Laubiericoncha, Cosel & Olu, 2008

Cosel, Rudo Von & Olu, Karine, 2008, A new genus and new species of Vesicomyidae (Mollusca, Bivalvia) from cold seeps on the Barbados accretionary prism, with comments on other species, Zoosystema 30 (4), pp. 929-944 : 931-932

Genus Laubiericoncha n. gen.

TYPE SPECIES. — Laubiericoncha myriamae n. sp., designated herein.

SPECIES INCLUDED. — Laubiericoncha myriamae n. sp., L. chuni ( Thiele & Jaeckel, 1931) n. comb.; L. angulata (Dall, 1896) n. comb.; L. suavis (Dall, 1913) n. comb., possibly Vesicomyidae gen.et sp. indet. (Edison Seamount, see Krylova & Janssen 2006: 246, pl. 4, fig. 25).

ETYMOLOGY. — This new genus is named in memory of Prof. Lucien Laubier, Membre de l’Institut, who was one of the leading oceanographers in France and who passed away when this manuscript was in press.

DISTRIBUTION. — Western Atlantic: SW of Barbados, Barbados accretionary prism. Eastern Atlantic: Gulf of Guinea. Eastern Pacific: Panama Bay, Baja California.

DIAGNOSIS. — Shells medium-sized to large, subovaltriangular to oval-oblong (length: 35 to 120 mm, height about 21 to 62 mm), with subparallel dorsal and ventral margins, moderately inflated, with prosogyrate umbones situated well in front of the vertical midline. Posterior margin rounded or tapering, posteriormost point at or just above the horizontal midline. Postero-dorsal area with two shallow and rounded ridges, the more ventral of them being the posterior angle. No lunule and escutcheon. Umbones rather flattened and not prominent. Hinge with a long, thin and curved cardinal 1 and a fused cardinal 3a and 3b in the right valve and a long and strong cardinal, assumed to consist of fused 2a and 2b, and posterior short and laminar cardinal (4b) in the left valve. No posterior nymphal ridge. Ligament rather short, external. Subumbonal pit present but very small and narrow to indistinct. Pallial sinus well marked, triangular, without or occasionally with very short prolongation of the pallial line at the end of it. Ventral limb of the pallial sinus in general slightly broadened. Pallial line not completely parallel to the ventral margin but in its anterior part slightly more distant from it than in its posterior part. Anterior pedal retractor scar deep to very deep. Ctenidia with outer and inner demibranchs. Exhalant and inhalant siphonal tubes small but rather long, in correspondence to the pallial sinus, fused over their whole length, with numerous very short tentacles. Mantle lobes with outer, middle and inner mantle folds. Foot muscular and well developed, triangular. Mantle ventrally open from in front of the anterior adductor to the fusion just under the siphonal tubes.

REMARKS

Laubiericoncha n. gen. is distinguished from other large Vesicomyidae by a combination of characters not seen elsewhere: its outline with broadly tapering posterior part with the posteriormost point (homologue to the postero-ventral corner in Calyptogena and other genera) situated in or just above the horizontal midline giving the valves the form of a Lutraria , the short, broad, acute and triangular pallial sinus, the two “posterior angles” and the typical hinge with diverging, rather long and thin but prominent cardinals, in the right valve a curved cardinal (fused 3a and 3b), a narrow anterior cardinal (1) situated under it and a very small to indistinct subumbonal pit. Species of the genus Callogonia , also with a welldeveloped and even deeper triangular pallial sinus, are smaller, somewhat shorter, the postero-ventral

L. suavis (Dall, 1913) n. comb. 35 mm L. sp. indet. (Edison Seamount) 47.6 mm L. angulata (Dall, 1895) n. comb. 58 mm L. myriamae n. gen., n. sp. 96 mm L. chuni ( Thiele & Jaeckel, 1931) n. comb. 120 mm

corner is situated more ventrally, and in the hinge of the right valve, the cardinals 1, 3a and 3b are situated in a line, a hinge configuration also typical for Vesicomya s.s. and Isorropodon (for details, see Cosel & Salas 2001). The hinge configuration of Laubiericoncha n. gen. is very close to that of Calyptogena s.s. and other Vesicomyidae hitherto grouped in Calyptogena . In Calyptogena pacifica Dall, 1891 (cf. Boss & Turner 1980: fig. 10B) the type species of Calyptogena , the cardinals 1 and 3a and 3b in the right valve and the cardinals 2a and 2b in the left valve are in the same position but shorter and thicker, while the cardinal 4b in the left valve is much longer. The gills of Calyptogena s.s. have only one demibranch, whereas in Laubiericoncha n. gen. they have two. Calyptogena s.s. lacks a pallial sinus, there is no subumbonal pit, but a posterior nymphal ridge in the right valve is present.We follow Krylova & Sahling (2006) in using Calyptogena in a more restricted concept.

