Scacchia rodriguesensis, 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 : 3208-3209

Scacchia rodriguesensis n. sp.

( figures 106–111 )

HOLOTYPE: one specimen from Baie du Nord, 2 m, mud, 19 ‡ 42.1 ’ S, 63 ‡ 22.5 ’ E. NMW.Z.2001.061.00033.

PARATYPES: 7 rv, 4 lv and two shells, as holotype, NMW .Z.2001.061.00034.

Measurements (mm, L 6 H). Holotype: 1.90 6 1.49. Paratypes: 2.16 6 1.81, 1.49 6 1.29, 1.51 6 1.18, 2.14 6 1.66, 2.40 6 1.93, 2.08 6 1.64, 1.96 6 1.54, 1.96 6 1.64, 1.98 6 1.53, 1.85 6 1.53, 2.17 6 1.63, 2.29 6 1.78, 2.36 6 1.83, 2.28 6 1.84, 2.72 6 2.07.

Description. Shell to 3 mm in length. Thin. A little inflated. Equivalve. Inequilateral, beaks just behind the midline. Outline subovate, anterior slightly more expanded than posterior, umbos prominent. Dorsal margins set off from lateral margins by weak flexures ( figure 107 : af, pf); posterior margin sloping more steeply than anterior, rounded; ventral margin curved, anterior margin broadly curved. Sculpture smooth except for weak growth lines. Prodissoconchs I and II present, PdI 125 M m in diameter, weakly sculptured; PdII large, 320 M m in diameter, smooth with weak concentric lines. Hinge with small but distinct teeth; rv ( figure 109 ) with a single projecting cardinal peg-like tooth, lateral teeth not developed but the anterior and posterior dorsal margins slightly flexed to form pseudo-teeth; lv ( figure 110 ) with a pair of cardinal peg-like teeth, lateral areas not flexed. Internal ligament small, in a shallow oblique resilifer; external ligament thin on an indistinct nymph. Shell colour white, some with a pair of weak reddish radial rays or with a median tinted zone ( figure 111 ).

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

Comparisons. The hinge structure suggests that this species belongs to the Erycinidae and because of the flexures in the right valve it is most reminiscent of the genus Scacchia . This genus has to date been confined in its use to Mediterranean and North Atlantic species ( van Aartsen, 1996) but we could not find an alternative genus for the Rodrigues shells. If this species has been described before it could have been allocated to any number of genera including Erycina , Rochefortia , Kellia and Bornia . Such small taxa have generally been ignored but Turton (1932) and Bartsch (1915) described many small bivalves from South Africa. Despite extensive trawling through the Indo-Pacific literature we are unable to find any previous description of this species.

Figures

FIGS. 106–111. Scacchia rodriguesensis n. sp. (106–110) Holotype, NMW.Z.2001.061.00033 (SEM): (106) external left valve; (107, 108) internal left and right; (109, 110) hinge view, left and right valves. (111) Juvenile paratype, NMW.Z.2001.061.00034 (MontageTM).