Thermal performance and heat tolerance shape the distribution of body sizes in Girella laevifrons (Kyphosidae) along the intertidal gradient

Temperature is one of the key environmental factors affecting the ecology and distribution of species in nature. In the marine environment, transient intertidal fish species inhabiting rockpools must tolerate high temperature fluctuations, which vary in magnitude according to the level of the pools along the intertidal vertical gradient. Transient intertidal fishes recruit to upper pools and migrate to the lower intertidal rockpools as their body size increases. However, the physiological mechanisms that determine the ability of transient fish species to colonize highly variable environments remain unknown. In this study, we evaluated how thermal tolerance, estimated using thermal collapse time (TCT) curves and thermal performance curves (TPC), varies among body size categories located in different intertidal rockpools along the vertical gradient in one of the most abundant and ecologically relevant intertidal fishes occurring along the Chilean coast. Our results indicate that small fish exhibited higher metabolic performance and lower thermal sensitivity, which increased their thermal tolerance in highly variable environments, compared to medium- and large-sized fish present in rockpools closer to the subtidal zone. As a consequence of this physiological pattern, larger fish occupy lower intertidal pools (which are cooler and stable habitats), while smaller fish utilize higher intertidal rockpools (which are warmer and exposed to greater temperature variability). Our findings propose a clear mechanism for understanding animal distribution in highly variable habitats and highlight the role of thermal sensitivity as a key physiological factor determining the ability of fish to colonize highly stressful thermal environments.