Partitioning of the Estuarine Environment by Two Species of Cancer

There is a little mixing of Cancer irroratus and C. Borealis populations in Narragansett Bay, R. I. Cancer irroratus is found on sand. and C. borealis occurs in areas of course gavel and mixed rocky debris. The morphology, walking ability and serum of these crabs were examined to explain their separation in the bay. Walking ability was measured in a rotating jug. Cancer irroratus was at least five times more active and had a higher optimum temperature for walking than C. borealis. The optimum temperature for walking corresponded with distributions of both species on the east coast of North America: C. irroratus is most common in temperate waters, whereas C. borealis is a more northern, colder—water species. Cancer irroratus also had high concentrations of serum phosphate, which may be associated with its activity in nature. The concentrations of glucose, total nitrogen and chloride were the same in both species, presumably because their growth rates were similar and the salinity in both habitats was the same. Thus, C. irroratus is adapted for life on a flat, sand bottom where survival must depend upon an ability to run, catch food and escape from predators. C. borealis leads a reclusive life, hiding in crevices and relying upon its heavy claws for protection. Spatial partitioning of the bottom by Cancer is compared to temporal partitioning in the plankton by two species of Acartia. Competition within these groups might help to stabilize estuarine production.