After showing a friend and marine biologist some coral that had changed color due to algae covering it, we made our way back to the beach. We stopped and checked in on a frog fish and then headed towards shore. In about 5'-6' of water, I noticed some green form in a puka in the reef and while I was looking, it moved. I immediately suspected it was a sponge crab and on closer inspection, I could make out some of its legs in the shadow of its cave. I pointed it out to my friend and then I dove down and with a small probe I carry, I coaxed it out of the puka and into the light. There were a couple cameras there besides mine and I held back and then dove down for a couple shots:

I caught it while it was making its way back into its hole. About an hour or so later, some friends showed up to snorkel and I guided them along with my wife back to the crab. I again prodded it to come out and this time, it left its sponge back in the cave. Some fish swam over to check the situation out and the crab took a number of swipes at them with his claws.

You can see the specialized sponge claws in the close up zoomed in on the rear right of the image directly above, shown below:

Some time had transpired and the crab was still out and fending off some fish as well as apparently feeding out of some algae. I felt bad that its sponge was still in the hole so I dove down, grabbed it and placed it on the crab's back. It initially reacted in a defensive rejection but as I swam back to the surface, it embraced the sponge and rolled around with it while moving and positioning it where it wanted it.

With the sponge back in place, the crab proceeded to go back into its cave.

It was quite dark in there and I dove down to try to get a look at what the crab might be doing since it was facing inwards. What I saw in the shadows made me regret prodding the crab out of there. There was another smaller crab tucked way back in there and I can only assume that the pair were in the process of procreation! Oops! My bad.

Went back the next day on the outside chance the crabs had hung around. A concerning discovery was made in that a sponge was left in the hole but no crabs to be found. This sponge must have belonged to the other, smaller crab (female?).

A bit later on, I saw some friends and asked them if they wanted to see the sponge. While swimming over to the hole where I had left it (in case the owner returned) I happened to be looking down on the reef and discovered a small sponge crab, sans sponge, resting in some sand between the coral. I swam over and grabbed the sponge and back to the crab and dove down to "offer" it back. Unfortunately, it became obvious that the crab was dead.

A cropped detail from the above image:

Two cropped details from above:

I don't know if the break in the one leg was present before I found the crab or not. My suspicion is that it was.