Intra- and inter-specific differences of upper thermal limits in the sand darters (Percidae: Ammocrypta)

Intra- and inter-specific differences in upper thermal limits, such as thermal tolerance, can impact conservation decision-making, yet values are typically obtained from a single population. In this study, we assessed the intra- and inter-specific variation of critical thermal maximum (CT_max) and agitation temperature (T_ag) among two eastern sand darter (Ammocrypta pellucida) populations (intra-specific) and five species of sand darters (genus Ammocrypta; inter-specific). The Ammocrypta genus is a monophyletic clade of ecologically similar benthic fishes demonstrating high niche conservatism. Field based CT_max trials were conducted stream-side across seasons (April-August 2019 and 2023) to encompass a range of ambient water temperatures (12-28°C). No intra-specific differences in CT_max or T_ag between eastern sand darter populations were observed, but population was important in predicting T_ag. Minimal inter-specific differences were observed within Ammocrypta with only eastern and naked sand darters (A. pellucida and Ammocrypta beanii) having consistent differences in CT_max, while the southern sand darter (Ammocrypta meridiana) had lower acclimation capacity. Overall, study results provide a better understanding of thermal tolerance differences within Ammocrypta and underline the need for estimates of population- and species-specific tolerances for conservation-based decisions regarding species capacities to withstand climate change.