GALEOMMATIDAE, Gray, 1840

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 : 3196

GALEOMMATIDAE

The Galeommatidae contain a variety of taxa of which many can be likened to bivalved slugs in that the mantle is permanently or temporarily extended over the shell and the valves are capable of opening widely, often to 180 ‡. The valves may be unable to close completely and may gape ventrally, latero-dorsally or not at all. Many genera have been erected encompassing the above range of morphologies but their parameters are not clear. This was illustrated by Mikkelsen and Bieler (1992) who showed that species allocated to a single genus, in their example Scintilla , may not be monophyletic. Given these uncertainties it would be unwise here to attempt to refine the generic concepts based solely on Rodrigues material. We have restricted our use of genera to Galeomma , Amphilepida and Scintilla . Galeomma is used for species that cannot retract the mantle and maintain a wide gape; Amphilepida also for species that cannot fully retract the mantle but can partially close the valves; and Scintilla for those that can retract the mantle and close the valves. There is a trend from Scintilla through Amphilepida to Galeomma for reduction in the hinge teeth, with the latter showing little if any structures other than the ligament. All genera have decorated mantles in the form of tubercles to long tentacles and resemble nudibranchs when expanded and mobile. They live deep in coral rubble crevices or under slabs of coral rubble/rock, mostly those lying on a veneer of silt or sand.

The mantle decoration is a distinctive character but one which has seldom been used. Most of the described taxa date from the mid-19th century and are based on shells only and are not illustrated originally. Deshayes (1856a, 1856b) and Sowerby (1862) account for most of the described taxa and type specimens are available mostly in the Natural History Museum, London. Examination of these was not entirely satisfactory as differences in shell form could not be assessed due to the limited material available and from our observations similar shell forms may have different mantle decorations. We have therefore been faced with a dilemma over the following options.

(1) Allocate names from described taxa without matching mantle characters and based on few shells.

(2) Allocate no species names to the Rodrigues material.

(3) Allocate new species names to the Rodrigues material based on shell and mantle characters.

We have tried to match shells to existing descriptions especially where taxa have been described from the Mascarene region, but with little success, the only exceptions being Amphilepida aurantia (Lamarck, 1819) ( figures 86 , 94 ) and Scintilla pisum Sowerby, 1862 ( figures 85 , 93 , 95 ). Another two species we found had shell morphologies consistent enough with those described from the Philippines by Deshayes (1856b) to consider them conspecific. The remaining two taxa we could not link to any described taxon and these have been described as new. Another two, also allocated to Scintilla , known only from single valves are illustrated but not described as new.

Figures

FIGS. 81–88. (81) Scintilla lynchi, NMW.Z.2000.086.00011 [8.98 mm]. (82) Scintilla vitrea, NMW.Z.2000.086.00004 [7.37 mm]. (83) Scintilla larcombei, NMW.Z.2001.061.00028 [3.29 mm]. (84) Scintilla anomala, NMW.Z.2000.086.00007 [9.2mm]. (85) Scintilla pisum, NMW.Z.2001.061.00025 [8.5 mm]. (86) Amphilepida aurantia, NMW.Z.1999.073.00004 [10.1 mm]. (87) Scintilla sp. A, NMW.Z.2000.086.00012 [2.62 mm]. (88) Scintilla sp. B, NMW.Z.2000.086.00013 [14.91 mm]. All external (MontageTM).

FIGS. 89–94. (89) Scintilla larcombei, NMW.Z.2001.061.00028. (90) Scintilla lynchi, NMW.Z.2000.086.00011. (91), Scintilla anomala, NMW.Z.2000.086.00007. (92) Scintilla vitrea, NMW.Z.2000.086.00004. (93) Scintilla pisum, NMW.Z.2001.061. 00025. (94) Amphilepida aurantia, NMW.Z.1999.073.00004. All hinge views (SEM).

FIGS. 95–98. Line drawings of live Scintilla showing tentacles. (95) S. pisum, NMW.Z.2001.061.00025. (96) S. anomala, NMW.Z.2000.086.00007. (97) S. lynchi, NMW.Z.2000.086.00011. (98) S. vitrea, NMW.Z.2000.086.00004.