Morphology and reproduction of Nipponomysella subtruncata (Yokoyama), a galeommatoidean bivalve commensal with the sipunculan Siphonosoma cumanense (Keferstein) in Japan

The shell and anatomy of Nipponomysella subtruncata is described. The bival ve is attached singly or in groups of up to nine on Siphonosoma cumanense, a burrowing intertidal sipunculan in south-west Japan. The species is a pro tandrous hermaphrodite. Specimens 1.4-2.5 mm long are males, which between 2.1 and 3 mm in length reverse sex and remain females. Reproduction peaks i n summer and the annual number of clutches is small. Ripe oocytes, 84-88 mu m diameter, are spawned into the suprabranchial cavity where they develop i nto 107-mum-long straight-hinged veligers. Following a planktotrophic perio d of unknown duration, the c. 360-mum-long spat normally settle upon and at tach to the shells of larger, predominantly female, individuals. At a lengt h of 1-1.6 mm they detach again and live separately thereafter. Sperm are t ransferred in spermatophores and stored within paired, mushroom-shaped rece ptacles located posteriorly in the female's suprabranchial cavity. The rece ptacles first appear in large males or in specimens in the process of rever sing sex. Stored sperm probably survive long enough to fertilize more than one clutch. The anatomy of Nipponomysella is characteristic of the Montacut idae, and is of especial interest because of the unique structure of the sp erm receptacles.