Laubiericoncha chuni ( Thiele & Jaeckel, 1931 ) Cosel & Olu, 2008

Laubiericoncha chuni ( Thiele & Jaeckel, 1931) n. comb.

( Figs 3 View FIG ; 4 View FIG ; 5A, B View FIG ; 7B View FIG )

Vesicomya chuni Thiele & Jaeckel, 1931: 228 , pl. 9 (4), fig. 100.

Vesicomya (Callogonia) chuni – Boss 1970: 68, 69, figs 1, 2, 21, 24.

“ Vesicomya ” chuni – Cosel & Salas 2001: 356-358, figs 74, 75, 96, 97, 107.

TYPE MATERIAL. — Lectotype, a lv., selected by Boss (1970: 68) ( ZMB Moll. 77848a); paralectotype: same locality, a rv., ( ZMB Moll. 77848a), both trawled by RV Valdivia . The numerous additional valves from the same locality ( ZMB Moll. 77848b) were not mentioned by Boss (1970) apparently he may not have seen them. These valves are also paralectotypes.

TYPE LOCALITY. — W of Campo, Cameroon, Gulf of Guinea, 2°00’N, 8°4.3’E, 2492 m.

OTHER MATERIAL EXAMINED. — Cabinda, W of Congo river mouth, REGAB site, N of Congo Canyon, 5°46.89’S, 9°44.65’E, 3159- 3113 m, trawled by RV Atalante, BIOZAIRE 3 cruise, stn CP 20, 2.I.2004, leg. R. von Cosel, numerous sh., valves and fragments. W of Congo (Brazzaville), ASTRID site, ZAIROV 2 cruise, ROV Victor, stn Pl 73-13 (CT 0), 4°57’S, 10°09.5’E, 2820-2840 m, 1 juv. sh. (all MNHN).

DISTRIBUTION. — Gulf of Guinea from Cameroon (2°N) to the Congo Canyon (5°46’S). Boss (1970) mentions an additional record from SE of Accra, Ghana at 4°58’N, 3°48’E, 2268-2332 m, RV Pillsbury (University of Miami), stn 314.

DESCRIPTION

Shell to 120 mm long (see Table 3), oblong-oval, quite variable in shell shape and outline, moderately

tumid, rather light and thin-shelled and only in large fully grown specimens occasionally becoming thicker. Anterior margin broadly rounded. Ventral margin convex, in the middle part slightly convex, in very large specimens occasionally almost straight. Posterior part broadly tapering, ending in a rounded corner close to the horizontal midline. Umbones very shallow; beaks prosogyrous, well in front of the vertical midline, in fully grown specimens just before the end of the anterior third of the valve.

Surface dull, chalky, with fine, very densely spaced, irregular commarginal, slightly laminar ridges which on the anterior, ventral and posterior part of the valves become more or less wrinkly and oblique. Earliest part almost smooth, only with fine to indistinct growth lines. Lunule and escutcheon absent.In place of the escutcheon and parallel to the postero-dorsal margin a sulcus with a rounded delimitation which can be characterized as a second posterior angle (see also Boss [1970]). Posterior angle closely below this ridge rounded but well marked and ending at the posteriormost margin. Between the two “angles” a shallow radial depression, in which the wrinkly sculpture is denser than on the rest of the shell. On juvenile and subadult specimens periostracum glossy, pale straw-coloured, especially on the earlier parts, on fully grown specimens more brownish on the ventral part and more or less eroded on the rest of the shell, close to the margins becoming slightly leafy along the lamellae.

