|Jonah Crab carapace Cancer borealis|
|Atlantic Rock Crab Cancer irroratus|
As far as I know, from reading several books from the library and trying to find information online, one of the main diagnostic differences between Atlantic Rock Crabs and Jonah Crabs is the "toothed" edge to the sides of the Atlantic Rock Crab. By contrast, Jonah Crabs have a much more rounded side to the carapace, as the two imagines above illustrate. These two individual carapaces seemed to be on the extremes of the toothed/rounded spectrum; there were some that seemed to be much more intermediate between the two.
|Lady Crab or Calico Crab Ovalipes ocellatus|