Propeamussium regillum, 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 : 474-476

Propeamussium regillum n. sp.

( Fig. 1 A-G)

TYPE MATERIAL. — Papua New Guinea. KAVIENG 2014, stn CP4441, 02°17’S, 150°38’E, 684-791 m, alive, holotype spm (MNHN-IM-2013-58330). — Same data, paratype spm (MNHN-IM-2013-58329). — BIOPAPUA 2010, stn DW3680, 04°37’S, 149°27’E, 615-647 m, alive, paratype spm ( MNHN- IM-2013-57983). — PAPUA NIUGINI 2012, stn CP3981, 05°11’S, 147°03’E, 688 m, alive, paratype spm (MNHN-IM-2000-33120). — Stn CP 4014, 05°35’S, 148°13’E, 630-870 m, alive, 2 paratypes spms (MNHN-IM-2000-33119; RMNH.MOL.290821) .

TYPE LOCALITY. — Papua New Guinea, New Ireland, 02°17’S, 150°38’E, 684-791 m (KAVIENG 2014, stn CP441).

DISTRIBUTION. — Bismarck Sea, live in 615-870 m (maximum depth range), 647-688 m (minimum depth range).

ETYMOLOGY. — Left valve with lamellose radials (Latin regillus, adjective meaning vertical rows of warp-threads).

DESCRIPTION

Shell

Up to 31.1 mm high, fragile and transparent, almost circular, equilateral, inequivalve, slightly convex, left valve somewhat larger than right valve, auricles of left valve equal in shape and size, of right valve slightly unequal in shape, umbonal angle 100-110°.

Prodissoconch

240 µm long, larval shell inflated and smooth. Left valve

Smooth in early ontogeny to c. 10 mm below umbonal top, irregularly spaced radial ribbing commencing after that (c. 10- 15 ribs), increasing by intermediate ribs in later growth stage to the periphery (c. 50 ribs). Ribs with prominent lamellae; weak intercostal commarginal ridges on adults.

Auricles

With 3-4 delicate squamous radial riblets. Right valve

Sculptured with fine, regularly spaced, close-set commarginal lamellae (5 per mm). Internal ribs 8 (2 central ribs weak or lacking), and 1-4 intercostal rudimentary riblets marginally. Marginal apron broad (c. 6-8 mm) and partly broken off. Hinge line straight. Resilifer triangular. No byssal fasciole or byssal notch, ctenolium lacking. Colour of left valve creamy, right valve whitish.

Dimension of holotype

H 31.1 mm, W 30 mm, D 4.9 mm.

REMARKS

The present species is morphologically closest to the superficially similar (shape, colour) congener Propeamussium investigatoris (E. A. Smith, 1906) , known from the bathyal northern Indian Ocean and south-western Pacific ( Dijkstra & Kastoro 1997: 248, figs 11-15; Dijkstra & Maestrati 2008: 82, 2012: 391; Dijkstra & Marshall 2008: 2).

Both species differ in size ( P. regillum n. sp. up to 31 mm, P. investigatoris up to 26 mm but most specimens smaller, up to 20 mm in height).

The sculpture of the left valve of both species is also different, viz. the early growth stage of P. regillum n. sp. is smooth, P. investigatoris has a fine commarginal sculpture.

P. regillum n. sp. has finer and narrower radial ribs (c. 30 in adult growth stage) with prominent lamellae commencing 10 mm below umbonal top, and P. investigatoris has many more small radial ribs (c. 50 in adult growth stage) with fine lamellae ( Dijkstra & Kastoro 1997: fig. 15) commencing 1-2 mm below the umbonal top.

The internal ribbing of P. regillum n. sp. are more delicate with a broad marginal apron of the right valve (c. 6-8 mm wide), that of P. investigatoris are more solid and the apron is narrower (c. 2-3 mm wide).

Propeamussium jeffreysii (E. A. Smith, 1885) , known from the bathyal western Indian Ocean and western Pacific, is also similar in shape and colour, but differs from the present species by having an almost smooth shell disc of the left valve with delicate commarginal and radial sculpture in early ontogeny (smooth in P. regillum n. sp.) and commarginal lamellae in the later growth stage ( P. regillum n. sp. with coarse, irregularly spaced radial ribs with prominent lamellae).

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.