REPRODUCTION AND VARIABLE LARVAL DEVELOPMENT OF AN ECTOPARASITIC SNAIL, TURBONILLA SP (PYRAMIDELLIDAE, OPISTHOBRANCHIA), ON CULTURED GIANT CLAMS

Turbonilla sp. is an elongate while snail with a shell reaching 6-7 mm in length. It forms lethal infestations on juvenile giant clams, Trid acna gigas, in a mariculture facility at Orpheus Island Research Stati on (OIRS) near Townsville, Australia. In land-based mariculture tanks rapidly multiplying populations have caused reduced growth rates and s ubstantial mortality of juvenile cultured T. gigas. Turbonilla sp. is a simultaneous hermaphrodite. Eggs were laid in discrete gelatinous ma sses attached to hard substrates, most often the external shells of li ve clams. Sexual maturity was reached at 2.5-2.95 mm in length and egg production continued throughout life. Individuals 3 mm and larger pro duced one egg mass every few days. Egg mass size increased with shell size, hence fecundity increased with size. Egg production occurred thr oughout the year. The chronology of larval development is described. D evelopmental mode was variable and involved hatching of both veligers and metamorphosed juveniles from the same egg mass. Egg masses kept in petri dishes for observation of larval development produced free-swim ming veligers. Both veligers and metamorphosed juveniles were observed hatching from egg masses left undisturbed on clams in mariculture tan ks. This represented a small variation in hatching time relative to me tamorphosis and involved no differences in the size or appearance of e ggs or embryos. The possibility that hatching was promoted by mechanic al disturbance caused by the observation technique is discussed. All l ife-history characteristics identified in this study contribute to the species' efficient exploitation of mariculture tanks containing juven ile T. gigas. Variable larval development enables Turbonilla sp. to en ter mariculture tanks as free-swimming larvae, settle on juvenile T. g igas and initiate population explosions through intracapsular metamorp hosis. This is enhanced by the calm conditions and lack of predators i nside the tanks.