Friday, July 16, 2010

Shedding one Skin for another: Bigger is Better

Ever wonder why you find so many dead crabs on the beach during the summer? Most of these “dead crabs” are actually crab molts. A molt is an exact copy of a crab, complete with gill coverings, without the actual animal in it! In order to grow, crabs have to shed their hard outer shell, also known as their exoskeleton. When a crab is ready to molt, it backs out of its shell through a split at the back edge of its carapace (the large flat surface on its back). This molting reveals the crab’s new soft shell, which fills with water to make the shell bigger than the old one. During the week it takes for the new shell to harden, the crab hides and doesn't eat.  The crab then spends the next year growing to fill the shell.

Size of the Dungeness crab before it molted

Size of the Dungeness crab after it molted

So, next time you are at your local beach, pay attention to how many crab shells you see. A simple way to tell the difference between a dead crab and a molt is to smell it. If it smells like a dead crab, then it is one. However, if it smells like the ocean, it’s a molt. If you don’t want to risk smelling it, simply try to lifting the back edge of the shell. If it’s a molt, the back of the shell will be detached from body and you’ll be able to look inside!
Dungeness crab molt

Crab molt, complete with gill coverings!

Most of the crabs at the Marine Science Center have now molted and are ready to spend the year in their new shell!

For information about crabs and crabbing, visit the Washington Department of Fish and Wildlife:



  1. Nice post! I just took my class of kids tidepooling today and saw my first (that I know of) slender cancer crab. It was burrowed in the sand waaaaaaay out of the water. I thought it was maybe just the top carapace so I was going to show the kids, but it felt soft so I started digging in the sand around to see if the legs were underneath the sand. They were and, not only that, the legs moved! I ended up taking the crab (very, very carefully) back to the water near some rocks that might provide some shelter. Crabs are pretty darn interesting!

  2. In the annuals of tidal love encounters, the one that happens betwixt opposite sexed crabs is delightful. The Mr. helps the Mrs. shed her carpace by stroking her back whilst holding her - belly to belly. The gentleman does, however, take advantage of her vulnerability and impregnates her at this time. The larvae will develop on her underbelly beneath the abdominal flap while her carapace hardens up.

  3. This is great - went rock crabbing last week, and saw quite a few "shells", not to mention a few very "soft" shelled crabs... We left those on the beach!


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