Sexual reproduction in the tropical corallimorpharian Rhodactis rhodostoma

Polyps of the tropical corallimorpharian Rhodactis rhodostoma segregate sex es between center and edge positions within aggregations produced by clonal replication. On a reef flat at Eilat, northern Red Sea, infertile polyps a nd males occur mainly along the edges of clonal aggregations, while females mostly occupy central positions within each aggregation. In addition, on t he inner to middle reef flat where polyps of this species are abundant, agg regations consist mostly of females. On the outer reef flat, where polyps a re rare, a sampled aggregation consisted mostly of males and infertile poly ps. Male polyps are significantly smaller than females, and the smallest po lyps are infertile. Fecundity increases significantly with polyp size in fe males, but testis size and number do not vary with body size in males. Oocy tes are present in polyps during most of the year and gradually increase in size until annual spawning in June July during the period of maximum day l ength. Testes do not vary significantly in size during the year and remain a small proportion of body mass (<8%). In contrast, females invest up to 30 % of their body mass into gonads during the months immediately before spawn ing. The annual spawning of gametes coincides with a temporary drop in the frequency of clonal replication by polyps. We estimate that each female pol yp of R. rhodostoma may release up to 3000 large eggs (500 <mu>m in maximum diameter) each summer. The high investment of this corallimorpharian in se xual production of planktonic propagules may allow rapid dispersal to reef habitats distant from parent populations.