Parvamussium thyrideum, : Dijkstra & Knudsen, 1998

Dijkstra, H. H. & Maestrati, P., 2015, Pectinoidea (Bivalvia: Propeamussiidae and Cyclochlamydidae) from the southwestern Indian Ocean, African Invertebrates 56 (3), pp. 585-585 : 611-612

Parvamussium thyrideum (Melvill in Melvill & Standen, 1907)

Fig. 5J–L

Pecten thyrideus Melvill in Melvill & Standen, 1907: 809, pl. 55, figs 10–10a. TYpe localitY: Gulf of Oman

(24°58'N 56°54'E), - 285 m, dead, shell-sand.

Propeamussium (Parvamussium) thyrideum: Oliver 1992: 70 , 73, text-fig. 6.

Parvamussium thyrideum: Dijkstra & Knudsen 1998: 49 , pl. 2, fig. 9; Dijkstra & Janssen 2013: 194, figs

46–47.

Description: Shell up to 5 mm in height, flattened, subcircular, left valve slightlY more convex than right valve, opaque or semi-transparent, inequivalve, equilateral, auricles unequal, umbonal angle 110°, colour whitish or creamy with white dots. Prodissoconch 140 µm long. Left valve sculptured with 13–17 prominent, irregularly spaced, radial costae, commencing c. 1 mm below umbonal top and extending to the ventral margin, and weaker, more regularlY and widelY spaced commarginal lamellae, with fine scales on intersections. Secondary interstitial radial riblets are developed marginally. Preradial sculpture of delicate, close-set, commarginal lamellae. Auricles with prominent commarginal lamellae and weak antimarginal lirae. Right valve sculptured with very fine, closelY spaced, commarginal lirae.Anterior auricle with 5–8 delicate antimarginal lirae, near dorsal margin crossed by commarginal lamellae. Posterior auricle with prominent commarginal lamellae. Byssal notch moderately deep, byssal fasciole small, no ctenolium. Hinge line straight. Resilifer broadly triangular. Variable rudimentary internal radial riblets are developed submarginally or even absent.

Type material examined: Syntypes NHMUK 1907.5.33-37, NMW 1955.158.679. Several syntypes are deposited in other museums (Dijkstra, personal observation).

Other material examined: S MADAGASCAR: Between Lokaro and Sainte Luce (24°51.9'S 47°28'E), - 80–83 m, dead, expedition Atimo Vatae, stn DW3519, RV Nosy Bé 11, 30.iv.2010 ( MNHN); Manantenina (24°23'S 47°31'E), - 200–220 m, dead, expedition Atimo Vatae, stn DW3523, RV Nosy Bé 11, 01.v.2010 ( MNHN) .

Distribution and habitat: Red Sea, Gulf of Aden,Arabian Sea and Persian Gulf. Now also from southern Madagascar (new record). Upper bathyal depths. Living on soft substrata (mud, muddy sand or sand with shell grit). Present specimens dead at - 80– 220 m.

Remarks: The present specimens of southern Madagascar are almost similar to the type material. The specimen of stn DW3519 has internal rudimentary radial riblets, which are lacking in the type material. These riblets are strongly variable and only developed in adult specimens.

A close congener is Parvamussium cancellus (Dijkstra, 1991) , recorded from the southwest Pacific, which differs morphologicallY from P. thyrideum by having a more prominent reticulate sculpture on the left valve with fewer intercalated secondary radial riblets.

Parvamussium cancellus was previously placed in Cyclopecten , based on juvenile and immature material, which lack internal rudimentary riblets. However, examined adult specimens of both species have rudimentary internal riblets (Dijkstra, pers. obs.).

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).