Eirik Ryvoll Åsheim, Anna H. Andreassen, Rachael Morgan, Fredrik Jutfelt 

Global warming is predicted to increase both acute and prolonged thermal challenges for aquatic ectotherms. Severe short and medium-term thermal stress over hours to days may cause mortality, while longer sub-lethal thermal challenges may cause performance declines. The interrelationship between the responses to short, medium and longer thermal challenges is unresolved. We asked if the same individuals are tolerant to both rapid and slow warming challenges, a question which has so far received little attention. Additionally, we investigated the possibility of a thermal syndrome where individuals in a population are distributed along a warm-type to cold-type axis. We tested whether different thermal traits correlate across individuals by acclimating 200 juvenile zebrafish (Danio rerio) to sub- or supra- optimal temperatures for growth (22 and 34°C) for 40 days and measured growth and thermal tolerance at two different warming rates. We found that tolerance to rapid warming correlated with tolerance to slow warming. However, individual tolerance to neither rapid nor slow warming correlated with growth at the supra-optimal temperature. We thus find some support for a syndrome-like organisation of thermal traits, but the lack of connection between tolerance and growth-performance indicates a restricted generality of a thermal syndrome. The results suggest that tolerance to rapid warming may share underlying physiological mechanisms with tolerance to slower heating, and indicate that the relevance of acute critical thermal tolerance extends beyond the rapid ramping rates used to measure them.