Plasticity rates and capacities differ across traits and temperatures: Insights from physiological responses of aquatic organisms to salinity change

Abstract Phenotypic plasticity allows organisms to track changing trait optima under environmental variation. To what extent this is achieved will be jointly determined by the rate of and the capacity for plasticity. Despite the potential fitness implications of these two components of phenotypic plasticity, a quantitative understanding of their variance across trait types and environmental conditions is lacking. Here, we address this knowledge gap by compiling data from time‐course experiments that describe how a range of physiological traits of aquatic ectotherms changes in response to a shift in salinity. We then build upon recent analytical advances to produce estimates of plasticity rate and capacity from each experiment that are comparable across traits. A total of 324 estimates of plasticity rate and capacity originating from 59 species of fishes, sharks and crustaceans were available for analyses. For each experiment, the trait was assigned to one of five trait categories. Bivariate meta‐analyses were used to quantify temperature effects on rates and capacities across trait categories. Our analyses show how temperature effects on plastic responses to a change in salinity differ among trait categories, and that this interactive effect of temperature and trait category differs for plasticity rate and capacity. Temperature effects were more consistent for plasticity rate than for capacity, with higher temperatures being associated with faster plasticity for hormones, metabolites and metabolic and osmoregulatory enzymes. For plasticity capacity, a positive effect of temperature was only found for metabolic and osmoregulatory enzymes. Within the range of observed temperatures, hormones had consistently the fastest predicted rate of response to a change in salinity, whereas the two categories of enzymes were relatively slow in comparison. Plastic responses to salinity change were substantially faster than those previously reported for thermal tolerance. Our study adds to growing evidence that the rate of phenotypic plasticity has important axes of variation. Further progress in understanding how organisms can deal with fluctuating environments will likely benefit from increased theoretical and empirical efforts within this field of research. Read the free Plain Language Summary for this article on the Journal blog.