Rl. Cumming et Ra. Alford, POPULATION-DYNAMICS OF TURBONILLA SP (PYRAMIDELLIDAE, OPISTHOBRANCHIA), AN ECTOPARASITE OF GIANT CLAMS IN MARICULTURE, Journal of experimental marine biology and ecology, 183(1), 1994, pp. 91-111
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.