Sponges harbor genetically identical populations of the zoanthid Parazoanthus parasiticus

Many tropical sponges harbor zoanthid populations on their surfaces. Althou gh 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 clo nes. The purpose of the research presented here is to distinguish between t hese possibilities. I examined the spatial arrangement and genetic composit ion of populations of the zoanthid Parazoanthus parasiticus on sponges coll ected in the Florida Keys, USA. The pattern of dispersion of P. parasiticus on the sponge Callyspongia vaginalis was random for four of the six popula tions examined. Two populations exhibited overdispersion. Polyp density dec reased towards the growing edge of C. vaginalis, and approximately 12% of p olyps were in the process of budding. Electrophoretic variability at five e nzyme loci indicated that populations of P. parasiticus, collected from thr ee species of sponge, consisted of genetically identical individuals. Heter ozygosity at several loci indicated an absence of meiotic segregation, and therefore clonal reproduction. Based on these data, it appears that a singl e sexually derived larva settles on suitable hosts and colonizes space thro ugh asexual reproduction. The ecological and evolutionary consequences of t hese results are discussed as they relate to local adaptation.