The adult mangrove tree crab, Aratus pisonii, is responsible for significant portions of leaf herbivory in mangrove forests. Using the mixing model software, IsoSource, we compare 13C & 15N of three A. pisonii size classes to evaluate their food sources in a mangrove forest along the Indian River Lagoon, FL. We also developed a mass balance method for incorporation of nutrient concentration into the model. Our results suggest that the juvenile Aratus diet of MPB and freshly fallen leaves shifts to a diet dominated by senescent leaves and lichens as an adult. This pattern is driven by strong intraspecific restrictions that prevent small A. pisonii from entering the mangrove tree canopy. Thus, only the larger crabs have access to nitrogen-fixing, 15N depleted lichens in the branches which gives them an unusually low 15N value relative to smaller individuals. Similarly, large crab reliance on leaf material results in depleted 13C values compared to the small crabs who utilize the 13C rich microphytobenthos.