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Changes to the timing of key life‐cycle events can alter selection on life‐history traits and have the potential to drive a cascade of effects at the community level. We investigated how the growth rate and sexual maturation of two annual killifish species (Austrolebias bellottii and Austrolebias nigripinnis) were altered by a change in the pattern of precipitation. These are annual species, endemic to ephemeral pools, persist in desiccated sediment as partly developed embryos. We sampled 18 pools supporting populations of both species in a Pampa region adjacent to the Rio Negro in western Uruguay throughout the austral winter of 2015. Fish growth and maturation were monitored from pool inundation to habitat desiccation, along with environmental variables. The region experienced an unprecedented mid‐winter desiccation of pools that usually contain water from autumn to late spring, typically only desiccating in summer. Many desiccated pools were re‐inundated from later rains, generating a second cohort of killifish in some pools (53%) in response to atypical conditions. The second cohort developed more rapidly than the first, with 40% earlier attainment of asymptotic body size. Rapid development of second‐cohort fish was associated with earlier maturation and greater investment in reproductive tissue. The study demonstrated the capacity of annual fish to express developmental plasticity that buffered the negative consequences of an atypical seasonal climatic cycle. The capacity of these annual fishes to cope with alternations to their life cycle will depend on the ability of egg banks in the sediment to tolerate an increased unpredictability of precipitation. Overall, we demonstrate how phenotypic plasticity can mitigate the negative impacts of climate change and associated altered phenology.

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