Propeamussium boucheti, Dijkstra & Maestrati, 2008

Dijkstra, Henk H. & Maestrati, Philippe, 2008, New species and new records of deep-water Pectinoidea (Bivalvia: Propeamussiidae, Entoliidae and Pectinidae) from the South Pacific, Mémoires du Muséum national d'Histoire naturelle 196, pp. 77-114 : 79-82

Propeamussium boucheti n. sp.

Figs 1-4

TYPE MATERIAL. — Holotype (spm) MNHN 20441 and 25 paratypes MNHN 20442-20447 , 2 HD.

TYPE LOCALITY. — Fiji, Bligh Water , 16˚52.51’S, 177˚43.66’E, 500-614 m [MUSORSTOM 10: stn CP 1341] .

MATERIAL EXAMINED. — Fiji. MUSORSTOM 10: stn CP 1330, 17˚10’S, 177˚56’E, 567-699 m, 1 lv (paratype MNHN 20442 ); stn CP 1341, 16˚53’S, 177˚44’E, 500-614 m, 6 spms, 1 lv, 2 rv (holotype MNHN 20441 and 7 paratypes MNHN 20443 , 1 HD) . — BORDAU 1 : stn CP 1409, 16˚02’S, 179˚30’W, 557-558 m, 4 spms (paratypes MNHN 20444 ); stn CP 1420, 17˚05’S, 178˚57’W, 550-687 m, 6 spms (paratypes MNHN 20445 , Figs 3-4) .

Tonga. BORDAU 2: stn CP 1556, 20˚11’S, 174˚45’W, 589- 591 m, 2 spms, 1 lv (paratypes MNHN 20446); stn CP 1558, 20˚10’S, 174˚43’W, 580-593 m, 5 spms (4 paratypes MNHN 20447, 1 HD).

DISTRIBUTION. — This species is so far only known from the easternmost regions of Melanesia, around Fiji and Tonga; live in 558-589 m.

DESCRIPTION. — Shell up to c. 65 mm in height, rather solid, circular, inequivalve, somewhat inequilateral, umbonal angle c. 125˚, auricles short, small and equal in size, opaque to semi-translucent, left and right valve pale brownish, auricles of some specimens orange. Prodissoconch c. 250 Μm in height.

Left valve more convex than right, nearly smooth, bearing only delicate commarginal growth lines, more prominent and closely spaced near the ventral margin. Anterior and posterior auricles with close-set commarginal growth lines. Generally 12-14 interior ribs (on left valve slightly smaller than on right) and an auricular riblet on each side, commencing just below resilifer and extending to pallial line, with one or two intermediate rudimentary riblets.

Right valve with fine and regularly, closely spaced commarginal lirae and granulate interstitial microsculpture (prismatic calcite layer). Marginal apron broken off of many specimens. Anterior and posterior auricles with close-set commarginal growth lines. Resilifer triangular.

Dimensions of holotype: H 65.6, L 65.8, D 8.9 mm.

REMARKS. — Propeamussium boucheti is morphologically closest to Propeamussium watsoni , but differs in size, external sculpture, internal ribbing and colour. The left valve of P. boucheti is almost smooth (with only commarginal growth lines), while that of P. watsoni is radially sculptured in the early growth stages and commarginally sculptured in later ontogeny. Propeamussium boucheti has more internal ribs (12-14, sometimes more, with interstitial rudimentary riblets) than P. watsoni (8-10, generally 10, without rudimentary riblets). Propeamussium boucheti is usually creamy or orange in colour, rarely whitish, whereas P. watsoni is whitish. Both species are semi-transparent, sometimes opaque.

Propeamussium boucheti differs from P. alcocki in having a more solid shell ( P. alcocki is very fragile). It also differs in shape, P. boucheti being almost circular, somewhat wider than high, whereas P. alcocki is higher than wide. Both species lack external sculpture and only have commarginal growth lines on the left valve. The internal ribs of P. boucheti are prominent and more numerous than those of P. alcocki .

Propeamussium boucheti and P. sibogai are similar in shape but P. boucheti is more solid and larger. The two are similar in colour but P. sibogai is more transparent. Propeamussium boucheti has more internal ribs but they are less prominent than in P. sibogai . Moreover, the internal ribs of P. boucheti are whitish, while those of P. sibogai are brownish.

Propeamussium boucheti is compared with other species of Propeamussium as follows: P. boucheti P. alcocki P. watsoni P. sibogai

Shape circular elongate

Height 65 mm 50 mm

Shell semi-translucent translucent

Colour yellowish whitish circular circular

60 mm 55 mm

semi-translucent translucent

whitish yellowish

lv smooth commarginal growth lines radial and commarginal sculpture smooth

Int. ribs 12-14 8-10

Int. ribs strong delicate

8-10 6-7

delicate very strong

FIGS 1-10. 1-4, Propeamussium boucheti n. sp., holotype MNHN 20441, H 65.6 mm, L 65.8 mm, MUSORSTOM 10 stn CP 1341; 1, left valve, exterior; 2, right valve, exterior; 3, paratype MNHN 20445, BORDAU 1 stn CP 1420, left valve,interior, H 60.5 mm, L 61 mm; 4, right valve, interior, H 57 mm, L 50 mm (marginal apron broken off). 5-10, Parvamussium biformatum n. sp., holotype MNHN 20448, H 4.6 mm, L 4.9 mm, SALOMON 1 stn DW 1770; 5, left valve, exterior; 6, right valve, exterior; 7, left valve, interior; 8, right valve interior; 9, left valve, microsculpture, scale bar 200 Μm; 10, right valve, microsculpture, scale bar 500 Μm.

ETYMOLOGY. — Named after Dr Philippe Bouchet ( MNHN), leader of several French expeditions to the South Pacific, who made this pectinoid material available for study.