Repeatability of critical thermal maximum (CT_max) in two freshwater ectotherms across contexts

Critical thermal maximum (CT_max) is the most widely used method for quantifying acute upper thermal limits in ectotherms. CT_max protocol exposes 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_max 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 CT_max in two freshwater ectotherms (one decapod crustacean and one teleost fish): rusty crayfish Faxonius rusticus (n = 31) and pumpkinseed Lepomis gibbosus (n = 38). Individual repeatability was measured on a weekly basis 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 (ca. 0.4), similar to previous reports on within-context CT_max repeatability studies in fishes. In rusty crayfish, repeatability was much lower (ca. 0.16). This suggests CT_max repeatability may be both taxon- and context-dependent, thus further investigation into repeatability is needed across species for this important and widely used trait.