Plasticity of a coral reef fish, Abudefduf saxatilis, to thermal stress and habitat degradation

Abstract

Sea surface temperatures are rising at unprecedented rates due to anthropogenic activities. Warm conditions have the potential to push marine species beyond their physiological limits, which can lead to challenges on the individual and community level. Major changes are being observed in tropical and subtropical species, where many species are already living close to their thermal maxima. For example, reef-forming corals are particularly sensitive to warming, and increases in temperature can lead to bleaching and ultimately degradation of the complex three-dimensional habitats they form. Thus, in the case of organisms associated with coral reefs, oceanic warming will lead to detrimental effects that could be compounded by irreversible changes in reef structure. Studies have shown that, for marine ectotherms, thermal stress leads to increased aerobic demands, which in turn causes physiological, molecular, and behavioral changes. Despite recent advances evaluating the effect of temperature in ectotherms, questions remain on the behavioral responses to habitat complexity and warming, and how these traits also influence the molecular processes of the main sensory organ, the brain. Here, I evaluated how thermal stress and habitat loss are acting independently and synergistically as stressors in a damselfish of the Tropical Western Atlantic, Abudefduf saxatilis. Commonly known as the sergeant major, this species is omnivorous, territorial, and abundant in coral reefs, seagrass beds, and rocky habitats throughout the Atlantic. In chapter 1, I acutely exposed juvenile A. saxatilis to warm temperatures and simplified environments to evaluate molecular changes in the brain and the oxidative stress of liver and muscle. The results indicate that warming resulted in increased oxidative damage in the liver and changes in molecular pathways including neurotransmission, immune function, and DNA and tissue repair, while habitat loss did not have a significant effect on its own. In chapter 2, I exposed juveniles to habitat loss throughout 3 months of development and found that individuals in low complexity habitat became more active, bold, and aggressive than those in a high complexity habitat. The results found here are parallel to observations in the field, highlighting the relevance of controlled aquarium studies to address how marine fishes respond to abiotic stressors. This thesis addresses key gaps in the literature on how fishes are responding to coral reef degradation and thermal stress and enhances our understanding on the plastic responses that enable generalist species to persist in a changing world.