Coast & Sea Areas

Wild Harris

How do Crabs Grow?

All living things grow up by getting bigger. For animals without a skeleton, such as sea slugs, or animals with an internal skeleton, like fish, this is no problem.

What happens however when your skeleton is on the outside?

Crabs and other crustaceans (lobsters, shrimps, prawns etc) have a hard shell or exoskeleton. This is like a suit of armour that encases the whole body in a rigid case. While the animal can grow to fill the space inside, it can't grow any further than that.

The solution is to get a bigger suit of armour. At certain times in its life the crab will grow a new soft exoskeleton (also called a carapace) under the old one. The old shell splits open and the crab climbs out. It quickly pumps up its body by drinking lots of water before it sets hard. It can then grow into its new shell until it needs to make a bigger one all over again.

This is a dangerous time for the crab. While it is soft it has no protection against its enemies, so will need to hide away until the new shell has gone hard.

Anglers call these soft crabs 'peeler crabs' and use them for bait. 

edible crab soft coral.jpg Edible Crab    (Photo: Paul Tyler)

The hard shell of this crab makes it a very tough cookie indeed, but sooner or later it will have to climb out and grow itself a bigger one.  


Hermit Crabs have another trick. While they are heavily armoured at the front end, their rear end is quite soft. They borrow shells from dead sea snails such as whelks and winkles to protect themselves from danger. Not only does it protect the soft parts, they are also able to hide inside where they are safe from attack.

Hermit crabs still have to shed their carapace to grow bigger, but they will also have to find a bigger shell. A hermit crab will spend many hours examining shells for potential new homes before quickly slipping out of the old shell and into the new one. They will even quarrel with their neighbours over who should get the best shell!

If you pick up a hermit crab it might abandon its sea shell in fright. If you put the shell close to the crab it will soon find it and move back in.

hermit w pomatocerus crop.jpgHermit Crab     (Photo: Paul Tyler)

This crab has an additional layer of protection from the whelk shell, but it will still need to shed its skin to grow bigger, and may also have to find a bigger house to live in!

by Paul Tyler

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