Coral reef fishes from the world's most thermally extreme reefs as models for understanding future climate change

Abstract

An increase in anthropogenic greenhouse gasses is causing the Earth to warm, with current projections estimating an increase of 2 – 4.8°C by the end of the century. A multitude of climate change-attributed affects are already being seen across global ecosystems, as many plant and animal species, human settlements and environmental interactions are sensitive to elevated temperatures of only 1°C. One of the most vulnerable ecosystems to climate change effects are coral reefs, as increasing temperatures and a higher frequency of extreme weather events has already caused wide-spread devastation on reefs across the globe. Over 4000 species of teleost fish are associated with coral reefs, yet their responses to climate change are not well understood, although it is suggested that many reef species are likely already living close to their maximum temperature, with little capacity to acclimate. This PhD research is therefore aimed at increasing our knowledge of how reef fish species respond to increasing temperatures, to enable better predictions of future effects on fish species, populations, and coral reef ecosystems across the world. We use the Arabian Gulf (AG) as a natural proxy for climate change, as species here have evolved in a thermally extreme environment where sea surface temperatures range from as low as 12°C, up to >35.5°C each summer. Reef fish species within AG are therefore surviving temperatures even higher than those predicted for coral reefs elsewhere by the end of the century. We found that fishes within AG are not using behavioral thermoregulation as a strategy for avoiding the extreme summer temperatures as abundance was higher on the reefs during the summer (Chapter III). We therefore investigated metabolic traits and thermal tolerance of three common reef fish species (Scolopsis ghanam, Cheilodipterus novemstriatus and Escenius pulcher) across the natural temperature range of AG, comparing to the same species from the neighboring Gulf of Oman (GO) where conditions are less extreme (Chapter IV). We found that those from AG could consistently outperform the GO population, however there was evidence of trade-offs, and large species-specific differences in response to temperature, signifying ecological consequences as temperatures continue to rise. To assess whether the enhanced performance of AG fishes was enabled by the remodeling of aspects of the cardiorespiratory system, we investigated cardiac and gill morphology of S.ghanam from both AG and GO across the same temperature range. Although we found differences between the two populations, it does not appear evident that the parameters investigated within our study alone are facilitating greater performance of this species within AG. Understanding the impacts of future climate change on reef fish species is of paramount importance for better predicting population and ecosystem effects, and the use of multidisciplinary studies such as this PhD thesis, allows insight into mechanisms that may be underpinning performance responses. With continued research using natural proxies for climate change, we will gain better understanding into the future of coral reef ecosystems.

Date of Award	31 Dec 2023
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Holly Shiels (Supervisor)