Thermal Generalism Shapes Tolerance to Extreme Heatwaves in Estuarine Fishes.

Climate change is widely expected to push many taxa beyond their biological limits, yet some populations persist in environments that routinely experience climatic extremes. This raises a fundamental question: does their thermal tolerance reflect species-level thermal generalism (an evolved capacity to maintain performance across a broad thermal range) or population-specific acclimation to local conditions (phenotypic plasticity)? Distinguishing between these alternatives is key for predicting whether tolerance to climatic extremes will generalise across species distributions or remain restricted to heat-tolerant populations. To test these competing hypotheses, we quantified physiological performance (at cellular and organismal levels) in three economically important estuarine fish (Acanthopagrus australis, Sillago ciliata and Rhabdosargus sarba) collected from 20 sites across 15 estuaries along a ~1000-km latitudinal gradient spanning their core and poleward range edges. Sites differed in thermal variability, including some that intermittently exceeded the severest end-of-century climate projections. We then experimentally exposed two of the fish species from low and high thermal variability sites to moderate (+2°C, 14 days) or extreme (+4°C, 7 days) simulated marine heatwave events, assessing 12 physiological and behavioural proxies. All three species collected in situ maintained comparable physiological performance across sites of contrasting thermal variability. Both experimental species, each collected from a single high- and low-variability site, tolerated heatwave conditions regardless of thermal history or heatwave intensity, supporting species-level thermal generalism rather than population-specific acclimation. In the absence of heatwave conditions, S. ciliata from high-variability sites also showed steeper allometric scaling and faster growth at 26 versus 24°C experimental control temperatures, suggesting this species may benefit from the warmer baseline temperatures projected under gradual climate change. Combined, our findings support thermal generalism as the primary driver of tolerance in estuarine fishes to climatic extremes, with important implications for the fisheries they support under climate change.