Shrimp, of course, have an exoskeleton. That is to say, their skeleton is external rather than internal.
Moulting of the exoskeleton is a process needed in order for the shrimp to grow. It is necessary because, unlike skin, the chitinous cuticle of the shrimp is rigid and can not grow with the rest of the body.
There are 2 main stages to moulting:
- Apolysis – this is the separating of the cuticle from the body-cells underneath (the epidermis). This is achieved by the epidermal cells secreting fluid, causing the exoskeleton to loosen. Enzymatic fluid is also excreted and this begins to digest the old cuticle when the upper layers of the new cuticle have formed underneath. The new exoskeleton continues to form as the old is digested.
- Ecdysis – this is the expulsion of the old cuticle. When still encased by the old cuticle, the shrimp is described as being ‘pharate’. The shrimp bends its body into an ‘n’ shape to put stress on the weakened old cuticle and thus splits it. It then ‘springs’ out of the old exoskeleton. The expelled exoskeleton is called an ‘exuvia’.
Following moulting the shrimp is very vulnerable. This is because it is ‘teneral’; its new cuticle has not yet hardened. It is during this period that the shrimp grows, as the new exoskeleton is not yet rigid enough to prevent growth and can be stretched to accommodate.