Physical contact stress can trigger larval release in the brooding coral Siderastrea stellata

Abiotic and biotic stressors are known to trigger reproductive activities in several aquatic organisms. In reef environments, physical contact as a response to competition for space on the benthos is a common stressor among sessile organisms, often leading to severe tissue damage and even mortality due to biological and chemical mechanisms. However, the effect of physical stress on coral reproduction has received less attention. In this study, we observed colonies of the scleractinian coral Siderastrea stellata releasing larvae in response to physical contact with the zoantharian Palythoa caribaeorum. Organisms were collected from reefs in Brazil and taken to the laboratory, where competition through physical contact was simulated in tanks by placing the two species in direct contact for 72 h. During this period, seven out of eight corals that were in physical contact with the zoantharian released larvae, showing tissue discoloration and a marked decrease in photosynthetic efficiency. Only one of the other eight colonies held as a control with no physical contact released larvae, indicating that physical contact may have been the trigger for larval release. This is, to our knowledge, the first report of physical contact-induced larval release in a scleractinian species, providing grounds for further investigating the potential mechanisms involved in this phenomenon.