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Anybody who has visited one of the sandy beaches in California has probably seen kids running around digging up mole crabs (Emerita analoga). These crabs live in the swash zone at around the depth where the waves would be breaking over your ankles, moving up and down with the tide. They are bizarre little creatures, burrowing backwards into the sand with just their eyestalks and first antennae reaching up into the water.

Although it's called a mole crab, Emerita's external anatomy isn't very similar to that of other crabs. For one thing, it doesn't have claws. In fact, its legs are quite unlike the legs that you'd see in a typical crab. Check out Emerita's appendages:

External anatomy of Emerita analoga
External anatomy of Emerita analoga

The crab's head faces to the left in this diagram. The real surprise that these little crabs hide is the nature of the second antennae. Usually the crab keeps these long, delicate antennae protected under its outer (third) pair of maxillipeds. This is why you don't see them when you dig up a mole crab.

You do see them when the crabs are feeding. As a wave washes over the crab, it extends the second antennae and pivots them them around on ball-and-socket joints. The feathery antennae catch particles in the water, then are drawn underneath the maxillipeds so the food can be slurped off and eaten.

Here's a top-down view of two Emerita feeding. The purple-grayish thing in the field of view is a sand dollar (Dendraster excentricus).

This side view gives a better angle of what's going on:

I find these little crabs quite captivating. I love how they rise up when I put food into their tank.  Watching them feed always makes me smile.

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I was making my usual feeding and checking rounds at the marine lab last Wednesday, when I saw this:

Pugettia producta, molting.  Time 10:09:12

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:

Pugettia producta molting. Time 10:09:41
Pugettia producta molting. Time 10:12:18
Pugettia producta, molting. Time 10:13:57

Pretty nifty, eh?

 

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