Hyperosmosis induces polyp bail-out in pocilloporid corals
In this study, we applied hyperosmotic stress to Pocillopora acuta and observed polyp bail-out after 24 h, when seawater salinity reached ~46‰. In three other pocilloporid corals, i.e.,P damicornis, Seriatopora hystrix , and Stylophora pistillata, Shapiro et al. (2016) showed that polyp bail-out can be induced by evaporation-driven hyperosmotic stress. Although the species and salinity-changing profiles are different between this and previous studies, polyp bail-out was observed in both at relatively similar stress levels (10 – 14‰ higher than ambient conditions). Furthermore, our preliminary tests on P. verrucosa also displayed polyp bail-out under similar conditions and timing (data not shown). This observation implies that polyp bail-out may be a common response in pocilloporid corals and that these corals may experience strong osmotic fluctuations/stresses more frequently than we generally recognize. Although a salinity increase of >10‰ is relatively unlikely in waters in the subtidal zone (where most pocilloporid corals live), hyperosmotic environments can develop in tide pools and lagoons during low tides, due to evaporation, or when coral fragments get washed ashore occasionally at high tide. Since detached polyps are able to resettle and resume skeletogenesis after the hyperosmosis-induced polyp bail-out (Shapiro et al., 2016), this stress response can be considered as an escape mechanism for “lost” coral polyps to return to the ocean. Additional studies, however, are needed to further illuminate the ecological significance of this stress response.