Invertebrates
Crab
Brachyura · also called true crab
The crab body — flat, round, armoured, tail folded away, one pair of legs turned into pincers — is such a good design that evolution keeps arriving at it. At least five separate crustacean lineages have independently become crab-shaped, and several familiar “crabs”, including hermit crabs and king crabs, are really something else wearing the same form.
Overview
True crabs are the infraorder Brachyura, around 7,000 species, in every ocean and on many coasts, rivers, and tropical mountainsides. They range from the millimetre-wide pea crab to the Japanese spider crab, with a leg span near 4 m — the largest of any arthropod. Most are omnivores and scavengers, using the pincers to pick, tear, and crush.
Carcinisation
The repeated evolution of the crab form has its own name, carcinisation. A compact, flattened body with the abdomen tucked under is stable, well-protected, and good for squeezing into crevices, so lineages that started with a lobster-like shape have converged on it again and again. It is one of the clearest cases of convergent evolution in the animal kingdom.
Living in armour
A crab’s shell cannot grow, so the animal moults — splitting the old shell, backing out, swelling with water, and hardening a larger one over days while it hides, soft and defenceless. Limbs lost to a predator or rival are shed cleanly at a breakage point and regrown over later moults, which is how the stone crab claw fishery can take one claw and release the crab.
A scattered start in life
Most crabs hatch as a spiny, big-eyed zoea larva that drifts in the plankton through several moults, becomes a megalopa, and only then settles and turns into a miniature crab. The larval drift carries the young far from their parents and ties many coastal crab populations together.
Status
Crabs as a group are Not Evaluated. Most species are common, but some fished king and snow crab stocks have collapsed regionally with overfishing and warming water, several tiny-range freshwater crabs are among the most threatened crustaceans, and the coconut crab is declining on many islands.
Crab: common questions
Why do crabs walk sideways?
It comes down to the shape and hinging of the legs. A crab's walking legs are jointed so that they bend mainly up and down in a plane running across the body, which makes a sideways stride the quickest and most efficient way to move at speed. Going sideways also lets a crab slip into a narrow crevice or its burrow leading with one claw forward, keeping its armour and pincers facing any threat. Crabs are not locked into it, though. Many will walk slowly forwards or backwards when picking over food, and the swimming crabs, which have paddle-shaped back legs, can move and steer in any direction in open water.
What is the difference between a true crab and a hermit crab?
True crabs, the group Brachyura, have a short, hard, symmetrical abdomen folded flat under the body, so they are armoured all the way around. Hermit crabs belong to a separate group, the Anomura, and have a soft, curved, twisted abdomen with little armour, which is why they back it into an empty snail shell and carry the shell around as portable protection, moving to a bigger one as they grow. Hermit crabs also usually have only two obvious pairs of walking legs, with the rear pairs reduced and hidden in the shell. King crabs and porcelain crabs are also Anomura that merely look like true crabs.
How do crabs grow if they have a hard shell?
They moult. Because the exoskeleton cannot stretch, a crab periodically grows a soft new shell underneath the old one, then absorbs water to split the old shell along a line at the back and pulls its whole body, legs and all, out through the gap. The new shell is soft and wrinkled at first, and the crab pumps itself up with water to a larger size before the shell hardens with calcium over several days to a couple of weeks. During that time it is extremely vulnerable and stays hidden. Young crabs moult often, large adults perhaps once a year or less, and some stop growing altogether.
Can crabs feel pain?
The evidence has grown strong enough that several countries now include crabs and other decapod crustaceans in animal welfare law. Experiments show that crabs given a mild shock in one shelter will avoid that shelter afterwards, will trade off shock against the quality of a hiding place, tend and guard an injured limb, and show raised stress hormones after a harmful event, all responses that go beyond a simple reflex. This does not prove their experience resembles ours, but it is consistent with the animals having some capacity to suffer, which is why recommendations increasingly favour humane handling and killing methods for crabs used as food.