Development and survivorship of zooxanthellate and azooxanthellate primary polyps of the soft coral Heteroxenia fuscescens: Laboratory and field comparisons

The zooxanthellate Red Sea soft coral Heteroxenia fuscescens releases aposymbiotic planulae nearly all year round, with higher numbers in the summer than in other seasons. After metamorphosis, primary polyps become infected with symbiotic dinoflagellates (zooxanthellae), derived from the ambient seawater. This study compares aspects of development and survivorship of metamorphic stages of H. fuscescens in relation to onset of infection by the algal symbionts in the laboratory and the field. We revealed no distinct differences in the timing and sequence of morphogenetic events during metamorphosis between zooxanthellate and azooxanthellate primary polyps in the laboratory. In the field, the polyps exhibited higher developmental synchronization during the first days after metamorphosis compared to the laboratory-reared ones, probably as result of a rapid and synchronized onset of metamorphosis in the former. Later, there were no distinct differences in these parameters between the laboratory-reared zooxanthellate polyps and those in the field. Although the symbiotic state did not appear to affect developmental sequence or timing, it did increase host survivorship. In the laboratory, the survivorship of zooxanthellate primary polyps was significantly higher compared to azooxanthellate ones. Therefore, the symbiotic state appears to be ultimately important in overall survivorship. The survivorship of zooxanthellate primary polyps developed from planulae released during the summer months was higher compared to those from other seasons, thus strengthening existing evidence that the summer is the most favorable season for the breeding and development of H. fuscescens sexual progeny. Comparisons between laboratory-reared zooxanthellate primary polyps and naturally settled animals in the field showed a higher survivorship in the laboratory, possibly due to unfavorable conditions commonly occurring in the field. Our findings indicate that H. fuscescens represents an appropriate model system for study of acquisition of algae by aposymbiotic offspring and the impact of the process on their development and survivorship.