How do crabs grow? Moulting, shells and breathing explained

In a shallow creek, a blue crab backs slowly out of its own shell. It takes about two hours. What crawls out is the same crab, now soft, wrinkled and about a third bigger.

Crabs grow by moulting. A crab’s hard outer shell, its exoskeleton, cannot stretch, so the crab grows a new, soft shell underneath, splits the old one and climbs out. It then swells with water before the new shell hardens. A newly moulted blue crab is about a third larger than before, says the Chesapeake Bay Program.

Why do crabs have to moult to grow?

A crab’s skeleton is on the outside, and once it hardens it cannot get any bigger. The crab’s soft body keeps growing inside it, but the shell stays the same size, so sooner or later the crab must swap it for a larger one. Crabs, like insects and spiders, are arthropods, and moulting is how they all grow.

Moulting is controlled by hormones. A pair of small glands called Y-organs make the moulting hormones, called ecdysteroids. A hormone from the crab’s eyestalks, called moult-inhibiting hormone, acts as a brake on those glands. When the brake eases, moulting begins, explains biologist Donald Mykles in a 2021 review.

What happens when a crab moults?

A crab moults in three stages. First, over days or weeks, it grows a new soft shell under the old one and breaks down part of the old shell. Then it splits the old shell open and backs out, which takes a blue crab about two hours. Finally, it swells with water while its new shell hardens.

1. Getting ready (premoult). Under the old shell, the crab breaks down and reabsorbs part of the old exoskeleton and builds the new one, says Mykles.
2. Breaking out (ecdysis). The old shell splits open and the crab backs out, legs, claws and all. For a blue crab this takes about 2 hours (Chesapeake Bay Program).
3. Pumping up and hardening (postmoult). The fresh crab is wrinkled and soft. It reaches the texture sold as a “soft crab” within three to four hours, and needs to stay in water while its shell hardens over a day or two.

Watermen in Chesapeake Bay can tell when a blue crab is about to moult by a thin line on the edge of its back swimming paddle. A white line appears about 10 to 14 days before, turns pink about 5 days before, and red a day or two before. Crabs that are just hours away are called “rank.”

How often do crabs moult?

Young crabs moult often and older crabs much less. Small blue crabs can shed four or five times a month, while older ones go about 30 to 50 days between moults, says the Chesapeake Bay Program. Blue crabs mature 12 to 18 months after hatching, and few live longer than three years.

Big, long-lived crabs slow right down. Adult coconut crabs moult only once a year, usually in winter, and take 40 to 60 years to reach full size, according to a review by Peter Greenaway in the Memoirs of Museum Victoria. Before moulting, a coconut crab digs a burrow up to 1 m (3 ft) long and seals itself in for 3 to 16 weeks.

What happens to the old shell?

Some crabs eat it. After moulting, a coconut crab eats its old shell, which gives it the organic material and calcium salts it needs for the new one, Greenaway reports. Some land crabs instead pull calcium out of the old shell before they moult, store it in the body, and reuse it.

The empty shell, called an exuvia, can look exactly like a whole crab. If you find a perfect, empty crab on a beach, it may not be a dead crab at all, just a moult left behind.

Can crabs regrow lost legs?

Yes, but only by moulting. A crab that loses a leg grows a small new limb bud, and the new leg only becomes a working limb when it is pulled out at the next moult, says Mykles. Losing limbs early in the run-up to a moult can even delay the moult by about two to three weeks, giving the new legs time to grow.

How do crabs breathe?

Crabs breathe with gills hidden in chambers under the sides of the shell. A small beating paddle on each side, called the scaphognathite, pulls water or air through these chambers. In land hermit crabs and coconut crabs, the speed of this paddle sets how fast the crab breathes, Greenaway explains, and it speeds up mainly when carbon dioxide builds up.

On land, a crab has to get oxygen from air without drying out. Land crabs have solved this in different ways.

• Coconut crabs have very small gills that do little of the oxygen work. Instead, the lining of their gill chambers forms large “lungs” called branchiostegal lungs, which take oxygen from the air. The gills still help get rid of carbon dioxide (Greenaway 2003).
• Land hermit crabs have small lungs too, but living in a snail shell leaves little room for them. They also take in oxygen through thin skin on the top of the abdomen, inside the shell, and carry a store of water in the shell that can make up 30 to 50% of their wet weight.

Why do hermit crabs live in snail shells?

A hermit crab’s long, soft abdomen has no hard cover, so it borrows an empty snail shell for protection. As it grows, it needs bigger shells, and good ones are scarce. Land hermit crabs gather to swap shells, and one large empty shell can set off a chain of swaps from bigger crabs to smaller ones, says Greenaway.

The coconut crab is a hermit crab relative that gave this up. It uses shells only when young. Free of a shell, it can grow huge: it is the largest land arthropod, reaching about 3 kg (6.6 lb).

Why do moulting crabs matter to other animals?

A soft crab is an easy meal, so moulting feeds a lot of animals. Blue crabs are eaten by fish such as striped bass, Atlantic croaker and red drum, and smaller crabs by blue herons and sea turtles, says the Chesapeake Bay Program. Even hard-shelled blue crabs will eat freshly moulted ones.

Crabs also clean up. Blue crabs eat clams, oysters, mussels, smaller crustaceans, dead fish and detritus, the rotting bits of plants and animals on the bottom. That makes them both hunters and scavengers in bays and estuaries. Sea turtles are among the animals that hunt crabs; see what sea turtles eat.

Common mix-ups about crab shells

Mix-up: A soft-shell crab is a special kind of crab.

Fact: In Chesapeake Bay, a “soft crab” is a blue crab caught just after it has moulted, before its new shell hardens. Watermen spot crabs that are about to moult by the coloured line on the swimming paddle (Chesapeake Bay Program).

Mix-up: An empty crab shell on the beach means a crab has died.

Fact: It may be a moulted shell. The crab climbs out through a split in the old shell and leaves an almost perfect copy of itself behind.

Test yourself

1. Why can’t a crab just grow bigger inside its shell?

Because the hard exoskeleton cannot stretch. The crab has to shed it and grow a new, larger one.

2. About how much bigger is a blue crab after one moult?

About a third bigger.

3. What do coconut crabs use to breathe air?

Lung-like gill chambers called branchiostegal lungs. Their gills are small and mainly help get rid of carbon dioxide.

Frequently asked questions

Sources

1. Chesapeake Bay Program. Clues of a growing blue crab.
2. Chesapeake Bay Program. Field guide: blue crab (Callinectes sapidus).
3. Mykles, D. L. (2021). Signaling pathways that regulate the crustacean molting gland. Frontiers in Endocrinology 12: 674711.
4. Greenaway, P. (2003). Terrestrial adaptations in the Anomura (Crustacea: Decapoda). Memoirs of Museum Victoria 60(1): 13-26.

Sources checked 12 October 2026.