UC San Diego

Coral reefs are threatened worldwide due to anthropogenic activities resulting in the loss of these ecosystems at an alarming rate. In this dissertation I build upon fundamental ecological theory related to early life history characters in order to identify factors affecting coral reproductive success in response to environmental change. I found that apparently healthy corals on degraded reefs contained lower energetic lipid content than conspecifics on healthy reefs within the island of Curacao, which contains some of the Caribbean's healthiest coral reefs. This work also revealed that populations differed in the number of offspring produced, highlighting reef- specific differences in reproductive potential. Motivated by observed variation in offspring size within and among species, I found that large size enhanced larval survival, yet unexpectedly did not afford larvae greater tolerance of harsh environmental conditions. Surprisingly, a species that produces large larvae and tolerates marginal environmental conditions as adults was particularly sensitive to environmental stress during the larval stage. I concluded that this difference arose due to the presence of symbiotic algae in larvae of this species. I subsequently determined that the uptake of symbionts during the larval stage led to disadvantageous behaviors and gene expression patterns. Furthermore, symbionts did not provide energy to larvae, as they do in adults. These results led me to theorize that the timing of symbiont acquisition is most optimal when it occurs around the time larvae locate cues for metamorphosing into an adult body form. To substantiate my theory in an evolutionary context, I performed a character trait reconstruction with the known phylogeny of reef corals. Evolutionary transition rates differed depending on whether a species transfers symbionts directly to larvae or obtains them from the environment. The loss of direct transmission correlated with transitions from high to low bleaching susceptibility in adult corals, suggesting a previously unknown evolutionary transition that favors environmental tolerance. In summary, my work contributes to general understanding of how larval characters and early life history strategies affect survival and fitness in corals, and offers insights into the ecological and evolutionary mechanisms by which larval corals respond to human-induced environmental change