CTmax Repeatability Data Files & README

Critical thermal maximum (CT_max) is the most widely used method for quantifying upper thermal limits in ectotherms. CT_maxexposes animals to a consistent rate of environmental warming until they lose motor function. CT_max has been used to assess intraspecific variation among life stages, populations, or as a function of body size, often with the assumption that it is a durable and heritable trait at the individual-level. The existence of within-individual repeatability of CT_max has been used to infer the potential for thermal adaptation via the positive correlation between the repeatability of a trait and its heritability. However, for how widely used CT_maxhas become, surprisingly few studies have quantified within-individual repeatability in aquatic ectotherms, and none have assessed repeatability across contexts. We examined the cross-context repeatability of CT_maxin two freshwater ectotherms (one decapod crustacean and one teleost fish): rusty crayfish <i>Faxonius rusticus</i> (n = 31) and pumpkinseed <i>Lepomis gibbosus</i> (n = 39). Individual repeatability was measured across multiple trials (n = 5 pumpkinseed CT_max measurements, n = 7 rusty crayfish) that varied in acclimation temperature, oxygen saturation, and salinity. CT_max was most strongly influenced by acclimation temperature. Repeatability varied based on the statistical approach and between the two species. Pumpkinseed repeatability across contexts was moderate (<i>ca.</i> 0.4), similar to previous reports on within-context CT_maxrepeatability studies in fishes. In rusty crayfish, repeatability was much lower (<i>ca</i>. 0.15). This suggests CT_maxrepeatability may be both taxon- and context-dependent, thus further investigation is needed before inferring CT_max as a durable trait.