Hey, where's that shell going?

Have you ever been beach combing, spot a lovely clam shell, and go to pick it up only to have it wander away from you? Well, you may have seen this Crabday's crab:

Shellback Crab

Hypoconcha arcuata, H. parasitica, and H. spinosissima

this H. arcuata is a sneaky little surprise!

Did anyone else know these existed!?!? I didn't until I stumbled across a picture of them on The O'Keefe Family Website (no, I don't know the family; they just have really awesome crab eye candy!). The crabs pictured there are from North Carolina but all three species listed above are found all along the Atlantic coast of the US. Don't worry, west coasters, there's also a California shellback crab H. californiensis!

another H. arcuata showing how cozy a bivalve can be

While these guys rely on other animals' shells for protection, they are very different from hermit crabs. Shellback crabs are true crabs (infraorder Brachyuran) while hermit crabs are not considered 'true' crabs (infraorder Anomura). Also, while hermit crabs will protect their soft abdomens with 360° of a snail shell, shellback crabs only protect the top half of their "membranous" (aka soft) bodies by using old bivalve shells like clams or scallops.

this little juvie H. spinosissima is so small
it only needs a shell fragment for protection

What's really neat about these guys is they have modified 4th and 5th walking legs (kind of like how swimming crabs have leg modifications) that enable the shellback crabs to hook and almost suction themselves into the shell halves!

little known fact:
shellback crabs are incredible at yoga

I strongly advise you all to stretch before trying to impersonate the shellback crab! Safety first, am I right?