Costs of thermal adaptation: warm-adapted copepods show increased energetic demands under cold exposure

Marine ectotherms have the potential to adapt to ocean warming through evolutionary adaptation and phenotypic plasticity. Both mechanisms are fundamental to ensure species persistence in the face of climate change, but the adaptation-plasticity trade-off hypothesis claims that increased thermal tolerance reduces phenotypic plasticity in thermal tolerance thereby limiting the resilience of these populations to acute temperature variability. However, if such trade-off exists for other traits remains poorly explored. To test this in a marine intertidal ectotherm, we reared the copepod Tachidius discipes in two thermal environments for 7 months creating a cold (10 °C) and a warm line (20 °C) established over ∼8 and ∼13 generations respectively. Copepods were then reciprocally transplanted during one generation to assess the capacity for adaptation to warming in life history traits and identify trade-offs and costs associated with it. Males and females of both lines grew smaller at higher temperatures with no signs for adaptation. In contrast, adaptation to high temperatures in clutch size buffered the clutch size reduction by 20 % while plasticity was retained. T. discipes also showed adaptation to high temperature in sex ratios, with larger proportions of females in these conditions in the warm line. However, adaptation to warmer temperatures carried a loss of plasticity in metabolic rates since the warm line maintained an elevated metabolism when transplanted to a colder environment. In conclusion, adaptation to warmer temperatures in reproductive traits carried a plasticity trade-off in metabolic traits that can bring warm adapted populations at risk during colder periods, especially when co-occurring with periods of food limitation.