A superficially similar species is “ Calyptogena ” diagonalis Barry & Kochevar, 1999 from cold seeps off Oregon and Costa Rica (see Barry & Kochevar 1999). It is much larger (to 230 mm long) and much more elongate, but shares with Laubiericoncha n. gen. the short ligament, the ctenidia with two demibranchs, the shallow but acute pallial sinus and the inclined ventral pallial line, which, however, in “ C. ” diagonalis is not slightly but markedly oblique. Th e posterior angle is rounded but much more pronounced and ends in a postero-ventral corner but not as high as in the horizontal midline as in Laubiericoncha n. gen. In “ C. ” diagonalis , the basic arrangement of the teeth (see Barry & Kochevar 1999: fig. 1h) in the right valve is close to that of Laubiericoncha n. gen., however, in Laubiericoncha n. gen., the cardinals are much thinner and more elongately stretched. In the left valve, the dentition of “ C. ” diagonalis has a different, more “venerid” aspect, with longer 2a and stronger 2b, according to the images, there is a small but well-marked subumbonal pit. Krylova & Sahling (2006) exclude C. diagonalis from Calyptogena in the strict sense but do not place it in another genus.

The most similar genus is Archivesica Dall, 1908 (type species: Vesicomya gigas Dall, 1895 ). As in Laubiericoncha n. gen., the gills have two demibranchs, the subumbonal pit is also relatively small, but larger than in Laubiericoncha n. gen. However, the pallial sinus has a different form: it is shorter, broader and not acute but just concave; the siphons are thicker and shorter, without tentacles seen. The anterior pedal retractor scar is shallow, in contrast to the deep scar in Laubiericoncha n. gen. In Archivesica gigas , the shell is more subquadrate and more tumid, the umbones are more prominent, the posterior margin is not tapering, the ventral margin is straight, whereas in Laubiericoncha n. gen. it is slightly to markedly convex (see Fig. 6B , for further figures of Archivesica gigas see Cosel [2006: 144, as “ Calyptogena gigas ”])

Besides Laubiericoncha myriamae n. gen., n. sp. and L. chuni ( Thiele & Jaeckel, 1931) n. comb., Callogonia angulata Dall, 1895 from the Gulf of Panama (2320-3050 m) is the third species to be placed in Laubiericoncha n. gen. According to the figure in Dall (1908: fig. 263), it has a much more acute posterior margin than the two Atlantic species and is still more tapering posteriorly (see Fig. 6 ). The hinge of the figured right valve has the same configuration. Finally, following Krylova & Janssen (2006), we include also Vesicomya suavis Dall, 1913 from the W coast of Baja California, W of Tiburon Island (Animas) (25°N, 1345 m). Although Dall’s figure ( Dall 1925: pl. 27 fig. 1) shows only an external view, thanks to an unpublished sketch of the holotype (33.4 × 21.5 mm) by E. Krylova, the hinge dentition and pallial sinus prove its placement in the new genus. An undescribed species presumably also to be placed in the genus Laubiericoncha n. gen. has recently been collected on Edison Seamount (SW Pacific) by the German RV Sonne ( Krylova & Janssen 2006: 246, pl. 4 fig. 25). As the single valve obtained is of poor quality, we cannot positively confirm this until more and better material becomes available. A further undescribed species cited for the new genus by Krylova & Janssen (2006: 249) comes from the Holocene of Gakkel Ridge in the Arctic. The known maximum lengths of each species are marked in Table 1.

Figures

FIG. 6. — A, Laubiericoncha angulata (Dall, 1895) n. comb., semi-schematic drawing of the inside; holotype (USNM), a rv. (after Dall 1908: pl. 6, fig. 12), shell length 58 mm; B, Archivesica gigas (Dall, 1895), semi-schematic drawing of the inside of a rv., shell length 67.0 mm, Guaymas Basin, Gulf of California, Mexico, Claire site, 27°00,94’N, 111°24.66’W, 2025 m, GUAYNAUT cruise, stn Pl. 18, 30.XI.1991 (MNHN). Scale bar: B, 10 mm.