I was making my usual feeding and checking rounds at the marine lab last Wednesday, when I saw this:
This crab is a kelp crab, Pugettia producta. It is one of the common crab species on the California coast; you can find them in the low intertidal clinging to algae. Many of them are this golden-brown color, coincidentally(?) the same color of the kelp Macrocystis pyrifera. Juveniles are often reddish or dark brown in color, again matching or blending in with the algae where you see them. This particular crab has always been this color, at least since it has been in my care.
Crustaceans, as all arthropods, periodically molt their entire exoskeleton in one fell swoop. The exoskeleton splits along the transverse seam between the carapace and the abdomen, then the crab sort of slithers out backward. The entire exterior of the body, including legs, antennae, and mouthparts, is left behind as a larger version of the crab scuttles away to hide out for a few days until its new shell hardens.
I've kept lots of crabs and seen lots of molts show up in their tanks, but have never caught one in the act before. From when I started watching, in the photo above, to the final wiggle out of the old exoskeleton took no longer than 5 minutes. Here's the sequence of photos documenting the molt:
Pretty nifty, eh?