Parvamussium texturatum (Dautzenberg & Bavay, 1912)

Dijkstra, H. H. & Maestrati, P., 2015, Pectinoidea (Bivalvia: Propeamussiidae and Cyclochlamydidae) from the southwestern Indian Ocean, African Invertebrates 56 (3), pp. 585-585 : 610-611

Parvamussium texturatum (Dautzenberg & Bavay, 1912)

Fig. 5D–F

Amussium texturatum Dautzenberg & BavaY, 1912: 37, pl. 27, figs 19–22; Barnard, 1964: 432. TYpe localitY:

Philippines, Sulu Sea (6°08'N 121°19'E), - 275 m, live, coral bottom, dredge, 04.vii.1899 ( Siboga ,

stn 105).

Parvamussium texturatum: Dijkstra 1995: 34 , figs 103–106 [references, tYpe data, description, distribution];

Dijkstra & Maestrati 2013 b: 474.

Description: Shell up to 8 mm in height, fragile, almost circular, nearly opaque, weakly inflated, inequivalve, inequilateral, valves almost equallY convex, auricles unequal in shape and size (anterior much larger than posterior), umbonal angle 95°–110°. Colour whitish or creamy with a few white or brown spots. Prodissoconch 200 µm high. Left valve only sculptured with prominent commarginal lamellae in early growth stage (pre-radial stage below dissoconch). Irregularly arranged radial costae (c. 10) commence after 1–2 mm below the umbonal top, increasing by secondary intercostal radial costulae, enlarging towards the ventral margin. Strongly developed commarginal lamellae are crossing the radial costae, forming more or less prominent squamae on the intersections. Auricles with very prominent and close-set commarginal lamellae, somewhat weaker on posterior auricle. Right valve sculptured with regularly spaced, somewhat scaly, prominent commarginal lamellae. Anterior auricle with c. 4–7 radial riblets and weak overrunning commarginal lamellae. Posterior auricle with prominent close-set commarginal lamellae. Internal ribs, variable in length (in most specimens short) and number (c. 10 + 1–6 rudimentary), commencing generally in late ontogeny. Byssal notch narrow.

Type material examined: Holotype (pr) ZMA Moll. 3.12.022.

Other material examined: MOZAMBIQUE CHANNEL: Zambeze transect (19°34'S 36°45'E), - 352–357 m, dead, campaign Mainbaza, stn CC3151, 13.iv.2009 ( MNHN).S MADAGASCAR: S Pointe Barrow (25°32'S 44°16'E), - 549–576 m, dead, expedition Atimo Vatae, stn CP3585, RV Nosy Bé 11, 10.v.2010 ( MNHN) .

Distribution and habitat: Philippines, Indonesia, Solomon Islands, Coral Sea, New Caledonia, Loyalty Islands, and southwestern Indian Ocean (Dijkstra 1995: 34; Dijkstra & Maestrati 2013 b: 474). Living on soft substrata of mud and sand at - 240– 450 m. Present material only dead at - 352– 576 m.

Remarks: The present specimens are morphologically close to the holotype of the Sulu Sea, although the intercostal commarginal lamellae on the left valve are weaker, the scaly commarginal lamellae on the right valve lacking, and the internal ribs commencing in an earlier growth stage. However, the sculpture and also the internal ribs are rather variable and these variable characters are also examined in material from the southwestern Pacific (MNHN). A morphologically close congener is Parvamussium conspectum Dijkstra & Kastoro, 1997 , known from eastern Indonesia.This species differs from P. texturatum by having a more elongated shape ( P. texturatum is almost circular), coarser commarginal sculpture in early growth stage, and fewer internal ribs ( P. conspectum 9, P. texturatum 10 + 1–6 rudimentary).

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).