Phylogeny of respiratory adaptations and local temperature extremes shape the thermal vulnerability of intertidal crabs

1. Climate change is expected to significantly impact coastal populations worldwide. The macrophysiological factors that determine the upper thermal limits (UTL) of intertidal ectotherms, however, remain poorly understood. 2. In this study, we evaluated the UTL of air- and water- breathing intertidal decapods, juxtaposing these limits against the temperatures they usually experience at their latitudinal distribution. We explored the relationship between UTL, phylogenetic constraints and environmental temperatures in crabs from tropical and sub- tropical mangrove habitats through laboratory experiments. Additionally, we compiled and analysed existing data on the relationship between decapod thermal limits, latitudinal distribution and habitat temperatures. 3. Our study highlights that the UTL of Southeast Asian mangrove crabs is a phylogenetically conserved trait, heavily influenced by temperature and respiratory physiology. Additionally, analysis of existing data revealed a strong correlation between latitudinal distribution, respiratory strategy and thermal limits for decapods, reinforcing our experimental findings. 4. Furthermore, we found that mangrove crabs exhibit thermal safety margins (TSMs), calculated as the difference between UTL and maximum experienced temperatures that increase with latitude. 5. Our findings suggest that experienced temperature extremes are a major driver for thermal adaptation, and that the evolution of air- breathing enhanced the thermal plasticity in mid- latitude intertidal ectotherms. While our study found a positive correlation between TSM and latitudinal distribution, we also underscore the limitations of TSM as a standalone indicator of the vulnerability of intertidal ectotherms to global warming. The low- latitude species included here persist in habitats where environmental temperatures regularly exceed their predicted thermal limits. This mismatch between TSM estimates and observed performance highlights the need to incorporate both physiological and behavioural responses when forecasting species resilience to climate change.