Parvamussium torresi (E.A. Smith, 1885 )

Dijkstra, H. H. & Maestrati, P., 2015, Pectinoidea (Bivalvia: Propeamussiidae and Cyclochlamydidae) from the southwestern Indian Ocean, African Invertebrates 56 (3), pp. 585-585 : 612-613

Parvamussium torresi (E.A. Smith, 1885)

Fig. 5A–C

Amussium torresi E.A. Smith, 1885: 311 , pl. 23, figs 3–3b. TYpe localitY: Australia, Queensland, E of Cape

York (11°38'15"S 143°59'38"E) , - 285 m, live, coral sand (Challenger, stn 185B).

Parvamussium torresi: Dijkstra 1995: 36 , figs 51–54, 125–128 [References, tYpe data, description,

distribution, comparison]; Dijkstra & Kastoro 1997: 262, figs 90–94; Dijkstra & Maestrati

2008: 95.

Description: Shell up to 10 mm in height, fragile, subcircular, inequivalve, left valve slightly more convex than right valve, auricles unequal in size and shape, umbonal angle 100–105°. Internal riblets 10 in most specimens, some with 1 or 2 rudimentary interstitial riblets, plus 1 small auricular riblet on each side. Colour of most specimens semi-transparent white, dull whitish or stained with reddish or creamy spots. Prodissoconch 200 µm high. Left valve in early growth stage glossy and transparent (c. 2 mm of shell disc), after that dull throughout, very weakly sculptured with unevenly spaced commarginal growth lines or smooth; some specimens with minute, very close-set commarginal lamellae laterally, sometimes also ventrally. Auricles smooth or weakly sculptured with very closely spaced lamellae; anterior auricle considerable larger than posterior, with 2–4 radial riblets and close-set commarginal lamellae. Hinge line 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. Hinge line somewhat raised due to prominent lamellae on antero- and postero-dorsal margins. Resilifer triangular. Byssal notch narrow. TYpe material examined: LectotYpe (pr) NHMUK 1887.2.9.3316, designated bY Dijkstra (1995: 36, figs 125–128), 5 paralectotypes (v) NHMUK 1887.2.9.3317/1-5.

Other material examined: MOZAMBIQUE CHANNEL: Maputo transect (25°56'S 33°07'E), - 193–194 m, dead, campaign Mainbaza, stn CP3131, 09.iv.2009 ( MNHN) ; Maputo transect (25°11'S 35°10'E), - 200– 201 m, live, campaign MAINBAZA, stn CP3133, 10.iv.2009 ( MNHN) ; Maputo transect (23°32'S 35°46'E), - 264–277 m, live, campaign Mainbaza, stn CP3143, 11.iv.2009 ( MNHN) ; Maputo transect (23°33'S 35°41'E) ,

- 171–180 m, live, campaign Mainbaza, stn CP3144, 11.iv.2009 ( MNHN); Inhambane transect (23°59'S 35°39'E) , - 206–210 m, live, campaign MAINBAZA, stn CC3160, 15.iv.2009 ( MNHN); Inhambane transect (24°02'S 35°41'E) , - 266–267 m, live, campaign Mainbaza, stn CC3161, 15.iv.2009 ( MNHN) .

Distribution and habitat: Southern Philippine Islands: - 350 m ( Dijkstra 1990 a); 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), Hebrides: - 260–300 m, Wallis and Futuna Islands: - 250–330 m (dead) (Dijkstra 1995; 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 in the southwestern Indian Ocean, the Mozambique Channel (new record). The present species is free-living on soft substrata of mud or muddy sand at a bathyal depth of - 170– 583 m. Present material live at - 180–264 m (minimum depth range).

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 slightlY more inflated). The right valve of P. torresi is sculptured with weak commarginal lirae, which are lacking from most specimens of P. scitulum . The present material from the Mozambique Channel is morphologically almost identical to the type specimens from Torres Strait (NE Queensland), but are slightly different in colour (with reddish or creamy dots, typical specimens are whitish), and have a prominent transparent and glossy shell disc in early growth stage of the left valve (typical specimens are more dull).

Figures

Fig. 5. Parvamussium spp.: (A–C) P. torresi (E.A. Smith, 1885): campaign Mainbaza, stn CP3143, (A) external view of the left valve, H = 7.8 mm, scale bar = 2 mm; (B) external view of the right valve, H = 6.6 mm, scale bar = 2 mm; (C) details of the left valve, H = 7.8 mm, scale bar = 0.1 mm (MNHN).(D–F) P.texturatum (Dautzenberg & Bavay, 1912):campaign Mainbaza, stn CC3151, (D) external view of the left valve, H = 5.7 mm, scale bar 2 mm; (E) external view of the right valve, H = 5.7 mm, scale bar 2 mm; (F) details of the left valve, H = 5.7 mm, scale bar 0.1 mm (MNHN). (G–I) P. vesiculatum Dijkstra, 1995: campaign Miriky, stn DW3196, (G) external view of a left valve (typical), H = 6.6 mm, scale bar = 0.2 mm; (H) external view of a left valve (atypical), H = 7 mm, scale bar = 0.2 mm; (I) details of the left valve (typical), scale bar = 0.1 mm (MNHN). (J–L) P.thyrideum (Melvill inMelvill & Standen,1907):expeditionAtimoVatae,stn DW3523,(J) external view of one left valve, H = 5.2 mm, scale bar = 1 mm (K) external view of one right valve, H = 3.1 mm, scale bar = 1 mm;; (L) details of the left valve, H = 5.2 mm, scale bar = 0.1 mm (MNHN).(M) P.vidalense (Barnard, 1964), holotype: SouthAfrica, KwaZulu-Natal, off CapeVidal, external view of the left valve, H = 7.3 mm, scale bar = 2 mm (SAM A9495).