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

Abstract

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 (CTmax) and agitation temperature (Tag) 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 CTmax 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 CTmax or Tag between eastern sand darter populations were observed, but population was important in predicting Tag. Minimal inter-specific differences were observed within Ammocrypta with only eastern and naked sand darters (A. pellucida and Ammocrypta beanii) having consistent differences in CTmax, 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.