UC San Diego

At every moment, a tropical reef coral interacts with millions of microbial organisms from tens of thousands of different species. Among these viruses, bacteria, archaea, dinoflagellates, and protists are cooperative symbionts as well as pathogens and parasites. Each interaction between a coral and a microscopic organism has ecological consequences for the coral community. In this dissertation, I show that juveniles of the Caribbean coral Montastraea faveolata suffer distance-dependent mortality in the presence of adult corals of the same species which is caused by microbial communities near these adults. This previously undiscovered structuring force will affect the spatial patterning of reefs and appears strong enough to drive the evolution of habitat selection behavior by coral larvae. The behavior of pre-settlement larvae differs based on both the microbial environment and the manner in which that environment is altered, indicating that water column microbes may be used as navigational information by dispersing corals. Sterilized seawater and penicillin both increase larval swimming rates, perhaps because larvae cannot detect the microbial "smell" of a suitable reef habitat, however only penicillin inhibits settlement and metamorphosis. Five antibiotics each induce a different pattern of abnormal behavior, indicating that the behavior of coral larvae could form the foundation of a new model system in toxicology. Furthermore, settlement failure on reefs may have a large behavioral component that has been underappreciated until now. The coral-dinoflagellate symbiosis is the best-known coral-microbe interaction however coral bleaching remains enigmatic. Based on observations made after a tropical storm, I propose that corals evolved bleaching as an adaptive mechanism to readjust symbiont communities after storm damage moves corallites into new light regimes. In a survey of viral diversity in the coral Diploria strigosa, I demonstrate that coral-associated viruses likely infect all other cells in the coral holobiont, including symbionts and potential pathogens. I propose that interactions among these viral groups could help to stabilize mutualistic coral-microbe interactions. Four additional natural history observations demonstrate the diversity of behaviors and interactions corals exhibit at the micron to millimeter scales and reveal that we have many more coral- microbe interactions left to discover