Parvamussium scitulum (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 : 138-140

Parvamussium scitulum (E. A. Smith, 1885)

Figs 9, 10A–D,F

Amussium scitulum Smith, 1885: 312 , pl. 23, figs 4–4b.

Amusium (Propeamusium) [sic] scitulum var.? cmadoritinctum Kuroda, 1931: 77 , figs 81–82.

Ctenamusium (Ctenamusium) cmadoritinctum (Kuroda) .– Oyama, 1951: 80, pl. 13, figs 3–4.

Ctenammssium [sic] cmadoritinctum (Kuroda) .–Habe, 1964b: 173, pl. 53, fig. 4.

Ctenamussium [sic] scitulum (Smith) .– Koyama et al., 1981: 63.

Propeamussium cmadoritinctum (Kuroda) .–Hayami, 1988a: 479, figs 2–1, 2.

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

Parvamussium scitulum (Smith) .–Dijkstra, 1991: 16, figs 35–43; Dijkstra, 1995b: 31, figs 43–46, 153–154; Dijkstra, 1998a: 14, pl. 1, figs 3–6; Lamprell & Whitehead, 1992: [16], 179; Dijkstra & Kastoro, 1997: 261, figs 75–78; Hayami, 2000: 913, pl. 454, fig. 7; Dijkstra, 2001: 86; Dijkstra & Maestrati, 2008: 94; Xu & Zhang, 2008: 78, fig. 217; Dijkstra & Maestrati, 2010: 339; Huber, 2010: 224; Dijkstra, 2011: 42, pl. 1016, figs 5–8b; Dijkstra, 2013: 21, pl. 3, figs 2a–d, pl. 4, fig. 10; Dijkstra & Janssen, 2013: 192, figs 40–42; Dijkstra & Maestrati, 2013b: 474; Dijkstra & Maestrati, 2015: 605, figs 4A–C.

Comments on synonymy. Kuroda (1931: 77) described a possible new variation of the present species from Japan, Amusium scitulum var. cmadoritinctum , which is more colourful and much larger in size than the type material of Parvamussium scitulum . Smith (1885: 312) suggested that the type specimens from New Guinea are possibly juveniles. The present specimens are indeed larger than the type material and the colour is highly variable, so we agree with Kuroda (1931) that the larger, coloured specimens are part of the variation of P. scitulum .

Type data. Amussium scitulum Smith: lectotype (lv) designated by Dijkstra (1995b: 31) NHMUK 1887.2.9.3319/ 1; 6 paralectotypes (v) NHMUK 1887.2.9.3319/2–7. Type locality: S of New Guinea, 9°59'S 139°42'E, dead, 51 m (Challenger stn 188) .

Amusium scitulum var. cmadoritinctum Kuroda : holotype (lv) and paratype (rv), figured in Kuroda (1931: 72, fig. 81 [holotype] and fig. 82 [paratype]), Kikuchi Shell Museum, Kuroda collection, now Nishinomiya Shell Museum (not seen). Type locality: Japan, Kyushu , Kagoshima Prefecture, Yakushima Island .

