CTmax Repeatability Data Files & README

Critical thermal maximum (CTmax) is the most widely used method for quantifying upper thermal limits in ectotherms. CTmax exposes animals to a consistent rate of environmental warming until they lose motor function. CTmax 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 CTmax 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 CTmax has 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 CTmax in two freshwater ectotherms (one decapod crustacean and one teleost fish): rusty crayfish Faxonius rusticus (n = 31) and pumpkinseed Lepomis gibbosus (n = 39). Individual repeatability was measured across multiple trials (n = 5 pumpkinseed CTmax measurements, n = 7 rusty crayfish) that varied in acclimation temperature, oxygen saturation, and salinity. CTmax 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 (ca. 0.4), similar to previous reports on within-context CTmax repeatability studies in fishes. In rusty crayfish, repeatability was much lower (ca. 0.15). This suggests CTmax repeatability may be both taxon- and context-dependent, thus further investigation is needed before inferring CTmax as a durable trait.