Need help identifying an odd crab seen in Cozumel

Images: 
Cozumel crab.JPG

I'm hoping someone on the REEF forum can help identify and solve the mystery of a large crab I photographed on Nov. 10, 2007 while diving at Columbia in Cozumel.  I'm assuming that this is a Channel Clinging Crab which we've seen many times, but I am stumped by its odd looking abdomen as shown in this photo - the crab was sitting still under a ledge but the reddish-brown area seen under its body was moving/pulsating in a slow but regular manner, almost like gills breathing.  My dive buddy and I have never seen this before - is it a normal part of the crab's body that is usually hidden, or an egg case, abnormal growth or....?  Thanks for your help!

crab hygiene

It looks like a female that is flushing the sand out of her shorts.

Rhoda Green

female crab anatomy

I've noticed this structure on channel crabs in Cozumel myself & always assumed it was a gill structure of some kind, but in fact it's more like an egg compartment. Female crabs brood their eggs in this structure in their lower abdomen. It moves up & down to circulate seawater & keep the eggs and/or hatchlings fresh & healthy. If you enter "crab anatomy" in Google you'll find a number of sites with sketches & text describing this anatomical feature, along with more than you'll ever want to know about crabs.

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