Many tropical sponges harbor zoanthid populations on their surfaces. Although zoanthid polyps are known to colonize space by budding, it is not known whether an entire population is comprised of a single clone or multiple clones. The purpose of the research presented here is to distinguish
between these possibilities. I examined the spatial arrangement and genetic composition of populations of the zoanthid Parazoanthus parasiticus on sponges collected in the Florida Keys, USA. The pattern of dispersion of P. parasiticus on the sponge Callyspongia vaginalis
was random for four of the six populations examined. Two populations exhibited overdispersion. Polyp density decreased towards the growing edge of C. vaginalis, and approximately 12% of polyps were in the process of budding. Electrophoretic variability at five enzyme loci indicated
that populations of P. parasiticus, collected from three species of sponge, consisted of genetically identical individuals. Heterozygosity at several loci indicated an absence of meiotic segregation, and therefore clonal reproduction. Based on these data, it appears that a single
sexually derived larva settles on suitable hosts and colonizes space through asexual reproduction. The ecological and evolutionary consequences of these results are discussed as they relate to local adaptation.
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