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Omnivory is a widespread trophic strategy with variable impacts on survival and reproduction, even within closely related taxonomic groups. In coastal ecosystems experiencing extensive environmental changes, many decapod crustaceans employ omnivorous feeding strategies. Because animals initially respond to environmental changes with behavioral modifications that can alter their foraging habits, a mechanistic understanding of how diet influences fitness is essential to predict the impact of future environmental changes on species fitness and population dynamics. We investigated the impact of an omnivorous diet on the consumption, survivorship, physiological condition, and reproductive effort of the mangrove tree crab Aratus pisonii, a major mangrove consumer. A. pisonii engaged in compensatory feeding on plant material; crabs consumed more food and digested it less efficiently as the proportion of plant material in their diet increased. Although there were relatively few deaths, survival appeared to be negatively impacted by the consumption of large quantities of animal material and marginally negatively influenced by high levels of plant consumption. Physiological condition improved due to consumption of both plant and animal material, while only consumption of animal material increased reproductive investment. These results demonstrate that the opportunistic inclusion of animal material in the diet of A. pisonii significantly improves physiological condition and reproductive effort, and suggests that vitellogenic individuals cannot fully compensate for a lack of animal material in their diet by increasing plant consumption. This study provides a mechanistic framework to understand how potential diet changes by omnivorous crabs such as A. pisonii, which are facing numerous changes in their environment, may impact their fitness and population dynamics.

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