Physiological response & recovery capacity of key grazers to climate change

Abstract

Global climate change is altering environmental conditions in the world’s oceans through gradual increase in carbon dioxide (pCO2) and ocean warming, to short-term climatic events such as temperature spikes and heatwaves. How ecosystems are impacted is underpinned by biological responses and underlying physiological mechanisms. I examine the physiological responses of key subtidal benthic grazers to different rates of environmental change. First, I tested the hypothesis that the sea urchin Heliocidaris crassispina would be unable to resist acute thermal stress events as physiological systems would not be able recover. Subsequently, the response of dominant grazing sea urchin (H. crassispina) and gastropod species Trochus maculatus and Astralium haematragum to gradual increases in pCO2 and temperature was assessed, to test the hypothesis that these two groups of organisms, whom share similar roles in ecosystems, would have similar physiological responses to environmental stress. Noting that acute stress had the potential for larger biological effects, I then tested the response of the congeneric urchin, Heliocidaris erythrogramma, to longer, environmentally relevant heat stress scenarios to test the hypothesis that intense thermal stress would cause irreversible physical and physiological impairment, instigating mortality. Lastly, I tested the hypothesis that individual urchins that were able to survive heatwaves would pass thermal tolerance onto their offspring, though there would be negative trade-offs during the developmental period. While neuromuscular function of H. crassispina improved during moderate heating (+3 °C, 31 °C), it was compromised under extreme heat-spike conditions (34 °C, + 6 °C), but recovered after return to ambient conditions (28 °C), displaying physiological resilience to short, intense heating. Contrary to predictions, the urchins and the gastropods responded differently to altered conditions despite sharing similar functional roles. Urchin metabolism increased in response to temperature, whereas the gastropods had depressed metabolism in response to elevated pCO2. Importantly, all species substantially decreased consumption of food, with the urchins demonstrating an energetic mismatch under combined future pCO2 and temperatures. Although the metabolism of H. erythrogramma increased with temperature, food consumption did not, creating a metabolic imbalance, and likelihood of further damage. Subsequently, thermal stress increased the incidence of bald-patch disease, which elevated and primarily influenced mortality highlighting that whilst metabolic rates may recover, potentially fatal trade-offs can be triggered if energetic homeostasis is not maintained. Importantly, offspring from the adults exposed to extreme heatwave temperatures were larger and reached settlement faster when developed under higher temperatures. Far fewer progeny from control adults survived at higher temperatures, suggesting a possible inheritance of thermal tolerance. However, the progeny of heat stress conditioned parents ultimately suffered high mortality, a potential “live-fast-die-young” trade-off. In summary, species which share similar functional roles can display different responses to environmental stressors, and stress intensity and duration can have substantially different physiological consequences. By demonstrating how the physiology of different species will be affected by stressful environmental conditions, this thesis contributes new knowledge on the physiological mechanisms which may underly changes to benthic communities from temperate and tropical habitats under climate change.