Sexual reproduction in the tropical corallimorpharian Rhodactis rhodostoma

Abstract. Polyps of the tropical corallimorpharian Rhodactis rhodostoma segregate sexes between center and edge positions within aggregations produced by clonal replication. On a reef flat at Eilat, northern Red Sea, infertile polyps and males occur mainly along the edges of clonal aggregations, while females mostly occupy central positions within each aggregation. In addition, on the inner to middle reef flat where polyps of this species are abundant, aggregations consist mostly of females. On the outer reef flat, where polyps are rare, a sampled aggregation consisted mostly of males and infertile polyps. Male polyps are significantly smaller than females, and the smallest polyps are infertile. Fecundity increases significantly with polyp size in females, but testis size and number do not vary with body size in males. Oocytes 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 length. 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 spawning. The annual spawning of gametes coincides with a temporary drop in the frequency of clonal replication by polyps. We estimate that each female polyp of R. rhodostoma may release up to 3000 large eggs (500 μm in maximum diameter) each summer. The high investment of this corallimorpharian in sexual production of planktonic propagules may allow rapid dispersal to reef habitats distant from parent populations.