IMPACTS OF ECTOPARASITIC GASTROPODS ON GROWTH, SURVIVAL, AND PHYSIOLOGY OF JUVENILE GIANT CLAMS (TRIDACNA-GIGAS), INCLUDING A SIMULATION-MODEL OF MORTALITY AND REDUCED GROWTH-RATE

Turbonilla sp. is a small pyramidellid gastropod that is an ectoparasi te on the giant clam, Tridacna gigas. In field surveys of an intertida l culture site at Orpheus Island, Great Barrier Reef, 9-17% of the juv enile clams were found to be infested with Turbonilla sp. in an aggreg ated pattern. Growth experiments showed that the parasites were lethal in cases of heavy infestations on juvenile clams and that they reduce d clam growth in sublethal infestations. Reduction in growth rate was related to clam size and number of parasites, such that % reduction in growth = - 0.258 P-2 + 9.3 P + 0.006 L-2 - 1.71 L + 98.9 where P is t he number of parasites per clam (range 0-20) and L is clam shell lengt h (range 55-105 mm) (r = 0.95). From this equation, the estimated tota l losses of linear growth rate and productivity of clams were 2.8-4.2% and about 7%, respectively, due to Turbonilla sp. infestations at the Orpheus Island intertidal site. Despite these general effects on clam growth and survival, physiological studies revealed no significant ef fects of the parasites on clam filter feeding, respiration or photosyn thesis rates. Furthermore, there was no apparent immune reaction or ti ssue damage, as no inflammation of the clam's mantle was found where t he parasite proboscis penetrated its host. These observations were con sistent with the parasite gaining its nutrition by feeding purely on t he host's body fluids. A computer simulation model. which was based on clam and parasite energy budgets, reasonably predicted the decreased growth and mortality patterns of infested clams. (C) 1997 Elsevier Sci ence B.V.