Sexual reproduction of a soft coral: Synchronous and brief annual spawning of Sarcophyton glaucum (Quoy & Gaimard, 1833)

The sexual reproduction of the alcyonacean octocoral Sarcophyton glaucum (Quoy & Gaimard, 1833) was studied for a period of about four years on the coral reefs of the northern Red Sea. S. glaucum is a dioecious species with gonads borne in the autozooids. The smallest colonies bearing testes measure 11 cm³ (6-7 years old) while females attain maturity at a much larger colony size of at least 61 cm³ (> ten years old). Sex ratio of the population is 1:1. The annual development of the sperm sacs takes 10-12 months. Oogenesis occurs every year, however egg maturation requires 22-23 months, resulting in the presence of two cohorts of oocytes in each female. The mature eggs are large with a maximal diameter of 500-750 µm. S. glaucum has a brief annual spawning period which occurs in the majority of the population during a single night (in 1980, spawning was on 9 July). The large size of the eggs is not the ultimate cause for their prolonged period of oogenesis. Nevertheless, the synchronous maturation of numerous eggs produced by a colony during a brief spawning period demands high energy expenditure which is allocated during two years. Fertilization is external and fully developed planulae are obtained 36 h after spawning. The larvae swim actively for 14 days, hence, promoting wide dispersal of the species. Life history features of S. glaucum include large body size, late age at the onset of reproduction, prolonged oogenic cycle, slow growth of the colony, and long life span.