Abstract
The reproductive biology of Sphaerodorum gracilis was studied between August 1978 and July 1979. Gametogenesis in both sexes involves the production of unusual gonads consisting of small clusters of cells floating in the coelomic fluid. Oogenesis begins in October with a prolonged period of oocyte proliferation lasting at least five months followed by a period of rapid vitellogenic growth leading to a restricted spawning season in the following October. Mature oocytes measure 200–250 μm in diameter and contain large quantities of lipid droplets and yolk granules. Spermatozoa were only observed in abundance in October; they possess a simple, small, round head measuring 2 μm in diameter, and a long filamentous flagellum. Fertilization is assumed to be external and the larvae have a direct, lecithotrophic development in keeping with other sphaerodorids.