Parvamussium australanum, Dijkstra & Maestrati, 2010

Dijkstra, Henk H. & Maestrati, Philippe, 2010, Pectinoidea (Mollusca, Bivalvia, Propeamussiidae, Entoliidae and Pectinidae) from the Austral Islands (French Polynesia), Zoosystema 32 (2), pp. 333-358 : 335-336

Parvamussium australanum n. sp.

( Fig. 2 A-E)

TYPE MATERIAL. — Rapa. BENTHAUS, stn CP 1891, 27°37’S, 144°15’W, 800-850 m, holotype spm ( MNHN 21377). — Same data, paratypes, 14 spms, 31 lv, 6 rv (46 MNHN 21378, 3 ZMA Moll. 4.09.010, 2 NMNZ M.287777).

TYPE LOCALITY. — Austral Islands, east of Rapa Island, 27°37’S, 144°15’W, 800-850 m (BENTHAUS, stn CP 1891).

ETYMOLOGY. — After the Austral Islands.

OTHER MATERIAL EXAMINED. — Rapa. BENTHAUS, stn CP 1892, 27°39’S, 144°16’W, 742-1000 m, 11 spms, 6 lv. — Stn CP 1909, 27°39’S, 144°16’W, 783-1000 m, 1 spm.

Banc Nord-Est Rapa. BENTHAUS, stn DW 1904, 27°27’S, 144°03’W, 600-900 m, 1 lv.

Banc Président Thiers. BENTHAUS, stn DW 1932, 24°41’S, 146°02’W, 500-800 m, 1 rv.

Tubuai. BENTHAUS, stn CP 1965, 23°21’S, 149°34’W, 500-1200 m, 1 spm.

Banc Arago. BENTHAUS, stn DW 1969, 23°22’S, 150°43’W, 200-640 m, 1 lv. — Stn DW 1972, 23°22’S, 150°43’W, 500-1020 m, 1 spm, 5 lv, 1 rv. — Stn DW 1975, 23°24’S, 150°44.3’W, 600-691 m, 1 rv.

Rurutu, Mont du Lotus. BENTHAUS, stn DW 1991, 22°36’S, 151°00’W, 470-780 m, 4 lv, 4 rv. — Stn DW 1992, 22°34’S, 151°00’W, 442-444 m, 1 lv.

Rurutu. BENTHAUS, stn DW 2004, 22°28’S, 151°19’W, 430-850 m, 2 lv. — Stn DW 2010, 22°32’S, 151°21’W, 520-950 m, 1 spm, 5 lv, 1 rv.

DISTRIBUTION. — Austral Islands, 200-800 m, alive at 500-800 m depth.

DESCRIPTION

Shell up to c. 12 mm in height, fragile, opaque white to semi-transparent, glossy near umbonal area, left valve slightly more convex than right valve, nearly circular, somewhat higher than wide, inequivalve, nearly equilateral, auricles unequal in shape and size (anterior larger than posterior).

Left valve sculptured with ventrally curled, distinct narrow commarginal lamellae (sometimes to hollow bands), commencing at c. 4 mm from umbonal margin, widely spaced (c. 1 mm apart on central part of disc), extending and more closely spaced to the ventral area,transforming to very closely spaced commarginal lamellae near the periphery.Interstitial,closely spaced, weak radial threads commence on the central part of the disc, more prominent laterally. Anterior auricle with rather coarse commarginal lamellae, posterior auricle nearly smooth, weak closely spaced, delicate commarginal lamellae near the margin.

Right valve with closely spaced, regularly arranged weak commarginal lirae. Marginal apron (c. 1 mm high) pressed to left valve, broken off many specimens. Anterior auricle with coarse closely spaced commarginal lamellae, posterior more delicate. Hinge line straight. Byssal notch shallow, byssal fasciole narrow, ctenolium lacking. Internally 10 or 11 radial ribs (sometimes lacking on the central part and only rudimentary laterally), commencing c. 3-5 mm from umbonal margin, extending nearly to ventral margin.

Dimensions of holotype:H 11.5 mm, W 11.2 mm, D 2.4 mm.

REMARKS

The commarginal sculpture of the left valve of the present species strongly resembles the sculpture of Cyclopecten cincinnatus Dijkstra & Gofas, 2004 , from the northeastern Atlantic ( Dijkstra & Gofas 2004: 48, fig.11A, B, D), however the commarginal sculpture of C. cincinnatus already starts in early growth stage (c. 0.1 mm, 4 mm of P. australanum ). Moreover, C. cincinnatus lacks close-set commarginal lamellae near the ventral margin and also lacks internal ribs.

The commarginal sculpture of P. australanum also somewhat resembles two other bathyal Pacific species, P. multiliratum Dijkstra, 1995 (see above) and Parvamussium retiaculum Dijkstra, 1995 , but both species have more closely spaced delicate lamellae. Moreover, P. multiliratum lacks radial sculpture and has more interior ribs (c. 14 and rudimentary). P. retiaculum has more widely spaced radial riblets, a larger anterior auricle of the right valve and coarser interior ribs than P. australanum .

Figures

FIG. 2.— A-E, Parvamussium australanum n. sp.;A, B, holotype,external and internal views of the left valve;C, D, holotype,external and internal views of the right valve; E, details of the left valve; F-J, Parvamussium dautzenbergi (Dijkstra,1990); F, G, external and internal views of the left valve; H, I, external and internal views of the right valve; J, details of the left valve; K-P, Cyclopecten ambiguus n. sp.; K, L, holotype,external (SEM) and internal views of the left valve;M, N, external and internal views of the right valve;O, prodissoconch and early stage of the left valve (SEM); P, details of the left valve (SEM). Scale bars: O, 100 μm, P, 200 μm.