Understanding the Adaptive Ability of Corals to Changing Environments.

Abstract

Reduced water quality is a major local threat to coral reefs worldwide, and has caused severe declines in the health of coral reefs in Hawaii, especially the nearshore areas. The corals living in Maunalua Bay, Oahu are under continual stresses from sedimentation and toxicant laden runoff as a result of large-scale urbanization that has taken place in the last century. Despite prolonged exposure to these environmental stressors, some corals are able to thrive, suggesting selection (adaptation). My dissertation research investigated whether corals in the nearshore areas have genetically adapted to their reduced water quality environment. The first chapter analyzed the population genetic structure of P. lobata, which revealed clear genetic differentiation between the nearshore and offshore P. lobata populations in Maunalua Bay, as well as two reefs in West Maui. My second chapter investigated the phenotypic differences between the nearshore and offshore P. lobata genotypes, found in the first chapter, to determine if the observed genetic differentiation was formed by selection. The reciprocal transplant and common garden experimental results showed clear physiological and molecular response differences between the two genotypes, highlighting the stress resilient traits of the nearshore genotype and inherent differences in the metabolic state between the genotypes. The results from the first and second chapters, however, suggest this local adaptation might happen at the cost of genetic diversity. The Porites corals are a notoriously difficult genus to identify correctly, due to their highly variable skeletal architecture and unresolved phylogeny. In order to assess the intraspecific morphological and genetic variations in P. lobata, morphometrics and genomic (RAD-seq) analyses were conducted in my third chapter. The morphometric data revealed significant groupings of skeletal characters between the geographic locations, and population genomic analysis also supported the strong geographical signature. There was a significant correlation between the morphological and the genetic distances, suggesting the genetic basis for the skeletal morphology of P. lobata. Understanding the genetic basis of coral survival offers a critical insight into their adaptive ability, which is indispensable for protecting the essential reef-building corals from impending environmental and climate change.