Rhomboidiella rodriguesensis, Oliver, P. Graham & Holmes, Anna, 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 : 3189-3191

Rhomboidiella rodriguesensis n. sp.

( figures 53–59 )

HOLOTYPE: one specimen, live collected, south side of Cotton Bay, Rodrigues, in intertidal algal turf, Stn 36, 19 ‡ 41.009 ’ S, 63 ‡ 29.882 ’ E. NMW.Z.2000.086.00001.

PARATYPES: eight specimens as holotype (all dry), NMW . Z.2000.086.00002; 1z3 v, Stn 9A, Gravier , south-east Rodrigues, NMW .Z.2000.086.00003; 11 specimens in alcohol, Stn 34, Gravier , NMW .Z.2000.086.00028.

Measurements (mm, L 6 H (6 B)). Holotype: 1.52 6 2.23 6 1.72. Paratypes, Cotton Bay: 1.36 6 1.93 6 1.38, 1.53 6 2.26 6 1.67, 1.31 6 1.76 6 1.21, 1.49 6 2.05 6 1.53, 1.26 6 1.72 6 1.19, 1.38 6 2.07 6 1.54, 1.39 6 1.82 6 1.38, 1.39 6 2.05 ( SEM); Stn 9A: 1.45 6 1.91 6 1.29, 1.34 6 1.82, 1.67 6 2.45, 1.32 6 1.91.

Description. Minute to 3 mm length. Inflated. Equivalve. Inequilateral, beaks close to the anterior margin. Outline modioliform, anterior dorsal slope about 45 ‡ to median axis merging gently into curved dorsal margin which then merges with broadly rounded posterior, ventral margin slightly incurved, anterior ventral weakly protruding. Sculpture on dissoconch weakly decussate ( figure 54 ), radial elements stronger than concentric elements, radial sculpture of fine riblets about 80 in number in 2 mm shell, crossed by a few concentric growth lines and numerous very fine, raised, concentric, weakly crispate lines, about 15 per mm on median area in 2 mm shell. In small shells a single anterior ventral riblet is obsolete, creating a narrow smooth area ( figure 57 ). Nepioconch ( figure 58 ) oval, very large about 700 M m across, sculptured with numerous radial and divaricating raised lines. Prodissonch large, discoid, anterior margin straight, 315 M m in diameter and sculptured by irregular polygonal reticulation with an underlying concentric arrangement dorsally ( figure 58 ). Hinge ( figure 59 ), in 2 mm shell, with four poorly defined teeth below prodissoconch, provincular denticles eroded by short ligament, 18 dorsal teeth increasing in size away from the ligament. Inner margin crenulate. Shell transluscent greenish with white opaque patches.

Derivation of name. Latin suffix ensis denoting place of belonging and Rodrigues, the known range of the species.

Comparisons. Two taxa can be compared, R. capensis Thiele, 1931 from South Africa and R. malaccana Ockelmann, 1983 from Thailand. Rhomboidiella capensis is larger to 7 mm in length and the outline differs in that the dorsal margin is straight rather than curved and the anterior ventral margin does not protrude. Barnard (1964) considers Mytilus thalia Turton, 1932 to be synonymous with R. capensis . Rhomboidiella malaccana is more similar to the Rodrigues species but the type material is scant, making comparison uncertain. The hinge appears to be more strongly developed in R. rodriguensis in having formed anterior teeth and many more posterior teeth. The sculpture patterns of the nepioconch and dissoconch do not agree in that the concentric elements are much weaker in R. rodriguensis .

Habitat. Living specimens were relatively common in intertidal sandy, algal turf on exposed platforms on the east of the island.

Figures

FIGS. 53–59. Rhomboidiella rodriguesensis n. sp. (53, 54) Holotype, NMW.Z.2000.086.00001: (53) external (MontageTM); (54) external (SEM). (55–59) Paratype, NMW.Z.2000. 086.00002: (55) juvenile (MontageTM); (56, 57) juvenile (SEM); (58, 59) prodissoconch (SEM).