Divaricella (Egracina) chavani, 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 : 845-848

Divaricella (Egracina) chavani n. sp.

( Figs 25 ; 26 )

Lucina ornata Reeve, 1850 : sp. 48 (non Agassiz, 1845 nec C. B. Adams, 1847 [nom. nud.]).

Divaricella (Egracina) dentata var. collignoni Chavan, 1951: 20-22 , figs 23, 24 (paratype), not fig. 24bis (holotype) [partim].

Divaricella (Egracina) dentata collignoni – Chavan 1969: N 506, fig. 5a-c.

TYPE MATERIAL. — Holotype: Gabon, Port-Gentil , Île aux Pigeons, 0°40.5’S, 8°45.3’E, muddy sand near mangroves, 1980-1989, leg. C. Chevalier, 1 sh. ( MNHN).

Paratypes: same locality, 8 sh. (5 MNHN, 1 BMNH, 1 IRSNB, 1 ZMC).

Lucina ornata : lectotype (BMNH 1963.186), rv., selected by Dekker & Goud (1994: 127), no locality, ex Cuming coll.

Paralectotype (BMNH), a left valve.

D. dentata var. collignoni : type lot in Chavan collection (holotype, rv., Îles Glorieuses, coll. Chavan no. 1223; paratype, a sh., Corisco, coll. Chavan no. 668); the Chavan collection is now in University Lyon 1.

TYPE LOCALITY. — Île aux Pigeons, Port-Gentil, Gabon. ETYMOLOGY. — In honour of André Chavan, who had made the first attempt to revise the Divaricella group.

OTHER MATERIAL EXAMINED. — Senegal. Cap Vert Peninsula, coll. de Cessac, 1 lv. ( MNHN). — Off Saloum, Petite Côte, 35-37 m, dredged RV Gérard Tréca, 8.III.1955, leg. I. Marche-Marchad, 1 lv. ( MNHN). — Casamance, 12°46.9’N, 17°29.9’W, 45 m, fine sand, dredged RV Louis Sauger, 29.III.1988, leg. R. von Cosel, 1 juv. lv. ( MNHN). — Senegal (no precision), coll. Admiral de Hell, 1846, 1 sh. ( MNHN).

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

Gabon. Île Banié, southern part of Golfo de Corisco, sand, 3-6 m, 1986-1987, leg. P.Bernard, 2 sh. ( MNHN). — Cap Esterias, Libreville, 1-4 m, 1986-1987, leg. P. Bernard, 2 rv. ( MNHN). — Port-Gentil, sand, 2-5 m, 1986- 1987, leg. P. Bernard, several v. ( MNHN). — Port- Gentil, Plage de la Sogara, beach drift, 12.I.2004, leg. R. von Cosel, 1 sh., several v. ( MNHN). — Gabon (no precision), 1853, coll. Aubry Lecomte, 2 sh., 1 rv. ( MNHN). — 0°38.4’S, 8°46’E, sand with forams, 5 m, dredged RV Calypso , 10.VI.1956, leg. I. Marche-Marchad,

1 juv. rv. ( MNHN).

Congo (Brazzaville). Pointe-Noire, Plage Mondaine, beach drift, XII.1985, leg. R. von Cosel, 1 rv. ( MNHN).

DISTRIBUTION. — Mauritania (Quaternary only, [Cou-

lombel 1980]), Senegal (Saloum to Casamance); Sierra Leone (Sherbro), Côte d’Ivoire to the Congo (Pointe- Noire). There are no records from Guinea-Bissau to Liberia and a distribution gap is very probable. The species is common in Gabon and rare elsewhere.

DESCRIPTION

Shell 15-28 mm long, rather solid, inflated, almost circular, length/height ratio 1.1. Beaks about in the middle. Antero-dorsal margin divided into two slightly concave to almost straight parts with a rounded corner between them and another rounded corner to the anterior margin. Posterodorsal margin slightly convex, postero-dorsal corner more or less rounded, posterior margin somewhat truncated, ventral margin evenly and well rounded.

