Mud crabs (Xanthidae) in Chesapeake Bay: claw characteristics and predation on epifaunal bivalves

We examined claw characteristics of mud crabs (Eurypanopeus depressus, Rhithropanopeus harrisii) to determine if one crab species was potentially more powerful than the other. We related our findings to the abilities of individuals of each species to open epifaunal mytiliform bivalves (Ischadium recurvum; Mytilopsis leucophaeata) that occur on beds of eastern oysters (Crassostrea virginica) in mesohaline Chesapeake Bay. There were high correlations between claw width or height and claw length, and between claw length and carapace width for both mud crab species. The mechanical advantage or "grip strength" of the crusher and cutter claws of both species did not change with crab size (carapace width) and did not differ between sexes in each species, nor did the cutter data differ between species. However, individuals of E. depressus had a significantly stronger crusher claw grip than did those of R. harrisii. Data on mechanical advantage for both species were similar to values reported in the literature for members of other xanthid crab species. These values in turn overlapped those reported for calappid, cancrid, majid, and grapsid crabs, and were greater than those of various species of portunid crabs and individual species of fiddler crab, lobster, crayfish, and ghost shrimp. When simultaneously presented with the two species of bivalves, the mud crabs E. depressus chose mussels of M. leucophaeata first and crabs of R. harrisii chose mussels of I. recurvum first about two-thirds of the time; ultimately, the crabs ate both bivalve species in >50% of the choice experiments. The size range in E. depressus was greater than that in R. harrisii, and crabs of E. depressus opened larger bivalves than did crabs of R. harrisii, although similar-sized individuals of the two crab species overlapped in their ability to open bivalves of both species. In Mytilopsis leucophaeata, there is probably no size refuge from predation by the mud crabs whereas the larger mussels of I. recurvum do have a refuge in size.