HOST SELECTION AND DISTRIBUTION OF HYPERMASTUS PLACENTAE (EULIMIDAE),AN ECTOPARASITIC GASTROPOD ON THE SAND DOLLAR ARACHNOIDES PLACENTA (ECHINOIDEA)

The intertidal sand dollar Arachnoides placenta is parasitized by the eulimid Hypermastus placentae at Pallarenda Beach, Townsville, Queensl and. A. placenta is the only echinoderm species in the region with whi ch H. placentae associates. Sand dollars are distributed in an aggrega ted pattern within the littoral zone at Pallarenda Beach, and the dist ribution of parasites within the adult sand dollar population conforms closely to Poisson expectations. A. placenta adults in ah size classe s are equally susceptible to parasitization by H. placentae, whereas j uveniles are not parasitized; host selection may be determined by a re quirement for host gonadal tissue in the diet of the parasite. Infesta tion rate and number of parasites per host decrease as host density in creases. Parasite body size is not related to host body size.