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If there’s one crustacean that can be described as tank-like, this is it. The body is compact, the exoskeleton incredibly hard and the claws have evolved to form a perfect fit to cover the entire face. This “face”, actually the breathing apparatus and mouthparts, is the most vulnerable part of a crab’s body, but the shame-faced crab’s claws form an impenetrable barrier to this opening.

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The two other parts of a crab’s body most likely to get chewed off by a predator are the eyes and legs. Deep sutures in this crab’s body allow both the eyes and the legs to disappear into grooves, making them completely inaccessible.

Now, about those claws. Aside from being incredibly powerful -- if you get pinched by one you may have to resort to snapping the claw off because this guy will not let go -- you would be hard pressed to find an animal with a body part that is so specialized in so many different ways.

The shame-faced crab holds its claws out in front of it like most decapods do, but the top of each claw has a large flare to it. When pressed against the body it creates a perfect fit around the exoskeleton surrounding the mouthparts, forming a tight seal around the face. It looks as if it is hiding its head in shame, hence the name.

The left claw comes to a fine point and has sharp, serrated teeth. Once it grabs hold of prey there’s no escaping. The right claw is powerful as well, but it’s not the pincher itself that’s truly amazing, but the nodules you can see on the wrist.

As an aside, most crustaceans with asymmetrical claws, like lobsters and fiddler crabs, which side each claw is on is random. Shame-faced crabs always have the claw with these nodules on the right, the sharp pincher claw on the left.

Look closely at the right claw, which is on left side of the photo. Those two nodules you see are not the pinchers (in this photo the actual pinchers of the right claw are hidden behind the left claw). This is basically the crab’s can opener. This species specializes in feeding on snails and hermit crabs, which of course are protected by a very hard spiral shell. Even the opening of the snail is protected, by a door-like structure called an operculum, which closes tightly against this opening to keep predators out. In most species, this operculum fits so well that even moisture can’t escape when the snail is exposed to air during low tide.

Getting back to the can opener analogy, this is exactly how these nodules are used. The snail is picked up with the sharp left claw, and then rotated and manipulated so that the outer edge of the shell opening is inserted between the two nodules. The crab then bends its wrist, snapping a little piece off the shell. The left claw spins the snail slightly, and then another piece of the snail’s opening is chipped away. This continues until the shell is completely unwound, and all thats left is the columella. The columella is the central axis that the entire snail shell is wrapped around.

The snail on the right is a live periwinkle. The other two are what’s left minutes after the crab catches it. Obviously the snail is eaten bit by bit as the shell is chipped away.

And if all that wasn’t enough responsibility for these claws, they also are used for digging. While most digging crabs use their back legs to burrow backwards into the sand, the shame-faced crab uses the claws like little bulldozers, excavating the sand forward so the crab sinks downward until it is completely covered by the sediment.

Burrowing underground is fine and all, but how does it then breathe? Again, it’s the claws that solve this problem as well. When beneath the sand, the crab regularly shoots a stream of water from its mouth (when you remove one from the water it spits for a few seconds, creating a little fountain that can be up to half a foot long. The crabs themselves are only about two inches thick.) The stream of water richochetes off the inside of the claws, shooting upward and moving the overlaying sand grains, briefly allowing the crab to take a breath.

Before getting to the movie of my crab eating a snail, there is one other species in the North Atlantic (although there are many more throughout the world). This is the gladiator crab, and in addition to all the normal shame-faced features, this one also has two fearsome spikes on either side of the carapace. And yes, these are as sharp as they appear.

Finally, here’s a short movie showing what I’ve described above. Turn up the volume so you can hear the snapping sound as the crab chips away at the shell.

Other diaries in this series can be found here.

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Originally posted to Mark H on Sat Apr 27, 2013 at 10:46 AM PDT.

Also republished by Backyard Science and Community Spotlight.

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