Exterior with fine, regular, dense, contiguous, divaricate ribs, with the line of sharp, V-shaped divarication going from the beaks to the anteroventral margin. Sculpture more marked near the anterior and posterior margin than in the middle of the valve. Anterior area small, posterior area not distinguished.

Hinge in the right valve with a small, more or less pronounced anterior cardinal and a broad and bifid posterior cardinal. Left valve with a strong but rather thin anterior and a thin posterior cardinal. Both valves with an indistinct knob as anterior lateral under the distal end of the lunule, posterior lateral a small knob under the postero-dorsal corner. Lunule very small and short, somewhat sunken, asymmetrical, broad in the right valve, narrow in the left valve, no escutcheon. Ligament deeply sunken and only partly visible from the exterior at dorsal view. Anterior adductor scar with moderately long extension separate from the pallial line, this latter meeting the impression at the end of the upper third.

Inner margin crenulate in reflection of the external sculpture.

Valves entirely white, periostracum not seen.

Measurements are provided in Table 13.

BIOTOPE

Sandy bottom, mostly in shallow water (1-6 m) but apparently also deeper.

REMARKS

This species was long known as Lucina ornata Reeve, 1850 , but this name is preoccupied by Agassiz, 1845. Chavan (1951) considered his material from West Africa and from East Africa conspecific with the Caribbean Divaricella dentata (Wood, 1815) (syn. D. serrata (d’Orbigny, 1846)) , but he separated them as a “nov. var.”, Divaricella (Egracina) dentata var. collignoni . Moreover, Chavan designated this variety in the same entry as type species of his new subgenus Egracina . Chavan’s type lot, however, as he admits himself (“forme africaine du dentataserrata des Antilles”) is a mixed lot: it consists of a specimen from West Africa (“Corisco”) and a specimen of a close western Indian Ocean (“Îles Glorieuses”) species which in his figure legend he designated explicitly as holotype ( Chavan 1951: 21), the West African species being the paratype. Chavan’s name is hence not available for the West African species.

Reeve’s type lot in BMNH (no locality) consists of two syntypes (single valves) from which the right valve was selected lectotype by Dekker & Goud (1994). This valve has a more sloping postero-dorsal margin than most specimens from West Africa but it still might fall within the variability of the West African species. The other (left) valve has a stronger sculpture and is more likely to belong to another species. Moreover, Dekker & Goud (1994) synonymized D. ornata ( Reeve, 1850) with D. irpex E. A. Smith, 1885 , a totally different Australian species.To avoid confusion, the West African species is here described as new, and the holotype is taken from “modern” and well localized material.

The distribution pattern of this species is still somewhat enigmatic: it is rare in the southern (weaker) part of the northern zone of Alternance ( Senegal: Petite Côte, Casamance, see Le Loeuff & Cosel 1998), it is absent from the northern typical tropical zone (Guinea-Liberia), and it is rare in the atypical tropical zone in the Gulf of Guinea ( Côte d’Ivoire). It has not been recorded from Cameroon, however, on the coast of Gabon situated in the southern part of the southern typical tropical zone it is common; whereas in the weaker part of the southern zone of Alternance ( Congo) it is present but rarer. Moreover, in the southern part of its range the species lives in quite shallow (1-6 m), whereas the few records from Senegal are from deeper water (35-45 m).

Figures

FIG. 25. — Divaricella (Egracina) chavani n. sp.: A, holotype, Île aux Pigeons, 0°40.5’S, 8°45.3’E, Port-Gentil, Gabon, muddy sand near mangroves, 1 sh., leg. C. Chevalier 1980-1989,27.4 mm (MNHN), exterior and interior of both v., dorsal view; B, paratype, same locality, 25.1 mm (MNHN), exterior of rv., exterior and interior of lv.

FIG. 26.— Divaricella (Egracina) chavanin.sp.,half schematic drawing of the inside of the rv. of the holotype. Scale bar: 10 mm.