Food and heat stress in the California mussel: evidence for an energetic trade-off between survival and growth.

In response to thermal stress, many rocky shore organisms exhibit characteristic physiological changes associated with increased tolerance to subsequent high temperatures. Although presumably adaptive, activation of the heat-shock response requires a significant energetic investment and therefore may impose a trade-off between survival and other life-history traits. We investigated the effects of chronic heat stress and variation in food availability on the relative allocation of resources to competing demographic parameters in the California mussel, Mytilus californianus. Our data support the idea that acclimatory responses to temperature stress can drive trade-offs among traits, as predicted by theory. Chronic heat stress invoked a cost to individuals, expressed as a reduction in shell growth or size-specific tissue mass in the field and laboratory, respectively. At the same time, prior thermal conditioning resulted in higher proportional survival after acute exposure to more extreme temperatures. Overall, mussels receiving less food exhibited poor condition and survival relative to individuals fed more, suggesting that individuals with limited access to resources are at greater risk because they are less able to mitigate potential costs of thermal stress through physiological mechanisms. Accurately forecasting the effects of climate change in rocky intertidal ecosystems will therefore require understanding not just how organisms respond to different temperature regimes, but also how variation in local resource availability modifies those responses.