Dynamics of thermal tolerance plasticity across fish species and life stages.

Climate warming with associated heat waves presents a concerning challenge for ectotherms such as fishes. During heatwaves, the ability to rapidly acclimate can be crucial for survival. However, surprisingly little is known about how different species and life stages vary in their acclimation dynamics, including the magnitude of change in thermal tolerance through acclimation (i.e. acclimation capacity; also known as the acclimation response ratio, ARR), the duration needed for the novel acclimation temperature to significantly alter thermal tolerance from the initial level (which we term the response induction time, t), or the duration needed to achieve the new acclimation steady state (which we term the time to full acclimation, t). To shed light on this knowledge gap, we studied the acclimation dynamics of three wild-caught fishes (goldsinny wrasse, three-spined stickleback and European flounder) by assessing upper thermal tolerance (CT) after different periods of time acclimating to a warmed environment. We also measured both CT and lower thermal tolerance (CT) in juvenile and adult lab-bred zebrafish acclimated to a warmed environment. Upper thermal tolerance of zebrafish and sticklebacks significantly increased after a 3 h exposure to a warm treatment, while t took six and 24 h in the wrasse and flounder, respectively. Goldsinny wrasse had the highest ARR, and did not reach full acclimation of CT within the duration of the study (10 days). All other species fully acclimated within 4-10 days. Juvenile zebrafish showed similar acclimation dynamics to adults for both upper and lower thermal tolerance, but had a higher CT for all acclimation durations. Our results demonstrate that acclimation dynamics of thermal tolerance vary across species, but can be similar between life stages within species. Understanding species-specific thermal plasticity is important for accurately modeling the projected impacts of climate change.