into the Indian Ocean from the Mozambique Channel, in 180– 264 m. Maximum depth range of live-taken specimens is 170– 583 m. Present material from Australia taken in 81–320 m (dead) and 495–583 m (alive).

Remarks. Juvenile specimens of Parvamussium scitulum and P. torresi are easily confused. Both species are very weakly sculptured or even smooth, but the convexity of the left valves is different ( P. scitulum flattened, P. torresi more convex). The right valve of Parvamussium torresi is sculptured with weak commarginal lirae, which are lacking from most specimens of P. scitulum . The present material from Australia is morphologically identical to the type specimens from Torres Strait (NE Queensland).