Additional material examined. —AUSTRALIA: QUEENSLAND: N Gulf of Carpentaria, 10°05.2'S 139°28.5'E, dead, 37 m (8 v, C.165156); off Bowen, 19°48'S 148°53'E, dead, 62 m (many v, C.165485); GBR, Swain Reefs, 3 km NE of W side of Gillett Cay, 21°70'S 152°43.3' E, dead, 64–73 m (many v, C.165488; many v, C.165489; many v, C.165519, in part); SE of Swain Reefs, 22°20'09"– 22°26'16"S 153°17'05"E, dead, 187 m (many v, C.462540); SE of Swain Reefs, 22°31.4' S 152°32.6' E, dead, 100 m (1 v, C.157661, in part; 13 v, C.157662); SE of Swain Reefs, 22°32.4'S 152°33.9'E, dead, 92 m (2 v, C.338461); 4 ml E of North Reef, 23°09'S 151°58'E, dead, 64 m (1 v, C.165157); Capricorn Group, One tree Island, 23°30'S 152°05'E, dead, 55 m (14 v, C.165486; 2 v, C.165158); E of Lady Musgrave Island, 23°50.1'S 152°32.1'E, dead, 132 m (6 v, C.157665); E of Lady Musgrave Island, 23°51.2'S 152°34.5'E, dead, 175–200 m (12 v, C.338460); off S end of Frazer Island, 25°48'S 153°46'E, dead, 73 m (21 v, C.165490). WESTERN AUSTRALIA: W of Broome, approx. 17°58'S 122°14'E, dead, 90 m (many v, WAM 474.91; many v, WAM 420.91); N of Broome, approx. 17°58'S 122°14'E, dead, 56 m (13 v, WAM 421.91); 94 n. ml NNE of Port Hedland, 18°48'S 119°0'E, dead, 92–94 m (many v, C.157691); NNW of Dampier, c. 19°34'S 116°08'E, dead, c. 101 m (13 v, WAM 489.91). NORTHERN TERRITORY: Arafura Sea, c. 245 ml NE of Croker Island, 8°36'S 135°08'E, dead, 82 m (1 v, C.157726; 2 v, C.157725); Arafura Sea, c. 330 km NW of Wessel Islands , 8°26'S 135°22' E, dead, 75 m (11 v, C.157727; 12 v, C.157728);Arafura Sea, 180 ml NW of Wessel Islands , 8°56'S 135°25'E, dead, 73 m Figure 10. A–D, F, Parvamussium scitulum (Smith) , separate valves, AM C.165490, HMAS Kimbla stn Q11, off S end of Frazer Island, QLD, 25°48'S 153°46'E, 73 m; rv anterior auricle exterior (A), rv interior (B), lv auricles exterior (C), rv exterior (D), lv exterior (F).E, J–M, Parvamussium squalidulum Dijkstra , separate valves, AM C.462546, Lord Howe Island shelf, 31°36'S 159°09'E, 35 m; rv anterior auricle exterior (E), rv exterior (J), rv interior (K), lv exterior (L), lv anterior auricle exterior (M). G–I, Parvamussium slacksmithae sp. nov., paratype (G, H) AM C.209769 and holotype (I), AM C.157685, FRV Soela stn 26-18, 72 nautical miles NNW of Dampier,WA, 19°28.9'– 19°29.0'S 116°29.4'– 116°29.0'E, 110 m; rv exterior (G), rv anterior auricle exterior (H), lv exterior (I). Scale bars represent 1 mm (A, C), 4 mm (B, D, F, I, K), 0.5 mm (E, H, M), 5 mm (G, J, L).

