Thermal plasticity over a marine-estuarine ecocline can buffer a tropical fish from warming.

Intraspecific variation in thermal tolerance can favor species persistence in a warmer ocean, but is often overlooked in fine-scale studies. Nonetheless, local drivers (e.g. salinity) interact with temperature to shape species' thermal response. Here, we acclimated juveniles of Brazilian silversides Atherinella brasiliensis captured at the limits of a marine-estuarine ecocline under reciprocal-cross conditions, to test for phenotypic plasticity in heat tolerance. We also tested whether silversides acclimated to temperatures predicted for 2100 (+3-4.5 °C). Fish in warm-brackish waters showed higher CTMax (Critical Thermal Maximum) than those in cold-marine conditions, regardless of their origin. Silversides' CTMax reached up to 40.6 °C, but it did not increase after exposure to temperatures predicted for 2100. Lack of acclimation response suggests that silversides heat tolerance has reached a "ceiling", despite thermal plasticity. Our findings show that fine-scale environmental heterogeneity can promote phenotypic plasticity for tropical species, reducing the risk of short-term extirpation.