Physiological Traits for Predicting Poleward Extensions in Tropical Fishes: From Lab to Management

Tropicalization, the phenomena by which tropical organisms are extending their distributions poleward into temperate latitudes in response to increasing temperatures and strengthening boundary currents, is occurring globally. Vagrant tropical species have large ecological and economic ramifications for the temperate habitats they invade. However, not all vagrants are able to persist long term in temperate habitats, with the first winter being a potential bottleneck for their persistence. This brings into question how some tropical vagrant species are successful at surviving temperate conditions and the physiology underpinning this success. This provides the opportunity to not only look at the available data introspectively but also forward-thinking by applying a range of holistic physiological traits relevant for biology and management. Therefore, the aim of our review is twofold: to review the current state-of-knowledge of the physiological mechanisms underpinning tropicalization and to develop a physiological framework by which current practices can complement new perspectives and tools. We use range-expanding tropical reef fishes as a model group of over 100 species undergoing climate-driven range shifts and eastern Australia as a case-study location due to it being a primary focal "living laboratory" for understanding tropicalization dynamics since the early 2000s. Current studies suggest that diet, behavior, and metabolic trade-offs may explain vagrant fish persistence, but these studies focus on whole-animal traits. Our framework helps expand upon focal traits, life stages, experimental design, physiological traits (e.g., we highlight the value of genetic and cellular markers for metabolic pathway changes under cold stress as potential biomarkers) and species to improve our understanding of the mechanisms underpinning tropicalization. Taken together, our framework places emphasis on measuring a suite of complimentary physiological traits, from cellular to whole-animal, to help guide future predictions of the long-term persistence of tropical species in temperate habitats.