Lamylucina exgaini, Cosel, 2006

Cosel, Rudo von, 2006, Taxonomy of tropical West African bivalves. VI. Remarks on Lucinidae (Mollusca, Bivalvia), with description of six new genera and eight new species, Zoosystema 28 (4), pp. 805-851 : 810-813

Lamylucina exgaini n. sp.

( Figs 2B, C ; 3 ; 4)

Phacoides (Lucinoma) gaini Lamy, 1920: 192-194 (partim).

TYPE MATERIAL. — Holotype: Guinean continental shelf, W of Kaporo, 9°42’N, 15°21’W, 32 m, RV André Nizery, SEDIGUI, stn 344, 22.V.1988, complete shell ( MNHN).

Paratype: same locality, 1 rv. ( MNHN).

TYPE LOCALITY. — Kaporo, Guinea.

ETYMOLOGY. — The fact that this species was hitherto always identified as L. gaini n. comb. is expressed in the name.

OTHER MATERIAL EXAMINED. — Senegal. Dakar region, S of Gorée, 110-112 m, dredged RV Gérard Tréca, 18.II.1954, leg. I. Marche-Marchad, 1 rv. (MNHN). — Same locality, 95-98 m, 1 lv. (MNHN). — Dakar region (no details), 129-150 m, dredged RV Gérard Tréca, 24.I.1958, leg. I. Marche-Marchad, 1 rv., 1 lv. (MNHN). — N Casamance, 12°44.5’N, 17°37.3’W, 40 m, dredged RV Louis Sauger, III.1988, leg. R. von Cosel, 1 rv., 1 lv. (MNHN) — N Casamance, Kafountine, 12°57.5’N, 17°16.8’W, 35 m, dredged RV Louis Sauger, III.1988, leg. R. von Cosel, 3 lv. (MNHN).

Guinea-Bissau. Southern part, 11°11’N, 16°59’W, 26 m, dredged RV André Nizery, 22.IV.1988, leg. R. von Cosel, 1 rv. ( MNHN).

Guinea (Conakry). Conakry (no details, paralectotype of Phacoides gaini Lamy, 1920 ), 1 lv. ( MNHN). — Guinean continental shelf, SEDIGUI cruises, 9°03’N- 10°12’N, 24-57 m, bottom grab or dredge, RV André Nizery, V.1988 and X.1988, leg. R. von Cosel, 36 lots with numerous sh. and v. ( MNHN).

Sierra Leone. Off Sherbro Island, 7°15.5’N, 12°61’W, 64 m, dredged RV Calypso, Golfe de Guinée, stn 10, 14.V.1956, leg. I. Marche-Marchad, 1 rv., 3 lv. ( MNHN).

Côte d’Ivoire. Abidjan region (no precision), leg. I. Marche-Marchad, 1 lv. ( MNHN).

Ghana. 4°37’N, 0°50’ W, 90-100 m, dredged RV Calypso, Golfe de Guinée, stn 26, 24.V.1956, leg. I. Marche- Marchad, 1 rv. ( MNHN).

Ilha do Principe. Off Praia Grande, 12 m, dredged RV Calypso, Golfe de Guinée, stn P 17, 3.VII.1956, leg. I. Marche-Marchad, 1 rv. ( MNHN).

São Tomé. Off Praia Lagarto, 5-6 m, dredged RV Calypso, Golfe de Guinée, stn T 18, leg. I. Marche-Marchad, 11.VI.1958, 1 rv. ( MNHN).

Gabon. Cap Esterias, Pointe Idolo, on sandbank, 14.XI.1988, leg. R. von Cosel, 1 v. ( MNHN). — Port- Gentil, Plage de la Sogara, beachdrift, 11.I.2004, leg. R. von Cosel, 2 v. ( MNHN).

DISTRIBUTION. — Senegal (Dakar region) to Gabon, São Tomé, Ilha do Principe.

DESCRIPTION

Shell small, 9-20 mm, rather thin but appearing solid, circular, almost as long as high (length/height ratio 1.0-1.1), slightly inequilateral, equivalve, very compressed. Beaks about in or just in front of the vertical midline. Anterior part broadly rounded, with a very shallow inflexion between the middle part and the rounded corner to the short anterodorsal margin. Postero-dorsal margin more or less convex, posterior margin evenly rounded, with an indistinct, rounded postero-dorsal corner. Ventral margin rounded.

