Natural biotic and anthropogenic stressors can interact to alter contaminant toxicity. Energetic restrictions are potential mechanisms causing this pattern. To identify processes underlying observed effects of predation risk and copper (Cu) on delayed copepod age at maturity, we examined how these two stressors affect respiration rates. We tested two very different copepod species: the large, pelagic calanoid Calanus finmarchicus, and the small, semi-benthic harpacticoid Tigriopus brevicornis. Adult individuals were exposed for 12 h to the treatments: predation risk, Cu (23 µg L