Synergistic Effects of UVR and Temperature on the Survival of Azooxanthellate and Zooxanthellate Early Developmental Stages of Soft Corals

The interaction between ambient water temperature and UVR and its possible effect on the survival of coral early life-history stages was investigated in the Red Sea soft corals Heteroxenia fuscescens Eherenberg, 1834, and Rhytisma fulvum fulvum Forsskål, 1775. Planulae and azoo- and zooxanthellate polyps were obtained from colonies collected from the coral reefs at Eilat (Gulf of Aqaba, northern Red Sea). Survival of the different developmental stages under different UVR regimes was examined in the laboratory and monitored every 12 hrs until 50% survival (i.e., LD₅₀) was attained. Maximal LD₅₀ value for H. fuscescens planulae under exposure to UVR was only 70 hrs whereas the unexposed planulae showed 100% survival over the same time period. Survival of Azooxanthellate primary polyps of H. fuscescens increased with higher temperatures under UVR exposure, and zooxanthellate polyps had higher rates of survival than azooxanthellate polyps at 26 °C. Rhytisma fulvum fulvum planulae had higher survival rates compared to those of H. fuscescens planulae, and azoo- and zooxanthellate primary polyps. Our results demonstrate that UVR and elevated temperature may have a synergistic effect on the survival of soft corals at early life-history stages. In addition, Mycosporine-like amino acids (MAAs) may have played a role in the survival of the different juvenile stages of the two studied species.