CrabsBrachyura

Being a crab is less a family than a solution, and the sea keeps arriving at it. At least five separate crustacean lineages have independently flattened the body, folded the tail underneath and finished up crab-shaped without being crabs at all: king crabs came out of the hermit crabs, porcelain crabs out of the squat lobsters. The animals in this section, the Brachyura, are the ones that got there first and never left.

The shape is better engineering than it looks. Tucking the abdomen flat under the body removes the long soft rear end that a lobster has to spend its life defending, and leaves a compact armored disc with nothing worth grabbing. The legs hinge in a single plane, which is why sideways is the efficient direction of travel: the leading legs pull, the trailing legs push, and the animal slides into a crack narrower than its own body length without having to turn around in it.

The claws are usually not a matched pair. On many reef species one is heavy and blunt-toothed for crushing shell and the other is finer and bladed for cutting and picking, and the animal uses them for different work. Size counts as much as function, because a claw held up is a statement about the animal behind it, and most arguments over a crevice are settled by display rather than by contact.

And nearly all of them are small. The heavy crabs get the attention, but the ones doing the work on a reef are a centimeter or two across and spend entire lives inside a single coral head, a sponge, a sea urchin or one strand of wire coral, never crossing open rock at all. Point a torch into a coral colony after dark and the number of crabs on that reef goes up sharply.