Depth-independent phenotypic variation of massive Porites coral color morphs

As coral reefs continue to decline due to anthropogenic stressors, community characterizations will reveal both historical selection processes and adaptive potential to environmental change. To address the potential role of color in the distribution and resilience of massive Porites corals, we surveyed the distributions of two dominant color morphs (brown and purple), and a unique intermediate state, across a depth gradient in Guam, Micronesia. We found that brown colonies dominated populations across all depths, and larger colonies had higher rates of partial tissue mortality and active disease lesions. Despite the dominance of brown colonies, both brown and purple color morphs showed a high similarity in susceptibility, as indicated by the colony sizes, the severity of partial tissue mortality, and the prevalence of active disease lesions. This is a non-intuitive result given the presence of phenotypic plasticity between color morphs, evident by an intermediate, transitionary stage between brown and purple colonies that suggests a functional divergence between one color over the other. The study also revealed the dominance of small colonies at depth, which provides some insight into the ecological impacts that may have shaped Guam's current massive Porites population size structure over the past several decades. With this, we provide foundational insight into the adaptive strategies and historical pressures that have shaped the modern massive Porites population.