Parvamussium formosum, : 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 : 601-602

Parvamussium formosum (Melvill in Melvill & Standen, 1907)

Fig. 3A–B, J–K

Amussium formosum Melvill in Melvill & Standen, 1907: 807, pl. 55, figs 7, 7a. TYpe localitY: Persian Gulf , Gulf of Oman (24°58'N 56°54'E), - 285 m, dead, leg. F. W. Townsend.

Propeamussium sp. cf. scitulum (Smith) : Oliver 1992: 70, text-fig. 5, 73, pl. 12, fig. 4 [not Amussium scitulum E.A. Smith, 1885 ].

Parvamussium formosum: Dijkstra & Knudsen 1998: 45 , pl. 2, figs 5–8; Dijkstra & Janssen 2013: 191, figs 20–22, 39.

Description: Shell up to 10 mm in height, fragile, semi-transparent, subcircular, somewhat higher than wide, inequivalve, left valve slightly more convex than right valve, equilateral, auricles unequal, umbonal angle 100°–105°, colour whitish. Prodissoconch 120 µm long. Left valve almost smooth (central part of shell disc) or weakly sculptured with numerous, irregularlY spaced, fine radial riblets, somewhat more prominent laterallY, sometimes with very weak, closely spaced, commarginal lamellae near periphery or on radial riblets. Anterior auricle larger than posterior and sculptured with fine, close-set antimarginal and commarginal lamellae, posterior auricle nearly smooth or sculptured with very weak antimarginal lirae. Right valve with very weak, regularly spaced, commarginal lirae.Anterior auricle fine commarginallY or reticulate sculptured, posterior auricle smooth or with verY fine commarginal lirae. BYssal notch moderatelY deep, byssal fasciole rather small, no ctenolium. Hinge line straight. Resilifer triangular. Internal ribs usually 10 (sometimes with 1–7 rudimentary interstitial riblets in adult specimens) and 1 anterior and 2 posterior auricular riblets. Internal ribs commencing in late growth stage and extending to the submarginal area. Internal posteriorly placed ribs on left valve somewhat oblique.

Type material examined: Two syntypes (rv) NHMUK 1907.5.3.50, 26 syntypes (v) NMW 1955.158.674. Several syntypes deposited in other museums (Dijkstra, personal observation).

Other material examined: NW MADAGASCAR: West of Cap d’Ambre (12°08'S 48°56'E), - 238–249 m, dead, campaign Miriky, stn DW3196, 28.vi.2009 ( MNHN); west of Cap d’Ambre (12°07'S 48°58'E), - 362–431 m, dead, campaign Miriky, stn DW3197, 28.vi.2009 ( MNHN); between Nosy-Bé and Banc du Leven (12°55'S 48°11'E), - 260–319 m, live, campaign Miriky, stn DW3228, 02.vii.2009 ( MNHN); between Majunga and Cap Saint-André (15°35'S 45°43'E), - 177–199 m, dead, campaign Miriky, stn DW3259, 10.vii.2009 ( MNHN). S MADAGASCAR:Between Lokaro and Sainte Luce (24°53'S 47°28'E), - 184–203 m, dead, expedition Atimo Vatae, stn DW3515, 30.iv.2010 ( MNHN); Manantenina (24°23'S 47°31'E), - 200–220 m, dead, expedition Atimo Vatae, stn DW3523, 01.v.2010 ( MNHN); Manantenina (24°23'S 47°32'E), - 395–407 m, dead, expedition Atimo Vatae, stn DW3525, 01.v.2010 ( MNHN); S of Faux Cap (26°07'S 45°39'E), - 264–280 m, dead, expedition Atimo Vatae, stn DW3552, 05.v.2010 ( MNHN); S of Faux Cap (26°09'S 45°40'E), - 455– 458 m, dead, expedition ATIMO VATAE, stn DW3555, 05.v.2010 ( MNHN); E of Faux Cap (25°38'S 46°13'E), - 128–133 m, dead, expedition Atimo Vatae, stn CP3561, 06.v.2010 ( MNHN); E of Faux Cap (25°37'S 46°18'E), - 347–355 m, dead, expedition Atimo Vatae, stn DW3563, 06.v.2010 ( MNHN); E of Faux Cap (25°37'S 46°20'E), - 433–456 m, dead, expedition Atimo Vatae, stn DW3564, 06.v.2010 ( MNHN); E of Faux Cap (25°36'S 46°21'E), - 618–624 m, dead, expedition Atimo Vatae, stn DW3565, 06.v.2010 ( MNHN); E of Faux Cap (25°37'S 46°21'E), - 618–624 m, dead, expedition Atimo Vatae, stn CP3566, 06.v.2010 ( MNHN); S Pointe Barrow (25°30'S 44°16'E), - 209–229 m, dead, expedition Atimo Vatae, stn DW3581, 10.v.2010 ( MNHN); SE Pointe Barrow (25°28'S 44°25'E), - 203–210 m, dead, expedition Atimo Vatae, stn CP3584, 10.v.2010 ( MNHN); S Cap Sainte Marie (26°13'S 45°08'E), - 225–282 m, dead, expedition Atimo Vatae, stn CP3613, 14.v.2010 ( MNHN); S Cap Sainte Marie (26°13'S 45°08'E), - 250–300 m, dead, expedition Atimo Vatae, stn CP3614, 14.v.2010 ( MNHN); S Cap Sainte Marie (26°14'S 45°09'E), - 284–286 m, dead, expedition Atimo Vatae, stn CP3615, 14.v.2010 ( MNHN); SE Faux Cap (25°47'S 46°02'E), - 133–178 m, dead, expedition Atimo Vatae, stn CP3620, 15.v.2010 ( MNHN). MOZAMBIQUE CHANNEL: Inhambane transect (23°59'S 35°39'E), - 206–210 m, live, campaign Mainbaza, stn CC3160, 15.iv.2009 ( MNHN). SOUTH AFRICA: KwaZulu­Natal: Off Durban (29°53.4'S 31°11.2'E), - 270 m, dead, verY fine sand, solitarY corals, NMDP stn XX16, dredged RV Meiring Naudé, 15.vi.1983 .

