Nitrogen Limitation in the Herbivorous Land Crab Cardisoma guanhumi

The herbivorous land crab Cardisoma guanhumi is limited in growth when fed plants available in its natural habitat. Crabs maintained in the laboratory on a plant diet supplemented with casein contained more tissue and nitrogen than crabs trapped from the field or maintained on plants alone. Nitrogen limitation may explain the anomalously slow growth rate estimated for this species. Behaviors were observed that offset low nitrogen in a plant diet and that included predation and cannibalism. Although crabs were selective about the plants eaten in their habitat, the choice was not correlated with nitrogen content in the plants and indicates the importance of allochemical defense to both plant survival and actual plant availability to the crabs.