Exterior with more or less irregular, fine, very densely spaced and thin, rounded commarginal ridges or cords, giving the surface a somewhat glossy appearance. Occasionally narrow, smooth interspaces. Ridges in the middle part coalescing in some specimens resulting in some nearly smooth areas. Only few of the commarginal cords may become lamellate or somewhat foliate on the anterodorsal and postero-dorsal margin. Occasionally some deeper incised growth stages. Postero-dorsal area separated by a broad and very shallow radial depression. Antero-dorsal area very small and narrow, delimited by a sharp groove.

Hinge plate moderately narrow, right valve with a single small cardinal, no lateral. Left valve with one anterior and one posterior simple cardinal, no laterals visible. Hinge teeth very shallow and often reduced to completely missing. Lunule very small

FIG. 4. — Distribution of Lamylucina gaini ( Lamy, 1920) n. comb. (★) and Lamylucina exgaini n. gen., n. sp. (●) on the continental shelf of Guinea. Light grey area marks the sandy-muddy bottom, the clear area is pure sand, partly with gravel and shell debris.

and heart-shaped, assymmetrical, laterally slightly more prominent in the right valve, deeply inset; escutcheon almost nonexistent. Anterior adductor scar very small and narrow, with moderately long and narrow ventral part separate from the pallial line, the latter meeting the muscle impression above the middle. Inner margin of valves smooth.

Valves entirely white; periostracum not seen.

Measurements are provided in Table 2.

BIOTOPE

Fine and mixed sand, without muddy components, mostly from about 25 m downward to 150 m, only on the coast of Gabon and the islands of the Gulf of Guinea in shallower water.

REMARKS

Lamylucina exgaini n. gen., n. sp. is similar to L. gaini n. comb., with which it was previously confused. The principal difference from L. gaini n. comb. is the external sculpture with the much more close-set commarginal sculpture consisting of fine rounded and broader cords rather than ridges or lamellae. These cords often touch each other or even merge to a certain extent, whereas in L. gaini n. comb., the lamellae always have more or less large, clearly defined interspaces. In contrast to L. gaini n. comb., the leaf-like prolongations of a part of the commarginal ridges are very much reduced or absent, and the postero-dorsal margin is often more sloping; with the valves thus appearing subcircular. The anterior adductor scar, especially its diverging part is somewhat narrower. The cardinals are less developed and may be completely absent.

Both species were encountered sympatrically in some places ( Gabon, Ilha do Principe), without intergrades.

During the SEDIGUI cruises on the Guinean shelf, L. gaini n. comb. was found in more inshore waters on fine sandy substrates but always with a more or less high portion of mud, whereas L. exgaini n. gen., n. sp. is confined to more offshore localities, living in sandy bottom without mud (see Fig. 4). The two species have clearly different sediment preferences, and in certain localities like the Guinean shelf, they also seem to prefer different depths but this is not always the case.

Figures

FIG. 2. — Half schematic drawings of the insides of right valves:A, Lamylucina gaini (Lamy, 1920) n. comb.,lectotype; B, C, Lamylucina exgaini n. gen., n. sp.; B, holotype; C, Ilha do Principe, RV Calypso, Golfe de Guinée, stn P 17. Scale bars: 10 mm.

FIG. 3. — Lamylucina exgaini n. gen., n. sp.: A, holotype, W of Kaporo, Guinean continental shelf, 9°42’N, 15°21’W, 32 m, RV André Nizery, SEDIGUI,stn 344,22.V.1988,11.4 mm (MNHN),exterior and interior of both v., dorsal view;B, paratype,same locality, 13.1 mm (MNHN), exterior of lv.; C, paralectotype of Lucina gaini Lamy, 1920, Conakry, 9.4 mm (MNHN), exterior and interior of lv.; D, Guinea, W of Île Konebomby, 9°48’N, 15°02.5’W, 33 m, SEDIGUI, stn 399, 16.4 mm, exterior and interior of rv.