Reproduction in the Red Sea soft coral Heteroxenia fuscescens: seasonalityand long-term record (1991 to 1997)

Heteroxenia fuscescens is a common zooxanthellate soft coral on the shallow reefs in the Gulf of Eilat, northern Red Sea. In the Red Sea, during its p rolonged planulation, H. fuscescens is subjected to a seasonal environmenta l regime that alternates between stratified warm summer waters and upwellin g of low-temperature winter waters. To examine the possible relationship be tween these seasonal fluctuations and the reproductive characteristics of H . fuscescens, we monitored its breeding activity for a 6-year period, inclu ding the percentage of colonies releasing planulae, number of planulae rele ased per colony per night (fecundity), planula size and percentage of relea sed deformed planulae. During summer and fall the combined average percenta ge of planulating colonies of H. fuscescens was significantly higher than i n winter and spring. In addition, fecundity was greater during the summer t han the rest of the year. Planulae released during summer were longer, with almost zero percent deformation. During the rest of the year they were sho rter, with a higher percentage of deformation. The current study indicates that although H. fuscescens reproduces all year round, the quantity and qua lity of its reproductive features are subject to seasonal variability. In t he Gulf of Eilat seasonal changes in the abiotic features of the water may have an impact on its reproduction. During summer, primary productivity rea ches a distinctive maximum up to a depth of about 40 m. The winter upwellin g waters introduce nutrients from the depths into the upper 200 m, and espe cially into the major primary productivity zone, thereby triggering the ann ual phytoplankton bloom. Because colonies of H. fuscescens gain nutritional benefit from uptake of organic material dissolved in the water and by carb on fixation by zooxanthellae, we suggest that the seasonal fluctuations in the species' reproductive traits are related to these fluctuations in nutri ent and light levels.