Longevity, competence and energetic content in planulae of the soft coral Heteroxenia fuscescens

The longevity and competence period of the azooxanthellated planulae of the soft coral Heteroxenia fuscescens, a common shallow-reef inhabitant on the Red Sea reefs, were studied for the first time. The caloric content of the planulae in the course of their life is determined and its possible significance for the longevity and competence periods is considered. H. fuscescens planulae are able to metamorphose immediately upon release and tend to settle adjacent to their parent colonies. The current study presents evidence for absence of a pre-competence period. A maximal longevity of 50 days and a competence of 49 days were recorded, providing an effective dispersal time which encompasses virtually the entire lifespan of the planulae. A monthly variation was found in longevity and competence, possibly temperature related; significantly higher metamorphosis rates occurred during the hotter months of the year, indicating the occurrence of additional temporal variations related to the species reproduction. During late stages of the experiments most of the planulae underwent partial metamorphosis, probably due to a lack of necessary inducers for settlement. It is hypothesized that these polyps may obtain zooxanthellae through their mouth opening, thus acquiring the energy to extend their pelagic life, until they encounter such settlement inducers. Two-day old planulae had a caloric content of 0.58±0.05 cal x planula^-1; which decreased gradually with age, to a minimum of 0.35±0.005 cal x planula^-1 in 16-day old planulae. H. fuscescens has a wide distribution along the eastern coast of Africa, probably due m the longevity and competence of its planulae.