Alternate coral-bryozoan competitive superiority during coral bleaching

Bleaching of corals results from the loss of their symbiotic algae (zooxanthellae) and/or pigments. The supply of photoassimilates provided by the zooxanthellae to the coral declines during bleaching and reduces the ability to activate energy-costly processes such as maintenance, growth and reproduction. In the present study we compared the competitive outcomes, expressed as overgrowth and changes in colony sizes of Oculina patagonica (an encrusting Mediterranean stony coral) and the bryozoan Watersipora sp., growing in contact with each other, during and between bleaching events. Year-round observations of tagged colonies showed alternating competitive outcomes: O. patagonica wins over Watersipora sp. between bleaching events, but loses during bleaching events. Using the ¹⁴C-point-labeling technique on coral tissue, we examined intra-colonial translocation of photosynthetic products from the point-tissue labeling towards interaction zones. In non-bleached O. patagonica, competition resulted in preferentially oriented translocation of ¹⁴C products to the interaction zone located up to 8 cm away from the tissue-labeling site. Sites opposite the interaction zone received significantly less labeled photoassimilates compared to the interaction zone. In bleached colonies (40-85% bleached surface area), such translocation did not occur, probably explaining the failure to compete with the encrusting neighbor Watersipora sp. Our findings demonstrate the importance of colonial integration and resource orientation for the competitive superiority of O. patagonica.