The Biology and Functional Morphology ofCurvemysella paula(A. Adams, 1856) (Bivalvia: Galeommatoidea: Montacutidae) Commensal withDiogenes goniochirusForest, 1956 (Crustacea: Paguridea) on China Beach, Danang, Vietnam.

The Indo-West Pacific montacutid Curvemysella paula(A. Adams, 1856) is the sole representative of its genus and the only known bivalve that is an obligate commensal of hermit crabs. As such, studies on it are accordingly very few. It lives in small groups byssally attached to the internal columella of the host's adopted shell. This paper describes the species from a wave-exposed beach in central Vietnam at Danang. Here, it was living with Diogenes goniochirus, but elsewhere in Hong Kong and Japan other taxa serve as hosts. The adult shell of C. paula is antero-ventrally long, deeply concave ventrally and so inequivalve as to appear twisted. Smaller individuals are more equivalve and equilateral and become deformed with age and growth. The hinge plate has a structure typical of the Montacutidae with the ligament ventrally formed into a lithodesma. Anatomically, C. paula is also unremarkable and interest in it focuses on reproduction. The species lives in small groups, the oldest and largest being a female, the smallest and youngest males. Intermediate sized individuals are hermaphroditic. Curvemysella paula is thus a protandric consecutive hermaphrodite. Prodissoconch morphology suggests that C. paula does not ctenidially brood fertilised eggs or, if so, only briefly. Further, it seems likely that there is subsequently an abbreviated lecithotrophic period of development perhaps undertaken close to the sea bed - thereby keeping the larvae close to established hermit crab and parental bivalve populations.