Reproduction and Variable Larval Development of an Ectoparasitic Snail, Turbonilla sp. (Pyramidellidae, Opisthobranchia), on Cultured Giant Clams
Abstract:Turbonilla sp. is an elongate white snail with a shell reaching 6–7 mm in length. It forms lethal infestations on juvenile giant clams, Tridacna gigas, in a mariculture facility at Orpheus Island Research Station (OIRS) near Townsville, Australia. In land-based mariculture tanks rapidly multiplying populations have caused reduced growth rates and substantial mortality of juvenile cultured T. gigas. Turbonilla sp. is a simultaneous hermaphrodite. Eggs were laid in discrete gelatinous masses attached to hard substrates, most often the external shells of live clams. Sexual maturity was reached at 2.5–2.95 mm in length and egg production continued throughout life. Individuals 3 mm and larger produced one egg mass every few days. Egg mass size increased with shell size, hence fecundity increased with size. Egg production occurred throughout the year. The chronology of larval development is described. Developmental 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-swimming veligers. Both veligers and metamorphosed juveniles were observed hatching from egg masses left undisturbed on clams in mariculture tanks. This represented a small variation in hatching time relative to metamorphosis and involved no differences in the size or appearance of eggs or embryos. The possibility that hatching was promoted by mechanical disturbance caused by the observation technique is discussed. All life-history characteristics identified in this study contribute to the species' efficient exploitation of mariculture tanks containing juvenile T. gigas. Variable larval development enables Turbonilla sp. to enter mariculture tanks as free-swimming larvae, settle on juvenile T. gigas and initiate population explosions through intracapsular metamorphosis. This is enhanced by the calm conditions and lack of predators inside the tanks.
Document Type: Research Article
Publication date: March 1, 1993
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