Friday, September 9, 2011

Crab Carapaces of Plymouth Beach

I have a bit of a thing for Crustaceans, especially crabs. Lucky for me, we have a fairly large number of crab species here in the northwest Atlantic Ocean. Last fall, I went to Plymouth Beach, where there were a large number of crab carapaces (their "shells") washed up on the beach. I believe that October-November is one of the times of the year when they molt their carapaces, and I guess the carapaces stay around longer on the beach during the fall when there are less people braving the howling winds on the beach. I took some pictures of some of the crab carapaces, and I hope that this post may serve as a way for people to tell the larger species we have apart.

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

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