Thermal niche adaptations of common mudskipper (Periophthalmus kalolo) and barred mudskipper (Periophthalmus argentilineatus) in air and water.

Thermal tolerance niche analyses have been used extensively to identify adaptive thermal tactics used by wholly aquatic fishes, however no study to date has quantified thermal niche characteristics of air-breathing fishes. We use standardized thermal methodologies to estimate temperature acclimation ranges, upper and lower acclimation response ratios, and thermal niche areas in common (Periophthalmus kalolo) and barred (Periophthalmus argentilineatus) mudskippers in air and water. Common and barred mudskippers had an upper chronic limit of 37.0 °C, and respective low chronic temperatures of 14.0 and 11.4 °C, resulting in acclimation scope values of 23.0 °C and 25.6 °C. Both fishes had moderately large thermal niches, with barred mudskipper expressing larger niche areas in both water and air than common mudskipper (676.6 and 704.2 °C compared to 641.6 and 646.5 °C). Acclimation response ratios were relatively low, with fish gaining or losing between 0.10 and 0.43 °C of heat tolerance with each 1 °C change in acclimation temperature. Although intraspecific total niche areas remained largely unchanged between media (≤10%), both species showed a slight increase in heat tolerance but a notable upward shift in intrinsic tolerance when emerged. Media-dependent thermal niche adjustment is a unique, and thus far undescribed physiological adaptation that in combination with behavioral responses, allow mudskippers to thrive in some of the most austere thermal environments experienced by any fish.