The present and prospective ecology of a rocky shore predator : foraging, physiology and their microbiome

Abstract

Ocean heating and marine heatwaves (MHWs) negatively impact many marine ecosystems and the species supporting their functioning. Importantly, even under nominal temperatures, intertidal animals are living in a habitat with high thermal heterogeneity and potential thermal stress. For many, the present day and future impacts of thermal stress on their trophic ecology, behaviour, physiology and microbiome are unknown. Here, I investigated these impacts on the crab Eriphia ferox, a key intertidal predator, using simulated thermal scenarios under present and future conditions. First, I investigated the food web of E. ferox and hypothesised their diet would change between summer and winter when more algal species and biomass are present on the shore. Further, I used an experimental intertidal system in the laboratory to test the hypothesis that acclimation to high temperatures experienced in present-day rock pools increases thermal tolerance. As metabolism and energetic needs are driven by temperature, I then experimentally tested the effects of predicted ocean temperatures under climate change on their metabolic rate, food consumption, and whether changing metabolic needs at higher temperatures would alter prey choice to optimise energy intake. Subsequently, I tested the hypotheses that marine heatwaves would increase the metabolic rate and expression of physiological stress markers. Last, I described their microbiome and tested the hypothesis that temperatures under ocean warming and concomitant MHWs would negatively impact bacterial community structure. Contrary to expectations based on previous observational studies, the food web constrained by stable isotope analysis demonstrated that E. ferox consumes a wide prey selection which is seasonally variable. They primarily consume molluscs but forage algae in the winter months when it is highly abundant. I found that the thermal tolerance of E. ferox increased by 3.3°C when exposed to rock pool temperatures of ~39.5°C for only 10 days, which demonstrated their scope for rapid thermal acclimation. Long-term warming to future temperatures of +2°C and +4°C above current summer temperature (~28°C) caused an increase in metabolic rates but did not change prey-size preference. Contrary to predictions, however, consumption of prey did not change with increased temperature, indicating an energetic imbalance that may not be sustainable under extended warming. Interestingly, E. ferox may be resistant to high-temperature, short-term MHWs; even though the hottest predicted MHWs (34°C) caused more than a 2-fold increase in metabolic rates there were no significant changes to stress biomarkers (TAC, TBARS & Hsp70). The microbiome of E. ferox shows similar resistance to warming, as there was no impact of high temperatures on the host bacterial community structure. However, some changes to the abundance of individual microbial taxa could impact host digestion and health. By investigating their trophic ecology, behaviour, physiology and microbiome, this thesis highlights the resistance of an important tropical rocky shore predator to potential thermal stress, now and under future climate change. Identifying which species are climate change resistant will aid in adaptive management strategies, and directing limited conservation resources to species under threat from end of century global change.