BMLSS: Crustacea: moulting, changing shells

Crustacea

Moulting/Ecdysis

Crabs belong to a group of animals known as the Crustacea. The soft body is protected by a hard outside shell known as a exoskeleton. Unfortunately, this hard shell does not grow.

The hard shell, or exeskeleton of the Shore Crab does not grow

Instead, a soft shell grows inside the crab. Eventually, the crab grows too big and it has to shed its old hard shell. This process is called 'moulting' and it is a very dangerous time for the crab. The new shell is larger than the old one, but it is soft and takes time to harden.

While it remains soft, the crab tries to stay hidden so that it is not attacked by hungry fish and other animals. During the moult, the crab can grow back a lost claw, but it will be smaller than the one that was broken off.

The old discarded shell looks like a dead crab, but if you look closely you will find there are holes where the eyes should be. It is hollow inside and it will sometimes float to the surface where it may be washed ashore.

Demise of crabs and other crustaceans in captivity often occurs when the animal is in the process of changing its exoskeleton. Research into the moulting cycle indicates that several stages can be described. This is known by anglers that use soft-backs for bait.
The cycle is controlled by hormones and modified by environmental conditions. In the crabs, the various stages could be summarised as follows:

1. Premoult (Peeler) stage;
A new soft exoskeleton forms underneath the old outer shell, and the calcium is resorbed into the blood from the old skeleton.

2. Ecdysis:
The old exoskeleton splits open and the crab withdraws and begins to rapidly absorb water. Prawns 'jacknife' out of their skeleton leaving a spectre of their former selves.

Edible Crab changing its shell

3. Postmoult (Soft-back) stage:
The crab hides away until the new skeleton hardens. In large crab species, feeding stops for several days.

4. Intermoult
This is the intervening stage. However, it should be noted that changes are constantly in progress, the carapace becomes rigid first, before the legs, and tissue growth continues internally.

Edible Crab with its discarded exoskeleton

Discarded exoskeleton of a prawn

5. Terminal Anecdysis.
Some species, noted in the Spider Crabs especially, reach their full growth size. No further moulting occurs.

Discarded exoskeletons of the European Lobster
Shells collected from a captive Lobster by Len Nevell

Notes

Even Hermit Crabs have to discard their old shell (exoskeleton) despite living in a shell of a marine shell (gastropod).

Hormones (notes):