Distribution and habitat: Red Sea, Gulf of Aden, Arabian Gulf and Persian Gulf. Now also extended southwards to the Mozambique Channel, northwestern and southern Madagascar and South Africa (new record). Living bathyally on soft substrata (mixed sediments of mud and sand). Present specimens live at - 260– 319 m.

Remarks: The present specimens from northwestern and southern Madagascar and South Africa are almost identical to the type material.The present specimens are slightly different in having a more coarse commarginal sculpture on the anterior auricle of the left valve (typically more delicate and closer spaced), and in having in a few specimens faint commarginal sculpture on the central disc shell of the left valve (in typical specimens lacking), but other morphological characters are similar.

Smooth juvenile specimens of P. formosum and P. siebenrocki could be easily confused, although the latter species has more prominent radial and wider spaced commarginal sculpture laterally and on the auricles.

A closely resembling species is Parvamussium thetidis (Hedley, 1902) , recorded from tropical eastern Australia, the Coral Sea as far east as the New Hebrides Archipelago. This species is living bathyally deeper at - 500–650 m (Dijkstra 1995: 35). Parvamussium thetidis has a delicate radial and commarginal sculpture on the left valve ( P. formosum almost lacks commarginal sculpture), although the radial riblets are more prominently developed and the commarginal lamellae more closely spaced throughout the shell disc, and fewer internal ribs ( P. formosum 10 + 1–7 rudimentary, P. thetidis 10 + sometimes one rudimentary).

Figures

Fig.3. Parvamussiumspp.: (A, B, J, K) P.formosum (Melvill inMelvill &Standen,1907):campaign Miriky, stn CP3228, (A) external view of the left valve, H = 9 mm, scale bar = 2 mm; (B) external view of the right valve, H = 7.8 mm, scale bar = 2 mm; (J) details of the left valve, campaign Miriky, stn CP3228, H = 9 mm, scale bar = 0.1 mm; (K) details of the right valve, H = 7.8 mm, scale bar = 0.1 mm (MNHN). (C–I) P. kilburni sp. n., holotype: N. Zululand, off Dog Point, (C) external view of the left valve, H = 10.5 mm, scale bar = 2 mm; (D) external view of the right valve, H = 9.1 mm, scale bar = 2 mm; (E) internal view of the left valve, H = 10.5 mm, scale bar = 2 mm; (F) internal view of the right valve, H = 9.1 mm, scale bar = 2 mm; (G) prodissoconch of the left valve, H = 10.5 mm, scale bar = 0.2 mm; (H) details of the left valve, H = 10.5 mm, scale bar = 0.1 mm; (I) details of the right valve, N. Zululand, off Dog Point, H = 10.5 mm, scale bar = 0.1 mm (NMSA D7689).