Acar marsupialis, P. G. Oliver & Holmes, 2004

Oliver, P. Graham & Holmes, Anna, 2004, Cryptic bivalves with descriptions of new species from the Rodrigues lagoon, Journal of Natural History 38 (23), pp. 3175-3227 : 3176-3178

Acar marsupialis n. sp.

( figures 1–5 )

HOLOTYPE: 1 rv, Grand Pate´ , off Port Mathurin in shell gravel, 17 m, 19 ‡ 39.307 ’ S, 63 ‡ 24.950 ’ E. NMW.Z.2001.061.00004.

PARATYPES: 3 lvz1 rv as holotype, NMW .Z.2001.061.00005.

Measurements (mm, L 6 H). Holotype: 2.21 6 1.65. Paratypes: 2.19 6 1.56, 2.58 6 1.72, 2.34 6 1.54, 1.91 6 1.30.

Description. Shell minute to 2.58 mm. Equivalve. Inequilateral, beaks close to the anterior margin. Outline wedge-shaped, anterior reduced, posterior greatly expanded; dorsal margin long, straight; posterior margin long, slightly sinuate and angled about 90 ‡ to dorsal margin; anterior margin small smoothly merging into long, almost straight ventral margin. Sculpture of concentric lamellae that are regularly fluted by weaker radial riblets. Posterior ventral area arched but not carinate; median area flat, no byssal sinus apparent. Hinge plate long, taxodont teeth divided into two sets by distinct edentulous space; up to four anterior teeth and seven posterior teeth; anterior teeth more or less vertical, posterior teeth oblique. Ligament confined to a single chevron at the posterior end of the dorsal plate. Prodissoconch ( figure 5 ) of moderate size, 230 M m in diameter, apex pronounced, margin weakly rimmed. Adductor scars unequal, anterior about one-third size of posterior; anterior with distinct myophoric flanges, posterior flanges indistinct. Inner margin crenulate. In three valves the posterior ventral area is separated from the remainder of the cavity by a transverse ridge

( figure 2 ). Shell colour white.

Derivation of name. Latin marsupium ‘a pouch’, referring to the inner partitioned chamber.

Comparisons. The genus Acar in the Indo-Pacific is primarily represented by the widespread, common but variable Acar plicata (Dillwyn, 1817) ( figures 6, 7 ). Other species from the western Indian Ocean have been separated, most conclusively, by characters of the prodissoconch, e.g. A. petasion Kilburn, 1983 and A. abdita Oliver and Chesney, 1994 . Acar petasion has a large, 480 M m in diameter, collared and peaked prodissoconch ( Kilburn, 1983). The prodissoconch of Acar marsupialis is not very large ( figure 5 ) but the apex is pronounced and there is a weak rim that is distinct from that of A. plicata ( figure 7 ). The dimpled apex of the prodissoconch of A. plicata shown here does not agree with that illustrated by Kilburn (1983) and we assume this is due to the different angles of view used. If the South African A. plicata does truly have a domed apex then this suggests that A. plicata may comprise a complex of species. The wedge-shaped outline is unlike that of the other mentioned species, and additionally the inner partition is so far unique to this species.

Remarks. The internal partition observed in some valves is interpreted here to form a marsupial chamber for the brooding of larvae. Its presence in only some shells suggests that this may be a sex-related feature appearing only in females. These structures are relatively rare in bivalves with the best documented cases being those of Thecalia and Milneria [Carditoidea: Carditidae ] ( Yonge, 1969; Coan et al., 2000). We have considered that the partition could represent shell repair or be the result of parasitism but the partition is identical in form and position which suggests that it has not formed accidentally.

Figures

FIGS. 1–7. (1–5) Acar marsupialis n. sp. (1, 2) Holotype, NMW.Z.2001.061.00004: (1) external (MontageTM); (2) internal (SEM). (3–5) Paratype, NMW.Z.2001.061.00005 (SEM): (3) external; (4) internal; (5) prodissoconch. (6, 7) Acar plicata Rodrigues, NMW.Z.2000.086.00034 (SEM): (6) external left valve; (7) prodissoconch.