Cold thermal tolerance as a range-shift predictive trait: an essential link in the disparity of occurrence of tropical reef fishes in temperate waters

There is increasing empirical evidence of the shifting range distributions of tropical reef fishes as a response to persistent climatic warming. However, the link between the observed disparity of closely related species’ occurrence in temperate waters and their trait-based responses to winter thermal conditions is unclear. This study focuses on congeneric tropical reef fishes with similar dispersal potential (pelagic larval duration) but with varying occurrence in Kochi, southwestern Japan. The studied species include the abundantly occurring, overwintering, and reproductively established species in Kochi (adapted) and the less abundant species with no recorded overwintering and adult populations (non-adapted). This study assessed the responses of congeneric adapted and non-adapted species from two genera of Pomacentridae (Pomacentrus and Dascyllus) and one from Chaetodontidae (Chaetodon) to the winter seawater thermal range in Kochi by subjecting them to the decreasing temperature from 25 to 15 °C (1 °C d−1) in the laboratory. Both the adapted and non-adapted species demonstrated swimming and feeding reductions when temperatures approached 18 °C. However, stress-related behaviors were observed at 17 °C in the non-adapted species compared with 16 °C in their adapted congeners. The non-adapted species also exhibited relatively lower growth rates and higher mortality rates of > 50% during the 3-d exposure to 15 °C, indicating that the minimum winter temperature in Kochi is often lethal to these species. Thus, the present study highlights the species-specific cold thermal tolerance of tropical reef fishes to winter conditions at the leading edge as a useful predictor of a given specie’s range-shift capacity.