(8 v,C.157724); NW Gulf of Carpentaria , 8°48'S 137°01'E,dead (1 v, C.338462); 125 km N of Goulburn Island, 10°23'S 133°36'E, dead, 62 m (20 v, C.165483) ; 41 km NE of Goulburn Island , 11°10.5'S 133°43'E, dead, 47 m (many v,C.165482) ; 84 km NE of Goulburn Island , 10°48.5'S 133°48'E, dead, 60 m (17 v, C.165484) . TASMAN SEA: Middleton Reef , 29°24'S 159°04'E, dead, 273.5 m (many v, C.461378) ; Middleton Reef , 29°29'S 159°05'E, dead, 117.5 m (many v,C.461379) ; Middleton Reef , 29°24'S 159°06'E, dead, 121 m (13 v, C.461388) ; Middleton Reef , 29°25'S 159°02.E, dead, 298 m (many v, C.461382) ; Middleton Reef , 29°25'S 159°05'E, dead, 167 m (12 v, C.461389;25 v, C.461395) ; Middleton Reef , 29°30' S 159°03'E, dead, 254 m (many v, C.461384) ; Middleton Reef , 29°30'S 159°07'E,dead, 295 m (29 v, C.461381 [in part]) ; Lord Howe Shelf , 31°25'S 159°02'E,dead, 58.5 m (2 v, C.461411) ; Lord Howe Shelf , 31°25'S 159°03' E, dead, 64 m (4 v, C.461405) ; Lord Howe Shelf , 31°26'S 159°07' E, dead, 96 m (many v, C.461380;1 v, C.461398) ; Lord Howe Shelf , 31°27'S 159°08'E, dead, 57.5 m (5 v, C.461406) ; Lord Howe Shelf , 31°30'S 159°05'E, dead, 48 m (1 v, C.461403) ; Lord Howe Shelf , 31°31'S 159°07'E, dead, 51.5 m (1 v, C.461397) ; Lord Howe Shelf , 31°36'S 159°09'E, dead, 35 m (27 v, C.462546) ; Lord Howe Shelf , 31°43' S 159°04'E, dead, 98 m (11 v, C.461391) ; Lord Howe Shelf , 31°44'S 159°04'E, dead, 117.5 m (9 v, C.462547) ; Lord Howe Island, Balls Pyramid , 31°44'S 159°11'E, dead, 53.5 m (3 v, C.461408) ; Lord Howe Island, Balls Pyramid , 31°44' S 159°18'E, dead, 78 m (1 v, C.461404) ; Lord Howe Island, Balls Pyramyd , 31°46'S 159°09'E, dead, 157.5 m (8 v, C.461386 [in part]) ; off Lord Howe Island , 31°34.95'S 159°0.3'E, dead, 73 m (many v, C.165487).

Description. Shell fragile, small, up to c. 12 mm high, subcircular, compressed, inequivalve, inequilateral, right valve more convex than left, auricles rather small and unequal, umbonal angle c. 120°, prodissoconch c. 210 µm in height. Left valve stained, opaque, right valve semitranslucent white.

Left valve very weakly inflated, weakly sculptured with delicate, variable radial striations, somewhat stronger near posterior margin than elsewhere. Microscopic close-set commarginal striae developed on disc of some specimens, somewhat more widely spaced near umbonal area. Anterior auricle with prominent commarginal lamellae close to flank of disc, decreasing in prominence or absent near antero-dorsal margin; posterior auricle with fine radial and commarginal striations, almost absent from some specimens.

Right valve either smooth or weakly sculptured with a few growth lines. Internal riblets extending to submarginal area, 9 or 10 in most specimens, some specimens with 1 or 2 rudimentary interstitial riblets. Resilifer triangular. Dorsal margin straight. Byssal notch narrow.

Habitat. Living in the sublittoral and bathyal zones, free on soft sediment (sand, muddy sand or mud with coral rubble or gravel).

Distribution. Japan to tropical western Pacific, 40–500 m (?alive) (Hayami, 2000); Indonesia, 50–385 m (dead) (Dijkstra, 1990a, 1991; Dijkstra & Kastoro, 1997); Papua New Guinea, 60–125 m; Chesterfield Islands , Coral Sea, 300 m; New Caledonia, 250–350 m; Loyalty Islands , 70–200 m; Wallis and Futuna Islands , 200–240 m (dead) (Dijkstra, 1995b, 1998a, 2001; Dijkstra & Maestrati, 2008); Fiji, 80–120 m; Tonga, 327–360 m (dead) (Dijkstra & Maestrati, 2008); Austral Islands , 200–620 m (dead) (Dijkstra & Maestrati, 2010). Now also extended westwards into the Indian Ocean from the Mozambique Channel, northwestern and southern Madagascar and northeastern South Africa, in 53–112 m (Dijkstra & Maestrati, 2015). Maximum depth range of live-taken specimens is 60– 350 m. Lamprell & Whitehead (1992: [16], 179) recorded this species from deep water off Australia, but without any locality data. The present material from Australia taken only dead at 37– 200 m.

Remarks. The present material from Australia is somewhat more variable in sculpture (smooth or almost smooth to typically striate) than the type specimens from eastern Indonesia.

Figure 11. Distribution of Parvamussium slacksmithae sp. nov. (circles), P. squalidulum Dijkstra (stars), P. torresi (Smith) (triangles) and P. whissoni sp. nov. (square).