Northward expansion leads to cold tolerance? Investigating thermal adaptation of the non-native pike killifish (Belonesox belizanus) in Florida

For an introduced species to expand its range, individuals must be able to adapt to novel stressors (biotic and abiotic) that they may encounter. Florida has a distinct temperature gradient and rapid climate zone transitions from warmer subtropical regions to cooler temperate regions. Mean minimum temperatures are thought to prohibit northward movement of introduced fish species in south Florida. As the introduced Belonesox belizanus (pike killifish) expands its introduced range northward, it may be gaining cold tolerance. This study sought to compare thermal tolerances and cessation of feeding between adult B. belizanus from southern (initial, introduced population) and northern (spreading, fringe edge population) populations from its introduced range. To compare thermal tolerances between locations, southern individuals and northern individuals were acclimated to three temperatures (20, 25, and 30 °C) and their upper and lower lethal temperatures and cessation of feeding temperatures were measured. Two-way ANOVAs revealed that lower lethal temperatures and cessation of feeding temperatures were significantly different between northern and southern populations, with northern individuals having a lower lethal temperature approximately 0.5 °C lower and a cessation of feeding temperature approximately 2 °C lower compared with southern individuals. Directional selection may be influencing this non-native species’ thermal tolerance and feeding ability benefitting their continued effort to disperse from subtropical to temperate environments within Florida. This study indicates that B. belizanus may be gaining cold tolerance as it continues to spread northward in its introduced range.