Differential tolerance of species alters the seasonal response of marine epifauna to extreme warming.

Marine heatwaves are occurring with greater frequency and magnitude worldwide and can significantly alter community structure and ecosystem function. Predicting changes in community structure in extreme temperatures requires an understanding of variation among species in their thermal tolerance, and how potential acclimatization to recent temperatures influences survival. To address this, we determined the tolerance to extreme temperatures in a crustacean epifaunal assemblage that inhabits macroalgae in the southeast Australian ocean warming hotspot. Amphipods were the most abundant group and the thermal tolerance of the most abundant species (two in winter and four in summer) was tested to determine their thermal limits and probability of survival in near-future extreme temperatures. Survival, measured as time to immobilization, was compared across species, sexes, life stage and body size. The greatest variation in tolerance to extreme temperatures was among species (not body sizes or life stages), indicating that heatwaves could shift the composition of the macroalgal associated epifaunal assemblage. Comparison of recent thermal history (between 18 °C to 22 °C) revealed greater thermal tolerance of warm acclimatized individuals. Our results indicate that the impacts of a marine heatwave will depend on local species composition and their timing relative to recent climate conditions.

Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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