The relative abundance of the two crabs from Georgetown, South Carolina, compared to other areas characterizes Polyonyx as a more southernly distributed crab than Pinnixa. However, the zoeae and adults of Polyonyx are less tolerant of high temperatures than either the zoeae or adults of Pinnixa as indicated by both lethal limit and metabolic studies. Adult Polyonyx are also less tolerant of low temperatures than adult Pinnixa. Results indicate that Polyonyx is an obligate commensal of Chaetopterus, and Pinnixa is a facultative one.