Date of Award

Fall 12-2020

Document Type


Degree Name

Doctor of Philosophy (PhD)


Biological Sciences


Ecological Sciences

Committee Director

Daniel J. Barshis

Committee Member

David Gauthier

Committee Member

Richard Zimmerman

Committee Member

Mikhail Matz


Anthropogenic climate change is causing an increase in the frequency and severity of marine heat waves, resulting in declining health of coral reef ecosystems worldwide. Coral bleaching events – the breakdown in symbiosis between the coral host and their intracellular photosynthetic algae – are increasingly common in recent years and contribute to widespread losses in coral cover. However, bleaching and heat stress responses vary across spatial scales both within and among coral species. Coral populations native to highly variable environments can have greater bleaching resistance than corals from more stable habitats and corals transplanted into these variable reef sites can increase their thermal tolerance, providing promising evidence for the ability of corals to cope with rapid climate change. This dissertation investigates the physiological and genetic response of two massive corals, Porites lobata and Goniastrea retiformis, from a Moderately Variable (MV) and a Low Variability (LV) pool transplanted into a Highly Variable (HV) pool on Ofu Island in American Samoa. Paired transplant and native ramets were exposed to an acute thermal stress every six months (for 1.5 yrs) to evaluate changes in thermal tolerance. For both species, photosynthetic efficiency and chlorophyll loss following acute heat stress did not differ between ramets transplanted into the HV pool and respective native pool. Surprisingly, HV P. lobata exhibited the greatest bleaching susceptibility compared to MV and LV natives and there was no effect of acute heat stress on MV P. lobata. Genetic and gene expression patterns indicate shared responses to heat stress in the coral host, yet population-level differences were observed in response to acclimatization to a novel environment and symbionts had distinct variation in reacting to heat stress. During this study, Ofu’s backreef temperature regime surpassed historical records and fine scale temperature variation across reefs may have contributed to increased susceptibility of HV P. lobata. These results represent a stark contrast with other research on coral tolerance in variable environments, potentially underscoring species-specific mechanisms and regional thermal anomalies that may be equally important in shaping coral responses to extreme temperatures.


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