Variability of morphology-performance relationships under acute exposure to different temperatures in three strains of zebrafish

Locomotion is thermally sensitive in ectotherms and therefore it is typically expressed differently among thermally heterogenous environments. Locomotion is a complex function, and while physiological and behavioural traits that influence locomotor performance may respond to thermal variation throughout life, other contributing traits, like body shape, may have more restricted responses. How morphology affects locomotor performance under variable temperature conditions is unknown. Here, we investigated three genetically distinct strains of zebrafish (AB, WIK, and Tu) with a shared multi-generational history at 28°C. After rearing fish at a constant 28°C, we measured prolonged swimming speed ( U crit ) at each of six temperatures (between 16°C and 34°C). Speed was strongly positively correlated among temperatures, resulting in most among-individual variation being temperature-independent (i.e., fish were relatively fast or slow across all temperatures). However, we also detected significant variation along two axes reflecting temperature-dependent variation, including one reflecting differences among individuals consistent with a hotter-colder tradeoff (i.e., individuals that were relatively fast at cooler temperatures were relatively slow at hotter temperatures and vice versa). Although strains differed in mean swimming performance, within strain (among-individual) patterns of speed variation were markedly consistent. Body shape and size explained significant variation among individuals in both temperature-independent and temperature-dependent axes of swimming speed variation. Notably, morphological traits that were most strongly associated with temperature-independent performance variation (i.e., faster-slower) differed from those associated with temperature-dependent (i.e., hotter-colder) variation. Further, there were significant differences among strains in both the direction and strength of association for specific morphological traits. Our results suggest that thermally heterogenous environments could have complex effects on the evolution of traits that contribute to whole organism performance traits.