Award: OCE-2048589

Coral reefs support enormous biodiversity and provide food, coastal protection, and income for millions of people, but they are increasingly at risk from ocean warming. Corals are able to thrive in oligotrophic waters thanks to their symbiosis with algae, which provide them with carbon sugars. This project investigated why some colonies of the reef‑building coral Porites are more thermal tolerant while other, visually similar, colonies on the same reef lose their algae in a process called ‘coral bleaching’. Our work shows that what appears to be a single species is actually composed of multiple hidden (“cryptic”) genetic lineages with distinct biology, ecology, symbioses, and responses to stress.

During our field seasons we tagged and profiled the genetics of 90 Porites colonies across lagoon and outer‑reef environments in Palau. Genome‑wide sequencing revealed three genetically distinct lineages living side‑by‑side with no evidence of hybridization. Laboratory heat‑stress experiments showed that the lineage most common in warmer, more variable lagoon habitats tolerated higher temperatures than lineages dominant on outer reefs. These results demonstrate that cryptic genetic diversity can strongly determine how corals respond to warming seas.

We also examined the microscopic algal symbionts and bacterial communities that fuel coral growth. While all colonies hosted broadly similar algal types, lineages differed in the specific algal strains and bacterial assemblages they harbored and in how their skeletons capture and use light. These physiological and microbial differences indicate that the cryptic lineages function as distinct biological units rather than members within a single species.

The project produced the first successful ex situ spawning and larval rearing of massive Porites in Palau. Larvae were capable of remaining planktonic for more than two weeks, suggesting substantial dispersal potential, but delayed settlement reduced survival—especially under high temperatures. Additional experiments showed that newly settled corals face higher mortality under combined high light and heat, revealing trade‑offs between dispersal and thermal tolerance that can limit reef connectivity and promote local adaptation.

A year‑long reciprocal transplant experiment (lagoon ↔ outer reef), integrating physiology, genomics, and microbiome profiling, indicates that both inherited differences and environmental exposure shape coral performance. We also documented a natural bleaching event and are assessing whether lineages differ in bleaching susceptibility in situ.

Broader impacts include workforce and capacity building: the project trained a postdoctoral researcher (now an Assistant Professor), several graduate and undergraduate students, and a Palauan intern who has taken a permanent research role at the Palau International Coral Reef Center. We developed and publicly shared an optimized protocol for RNA extraction from heavily calcified corals and contributed findings to a Perspective in Nature Ecology & Evolution, helping inform conservation, restoration, and policy efforts. Recognizing and conserving cryptic diversity within reef species will be essential for accurate predictions of reef futures and for designing effective management and restoration strategies.

Last Modified: 02/27/2026
Modified by: Sarah W Davies