Hinge line rather narrow and short. Right valve with a lower anterior, strong and laminar cardinal tooth (1), which starts almost parallel to the antero-dorsal margin, then curves upwards and ends directly under the beaks, and a longer and stronger posterior tooth, assumed to be fused 3a and 3b. It starts above the anterior tooth parallel to it, ascends towards the umbo and then descends backwards to beneath the anterior part of the ligament, in form of an inverted “V”. Left valve anteriorly with a strong and somewhat irregular tooth 2a ascending towards the dorsal margin and situated beneath the umbo. Cardinal 2b thick and more or less triangular, fused with cardinal 2a under the umbo, descending vertically to the ventral margin of the hinge plate or somewhat inclined towards anteriorly. Posterior to 2a and 2b the well-inclined laminar posterior tooth (4b), inclined towards posteriorly. Ligament external, opisthodetic, rather short, on a narrow but strong nymphal plate. Subumbonal pit very short, shorter than 4a or 3a + 3b teeth, but deep and well marked. Pallial sinus variable, more or less deep, nearly triangular, acute or more rounded at the end and pointing to the upper limit of the anterior adductor scar; prolongation of the pallial line on the anterior side of the sinus extremity in general absent, in few specimens, however, a very small prolongation can be seen. Anterior pedal retractor scar deep to very deep, separate from the anterior adductor scar. Posterior pedal retractor scar united with the posterior adductor scar. Inside of the valves with very faint, irregularly sized and spaced radial vermiculations or waves and a very weak rib running from the umbonal cavity to the lower tip of the anterior adductor scar.

Valves whitish, with pale reddish brown hue on the interior within the limits of the pallial line.

Ctenidia with two demibranchs, large, moderately thick and fleshy, 52 mm long (inner demibranch) and about 45 mm (outer demibranch) in a 93 mm specimen (38 mm and 34 mm in a 88.4 mm specimen).Weak food-groove visible in both demibranchs. Siphons small but rather long and fused (in the preserved specimens retracted). Inhalant siphon with numerous very short tentacles, not well visible in preserved specimens. Blood with haemoglobin. Inner mantle fold in its posterior part with small blunt tentacles for a length of about 10 mm on both sides from the mantle fusion under the inhalant siphonal tube towards anterior.

BIOTOPE

Laubiericoncha chuni n. comb. has been sampled on the giant pockmark called REGAB (south of Gabon, 6°S) which is a very active cold seep site ( Ondreas et al. 2005; Olu-le Roy et al. 2007a), it is situated 8 km N of the Congo Canyon at 3150 m depth. Methane rich fluids are expelled at different places on the pockmark, and preferentially in its central zone where carbonate concretions

A

B

C, D

are developed. Laubiericoncha chuni n. comb. was collected in sedimentary areas at the southwestern part of the pockmark where a very large (100 × 50 m) vesicomyid field was observed. Other vesicomyid agregations occurred in the central area in dark and sulfide rich sediment around the concretions. Laubiericoncha chuni n. comb. co-occurs with another large vesicomyid species, which is currently under description by the present authors. During beam trawl operations on the northern part of the pockmark during the BIOZAIRE3 cruise (December 2003 - January 2004), the trawl accidentally hit a large agglomeration of mostly empty fresh shells of both species on rather sticky grey mud, among them hundreds of complete shells and a few live specimens of Laubiericoncha n. gen. The material taken by the Valdivia off Cameroon consisted of old and subrecent-looking valves only, and it cannot be excluded that the seeps at that very place are not active any more.

REMARKS

The type material of L. chuni n. comb. was already figured and the separation of Laubiericoncha n. gen. from Callogonia Dall, 1889 discussed by Cosel & Salas (2001: 356-358, figs 74, 75, 96, 97, 107). Subsequently, during the BIOZAIRE cruises, live-collected specimens of Laubiericoncha n. gen. already considered very close to L. chuni n. comb., were taken on the REGAB cold-seep site (see above). The specimens of this population show slight differences from the type material of L. chuni n. comb. originating from off Cameroon (2°N), which is situated at a distance of about 900 km to the North of REGAB. However, both populations are very variable in outline, and much overlap occurs. REGAB specimens are in general more elongate-lutrariform and more thin-shelled than the specimens from Cameroon, the beaks are situated more forward, the ventral margin is often less convex to almost straight in its middle. The hinge teeth are, within their basic configuration typical for the genus, quite variable in length and form. However, given the variability of the vast collected material from the REGAB site and all specimens from the type locality, we do not see any reason for morphologically separating the Gabon-Congo specimens from the Cameroon specimens of Laubiericoncha chuni n. comb. at species or subspecies level.