Uca

There are over 100 species in the genus Uca, commonly known as fiddler crabs.  I believe, from distribution information, that this is probably uca pugnax, but I'm not certain.  I can't quite seem to find a perfect match to ID this little guy.

Many are familiar with this semi-terrestrial crab as it inhabits sandy beaches and marsh flats of the northeast United States.  These crabs are a great example of what we call "sexual dimorphism."  This is where the male and female of the species look different, sometimes drastically so.  (Cardinals, and many other yardbirds, are another clear example of this).  Di = 2 and morph = form so dimorphism = 2 forms.  You sometimes see reference to polymorphism as well (poly = many) when species have more than two morphs.

In this case the dimporphism is the male's much larger claw.  This is a mate recognition/mate choice feature.  The male crab will wave his large claw, visually and often auditorily signalling to females and other males.  They will also use the large claws in combat with one another.  These little crabs live in pretty close quarters, making small burrows in mud or sand so presumably their lives are consumed by waving and fighting.

Oh, and feeding!  Something I did not know is that their common name comes from their appearance specifically during feeding.  They will use their smaller claw to pick up detritus and move it to their mouths so in the males it appears that they are bowing (with their small claw) a fiddle (their large claw).  I guess I always just assumed it was just an analogy between the large claw and a fiddle.

The day I took this photograph we found a great number of fiddler crabs under the wrack in a marsh flat so the next time you're at the coast poke around a bit and see if you can find these common arthropods.