Genetic differentiation underlies seasonal variation in thermal tolerance, body size, and plasticity in a short-lived copepod.

Organisms experience variation in the thermal environment on several different temporal scales, with seasonality being particularly prominent in temperate regions. For organisms with short generation times, seasonal variation is experienced across, rather than within, generations. How this affects the seasonal evolution of thermal tolerance and phenotypic plasticity is understudied, but has direct implications for the thermal ecology of these organisms. Here we document intra-annual patterns of thermal tolerance in two species of Acartia copepods (Crustacea) from a highly seasonal estuary, showing strong variation across the annual temperature cycle. Common garden, split-brood experiments indicate that this seasonal variation in thermal tolerance, along with seasonal variation in body size and phenotypic plasticity, is likely affected by genetic polymorphism. Our results show that adaptation to seasonal variation is important to consider when predicting how populations may respond to ongoing climate change.