Parvamussium minusculum, Dijkstra & Maestrati, 2017

Dijkstra, Henk H. & Maestrati, Philippe, 2017, New species and new records of littoral and bathyal living Pectinoidea (Bivalvia: Propeamussiidae, Cyclochlamydidae, Pectinidae) from the western and southwestern Pacific, Zoosystema 39 (4), pp. 473-485 : 476-477

Parvamussium minusculum n. sp.

( Fig. 1 H-N)

TYPE MATERIAL. — South China Sea. ZHONGSHA 2015, stn CP4157, 19°48’N, 116°29’E, 1205-1389 m, alive, holotype spm (MNHN-IM-2013-59427).

TYPE LOCALITY. — South China Sea, SW of DongSha, 19°48’N, 116°29’E, 1205-1389 m (ZHONGSHA 2015, stn CP4157).

ETYMOLOGY. — Latin minusculus, adjective meaning very small.

DESCRIPTION

Shell

Up to 3.1 mm high, fragile, transparent in early growth stage, after that opaque, anteriorly oblique, inequilateral, inequivalve, slightly convex, auricles unequal in shape, equal in size, umbonal angle 120°.

Prodissoconch

160 µm long, larval shell inflated and smooth.

Left valve

Smooth in early ontogeny with a few commarginal growth lines to 1.8 mm below umbonal top, after that regularly spaced, strongly raised commarginal lamellae commence (3 per mm)

with weak interstitial antimarginal riblets (8-10 per mm). Auricles with 5-6 commarginal lamellae, slightly more widely spaced on anterior than on posterior.

Right valve

With closely and regularly spaced, faint commarginal ridges, 10 per mm. Auricles smooth, anterior separated and posterior continuous with shell disc. Hinge line straight with 4 weak spines anteriorly.

Byssal notch

Moderately deep. Internal riblets 14, short and commencing in late growth stage, extending almost to the periphery.

Dimension of holotype

H 3.1 mm, W 3.6 mm, D 1.2 mm.

REMARKS

The present species has a unique oblique shape, which is seldom seen in Propeamussiidae . Parvamussium obliquum (E. A. Smith, 1885) , known from the lower bathyal tropical western Atlantic, is similar in size (3 mm), shape (anteriorly oblique) and internal ribbing (14 short riblets). However, P. minusculum n. sp. has a strongly sculptured left valve with commarginal lamellae and intercostal radial riblets in late ontogeny, whereas P. obliquum is smooth and translucent throughout.

A somewhat similar morphological congener (shape weakly anteriorly oblique, with short interior riblets) is Parvamussium propinquum (E. A. Smith, 1885) , known from the lower bathyal and upper abyssal eastern Atlantic, which differs from the present species in size ( P. minusculum n. sp. up to 3 mm in height, P. propinquum up to 7 mm), by having numerous delicate radial striae on the left valve and in internal ribbing ( P. minusculum n. sp. 14 ribs, P. propinquum 12).

Another somewhat close congener is Parvamussium permirum (Dautzenberg, 1925) , also known from the abyssal eastern Atlantic, which also differs from P. minusculum n. sp. in size ( P. minusculum n. sp. up to 3 mm, P. permirum up to 7 mm), by having a circular shape, by its sculpture of weak, widely spaced commarginal lamellae and numerous closely spaced radial riblets of similar strength, and by its 13 longer interior riblets commencing earlier in ontogeny and developing to the periphery ( P. minusculum n. sp. with 14 short riblets commencing in later growth stage and developing to the submarginal area).

Figures

FIG. 1. — A-D, F, G, holotype of Propeamussium regillum n. sp. (MNHN-IM-2013-58330): A, exterior rv; B, interior rv; C, exterior lv; D, interior lv; E, paratype (MNHN-IM-2013-58329), prodissoconch and early ontogeny of lv shell disc; F, exterior detail lv; G, exterior detail rv; H-N, holotype of Parvamussium minusculum n. sp.(MNHN-IM-2013-59427); H, exterior rv; I, interior rv; J, exterior lv; K, interior lv; L, prodissoconch and early ontogeny of lv shell disc; M, exterior detail lv; N, exterior detail rv. Abbreviations: see Material and methods. Scale bars: A-D, 10 mm; E, G, 200 µm; F, H-K, 1 mm; L, 50 µm; M, N, 100 µm.