Rapid thermal plasticity of cardiac and whole animal thermal tolerance in lab and wild populations of a eurythermal fish, the Mummichog (Fundulus heteroclitus)

Abstract

Nearshore aquatic habitats can experience large temperature changes over longer (months) but also shorter time scales (hours), potentially requiring fishes to adjust quickly. Warm acclimation can increase a fish’s upper thermal tolerance (CTmax) and improve cardiac heat tolerance, including greater peak heart rate and resistance to cardiac arrhythmia. In certain salmonids, these cardiac adjustments occur within days, but it remains unclear how widespread rapid thermal plasticity is among fishes, how it varies among species, or how it manifests in the wild. Using electrocardiogram and CTmax assays, I examined the time course of cardiac and whole-animal thermal acclimation in the eurythermal mummichog (Fundulus heteroclitus). During warm laboratory acclimation (30°C), mummichog showed the fastest cardiac thermal acclimation yet reported in fishes, with significant heat tolerance improvements occurring within 6-12 hours alongside increased CTmax. Comparisons of laboratory results with a seasonal time course of cardiac thermal tolerance and CTmax in wild mummichog revealed that field thermal tolerance was primarily shaped by the warmest temperatures experienced recently, with some cardiac traits matching fully acclimated laboratory phenotypes within a single tidal cycle. This rapid, reversible plasticity of thermal tolerance likely helps mummichog keep pace with environmental temperature variation and maximize thermal safety margins.