Parathyasira sp.

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 : 3195-3196

Parathyasira sp.

( figures 73, 74 )

Material examined. A single left valve taken from shell gravel at RDS 52, Grand Pate´ , off Port Mathurin, 19 ‡ 39.307 ’ S, 63 ‡ 24.950 ’ E, 29 September 2001, sand in surge channel, 17 m, scuba, coll. N. L. Bruce and J. Smith, NMW .Z.2001.061.00022.

Description. Small (2.0 mm), fragile, compressed. Equilateral, outline subcircular-rhomboidal with prominent umbos and angulate posterior. Posterior dorsal margin moderately sloping almost straight following incised submarginal sulcus with vertical edges; auricle very low; posterior sulcus weak as a flattening of the valves and expressed on the outline as a posterior truncation; ventral margin slightly angulate; lunule small, lunule margin short, concave. Sculpture weak, of growth lines and concentric striations; sparse minute pustules over posterior slope and ventral areas.

Remarks. This single valve signals the presence of the Thyasiridae in the Rodrigues fauna and is of note because of the general scarcity of thyasirids in tropical shallow-water faunas. Their symbiosis with sulphide- or methanereducing bacteria makes them typical of enriched environments, usually in rather tranquil offshore areas, and it is most likely that this valve has been transported from the shelf zone beyond the reef front.

This shell has a weak posterior sulcus and a sharply incised submarginal sulcus, characters typical of Parathyasira rather than Thyasira , but with only a single valve further comparison is not possible.

Figures

FIGS. 73, 74. Parathyasira sp., NMW.Z.2001.061.00022, external (SEM).