Can Crabs Regrow Their Legs? Autotomy and Limb Regeneration

How crabs drop a leg

Throwing off a body part to escape is called autotomy, from Greek words meaning self-cutting. In crabs, each leg has a pre-formed breakage plane near its base, a weak line where two segments are fused together. When the leg is grabbed or badly hurt, a special muscle contracts and the leg snaps off cleanly at that line.

The break is designed to limit damage. A thin, double membrane across the inside of the leg at the breakage plane closes almost immediately, so very little blood, called hemolymph, is lost. Within a short time the stump is sealed by a scar.

Crabs use autotomy in several situations:

• To escape when a predator such as a fish, bird or octopus seizes a leg.
• When a rival crab holds on during a fight.
• When a leg is crushed, trapped or infected.

For a blue crab or any other crab living among hungry fish, losing a leg is far better than losing its life. It is common to find wild crabs with a missing leg or a small new one growing in.

How a new leg grows

Regrowth begins soon after the loss. Under the scar, cells multiply to form a limb bud. Over the following weeks the bud develops into a complete miniature leg, with all its segments, but it stays folded up inside a thin, soft sac of cuticle tucked against the body.

The real change comes at the next molt. As the crab sheds its old shell, the new leg unfolds and hardens along with the rest of the body. It is fully working but noticeably smaller than the leg it replaces. With each later molt the regenerated leg grows a little more than the others, so after about two or three molts it is close to normal size.

Hormones tie this process to the molt cycle. Studies of the Atlantic sand fiddler crab, Uca pugilator, have shown that the same steroid hormones that trigger molting also control the growth of a regenerating limb. In some crabs, losing several legs at once can even speed up the next molt.

What regrowing a leg costs a crab

Regeneration is not free. Building new limbs takes energy and materials that would otherwise go into growth, so crabs regrowing several legs often grow less at each molt and end up smaller.

Until the new limb reaches full size, the crab is at a disadvantage. A missing or small claw makes it harder to crack shells, fight rivals, hold a mate or defend itself, and a crab missing several walking legs is slower to escape. Male crabs with missing claws usually have less success winning mates.

Timing also matters. A crab that has just molted has a soft shell for hours or days, which is one of the most dangerous times in its life. Regenerating a limb ties the crab to this risky molting schedule.

When crabs cannot regrow legs

Because regrowth happens only at a molt, a crab that has stopped molting cannot replace a lost limb. Many crabs have a final, or terminal, molt. Female blue crabs, for example, molt for the last time when they become adults and mate, and after that they keep the same shell, and the same legs, for the rest of their lives.

Very large, old crabs and lobsters may molt only once every few years, so they replace lost limbs slowly if at all. This is why big adult crabs are so often seen with a missing leg or claw.

Do regrown claws work as well?

Not always. A regenerated claw can look similar on the outside but be built differently. In a well-known study of fiddler crabs, published in 2000, males that had lost their huge courtship claw grew a replacement that looked like the original but was lighter and weaker. Rival males could not easily tell the difference, so the regrown claw worked as a bluff in contests.

Commercial harvesting shows the limits of regrowth too. In Florida, fishers remove the claws of stone crabs and return the crabs to the water, relying on them to grow new ones. Experiments found that many crabs died after losing claws, especially when both were taken, and only a small share of the claws landed in the fishery show signs of being regrown.

Other animals that regrow limbs

Crabs share this ability with many other animals, though the details differ.

• Lobsters, crayfish and shrimp regrow legs and claws through molting, just like crabs. The American lobster can replace a lost crusher claw over several molts.
• Spiders, including tarantulas, can drop legs and grow new ones at their next molts, as described in how do tarantulas molt?
• Sea stars can regrow whole arms, and in some species a single arm can grow a new body; see can starfish regrow arms?
• Axolotls can rebuild entire limbs with bones, muscles and nerves, explained in how do axolotls regenerate?
• Lizards can regrow a lost tail, but the new one has a rod of cartilage instead of bones.

Mammals, including humans, cannot regrow limbs. The closest example among mammals is a deer growing a new set of antlers every year.