INTERTIDAL MUSSELS EXHIBIT ENERGETIC TRADE‐OFFS BETWEEN REPRODUCTION AND STRESS RESISTANCE

Life‐history theory suggests that trade‐offs exist between fitness components, with organisms balancing investment in reproduction against survival and future reproduction. This study examined the influence of stress on physiological trade‐offs in the dominant rocky intertidal mussel Mytilus californianus on the central Oregon coast, USA. The intertidal zone is a highly heterogeneous thermal environment that could lead to intrapopulation variation in stress responses. Stress increases along a vertical gradient, with higher physical stress occurring in the higher intertidal zone, both due to reduced feeding time and longer exposure to aerial conditions. Reproduction and carotenoid content were compared in mussels from the low and high vertical edges of the mussel bed. High‐edge mussels invested less relative energy in reproduction and also spawned all of their gametes in the early summer, whereas low‐edge mussels continuously spawned small batches of gametes throughout the year. High‐edge mussels accumulated high concentrations of carotenoid pigments into their gonadal tissues, potentially to protect gametes from damaging oxidative stress experienced during aerial exposure. A reciprocal transplant experiment revealed plastic responses in growth and reproduction to increased stress. In contrast, carotenoid content did not increase in response to stress, suggesting that carotenoids may not change rapidly or may not be easily lost or gained. Our results indicate that mussels exhibit physiological trade‐offs and, under increased stress predicted from climate change scenarios, may allocate energy away from reproduction toward costly physiological defenses.