POPULATION-DYNAMICS OF TURBONILLA SP (PYRAMIDELLIDAE, OPISTHOBRANCHIA), AN ECTOPARASITE OF GIANT CLAMS IN MARICULTURE

Epidemics of an ectoparasitic gastropod, Turbonilla sp., have caused s ubstantial mortality of juvenile Tridacna gigas kept in land-based sea water tanks and on racks elevated off the substrate on a fringing cora l reef. Experimental populations of Turbonilla were established to inv estigate their dynamics in these two habitats. Five life-tables were d eveloped. Two were based on sets of four populations maintained in lan d-based seawater tanks. Three were based on sets of three populations with different initial densities kept on trays elevated above the subs trate on a fringing reef. Projection matrices based on the life-tables were used to simulate population growth as a test of the accuracy of the methodology. Survival and recruitment rates were calculated from m onthly size-frequency distributions. Survival rates were relatively co nstant throughout life. Individuals reared from the veliger stage thro ugh metamorphosis followed Gompertz growth curves. Life-span of Turbon illa was 3-3.5 months. Sexual maturity was reached at 30-40 days, afte r which reproduction was continuous, resulting in a life-time fecundit y of 600-800 eggs. Average generation time was 72 days. Recruitment on to new hosts was by immigrating veligers. The offspring of initial imm igrants did not disperse and recruited within the parent population. T his caused very rapid population growth and highly aggregated spatial distributions. Recruitment and growth rates were similar in tank and r eef populations with the same initial population sizes but survival wa s 70% lower on the reef. Growth, survival and recruitment rates decrea sed as density increased, but not enough to prevent death of the hosts . Our observations of Turbonilla in the mariculture environment reveal ed a suite of demographic attributes which contribute to their opportu nistic ectoparasitic lifestyle, enhancing Turbonilla's ability to take maximum advantage of patchily distributed hosts in the natural enviro nment.