Cyclopecten ambiguus, 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 : 340-341

Cyclopecten ambiguus n. sp.

( Fig. 2 K-P)

Cyclopecten cancellus – Tröndlé & Boutet 2009: 6 [listed from Austral Is as C. cancellus ].

TYPE MATERIAL. — Rapa. Atelier RAPA stn 43, 27°36.8’S, 144°18.3’W, 45 m, holotype lv ( MNHN 21385).— Same data, paratypes, 3 lv, 4 rv (4 MNHN 21386, 2 ZMA Moll. 4.09.011, 1 NMNZ M.287778).

TYPE LOCALITY. — Austral Islands, Rapa Island, Baie de Haurei, 27°36.8’S, 144°18.3’W, 45 m (Atelier RAPA, stn 43).

ETYMOLOGY. — From the transforming sculpture (commarginal in juvenile stage, reticular in adult stage) on the left valve (Latin ambiguus , adjective meaning changeable, variable or inconstant).

OTHER MATERIAL EXAMINED. — Rapa. Atelier RAPA, stn 30, 27°38.2’S, 144°18.2’W, 16-20 m, 7 lv, 2 rv. — Stn 44, 27°36.3’S, 144°18.2’W, 30 m, 4 lv. — Stn 47, 27°36.7’S, 144°19.1’W, 33 m, 1 rv. — Stn 48, 27°34.1’S, 144°22.1’W, 36 m, 5 lv, 3 rv (1 ZMA). Banc Arago. BENTHAUS, stn DW 1979, 23°22’S, 150°44’W, 176-340 m, 1 lv.

DISTRIBUTION. — Austral Islands, only single valves at 16-176 m depth.

DESCRIPTION

Shell up to c. 4 mm high, circular, inequivalve, equilateral, left valve slightly more inflated than right valve (nearly flat), auricles nearly equal in size, unequal in shape, opaque whitish (lv), translucent (rv).

Left valve disc sculpture commencing immediately by very closely spaced commarginal lirae (c. 20 per mm) to 1 mm from the top margin, followed by a reticulate sculpture of widely spaced radial lirae (12 or 13) and overrunning commarginal lirae (c. 5 per mm), forming weak nodules on the intersections. Secondary intercalated radial lirae commence on the central part of the disc. Anterior and posterior auricle with 4 or 5 strongly developed commarginal lirae and one weak radial lira.

Right valve disc with very closely spaced commarginal lirae in early growth stage, gradually slightly wider to the ventral margin. Auricles with strong commarginal lirae (numerous on anterior, 9 or 10 on posterior).Hinge line straight. Byssal notch relatively deep, byssal fasciole rather broad. Internal rudimentary riblets near ventral area in adult stage.

Dimensions of holotype: H 3.0 mm, W 3.0 mm.

REMARKS

The present species is morphologically closest (similar size, shape and reticulated sculpture) to Cyclopecten cancellus Dijkstra, 1991 , from the southwestern Pacific (and was indeed listed from the Australs by Tröndlé & Boutet as C. cancellus ), but differs in having very closely spaced lirae in early growth stage (c. 20 per mm in C. ambiguus , vs c. 6 or 7 in C. cancellus ), a weaker sculpture of radial lirae, lacking radial sculpture on the auricles ( C. cancellus 2 or 3), and rudimentary inner riblets (usually lacking in typical C. cancellus ).

Cyclopecten thyrideus ( Melvill, 1907) , from the northwestern Indian Ocean is also close to C. ambiguus (somewhat similar in size, shape, sculpture and internal rudimenrary riblets), but differs in having a fine microscopic radial sculpture in early growth stage immediately below the dissoconch stage (a commarginal sculpture in C. ambiguus ), weaker commarginal lirae on the left valve and fewer commarginal lirae on the anterior auricle of the right valve).

Cyclopecten ryukyuensis Hayami & Kase, 1993 , from southern Japan and the Philippines is similar in size, shape and sculpture, but differs in having widely spaced commarginal lirae and irregular antimarginal intercalated scratches (see Hayami & Kase 1993: figs 195, 197) in early growth stage on the left valve (only closely spaced commarginal lirae in C. ambiguus ), fewer intermediate secondary radial lirae, and in lacking internal rudimentary riblets.

Finally, Cyclopecten secundus (Finlay, 1926) , from New Zealand, also has a somewhat similar sculpture in early growth stage as C. ryukyuensis , although with more closely spaced (but much wider and stronger than in C. ambiguus ), more prominent commarginal lirae with strong lamellae on the intersections (almost lacking in C. ambiguus ) and sometimes 1 or 2 internal rudimentary riblets near the periphery (usually and more numerous in C. ambiguus ).

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.