What Happened To This Crab?

Assateague Island National Seashore

After a morning of crabbing, a visitor and his young daughter brought this crab into the visitor center. Check out its deformed claw. What happened?

Crustaceans are invertebrates and have skeletons that are on the outside. In order to grow they have to molt or shed their exoskeleton. Before it can molt, the crab must create a new, soft, paper thin shell beneath the existing hard one. When the new shell is fully formed under the old shell, the crab stops eating and finds a safe place to begin molting.

Prior to coming out of its old, hard shell, the crab begins to absorb water into its tissues, causing it to swell up like a balloon. Eventually the outer shell splits at a seam that is located at the back of the crab where the carapace (top) and abdominal shells meet. Very gradually, as the crab pushes and compresses its appendages inside the old shell, the carapace begins to lift up. At this point, the the blue crab slides out of the back of the old shell. When it emerges, it is pillow soft. The crab immediately begins to pump water into its tissues again to make the new soft shell grow; it remains this way for up to 72 hours. During this period, we suspect that the crab in the picture below got attacked by a predator.

When threatened, the crab’s first instinct is to raise its claws as a warning to the potential predator. It is very likely that the predator called the crab’s bluff and grabbed its soft claw. In doing so, the claw ripped in multiple places. Obviously the crab fought off its predator and its shell hardened up. When the crab molts again a new claw will emerge in place of the damaged one.