Exoskeleton and moulting

Arthropods have evolved a hard, rigid external skeleton (exoskeleton/cuticle) that protects and gives support to the organism’s soft tissue and acts as a site for muscle attachment. The crustacean exoskeleton has two distinct layers; the procuticle and epicuticle. The epicuticle is the thin outermost layer, containing protein and lacking the polysaccharide chitin. The procuticle is much thicker and can be further divided into two sub-layers, the endocuticle and the exocuticle. Cross-bonding of chitin-protein chains in the exocuticle greatly contributes to the overall strength of the exoskeleton, as well as the impregnation of the procuticle with calcium salts such as calcium carbonate.

The obvious problem with having a hard, rigid outer skeleton is that it can not grow with the rest of the body. Arthropods have resolved this problem through moulting of the exoskeleton and growing whilst the new exoskeleton is still malleable. The cycle of moulting is regulated by hormones, and consists of 4 stages:

Proecdysis

The events that occur during this stage are collectively termed apolysis.

The epidermis separates from the procuticle by secreting enzymes that begin to digest the endocuticle. As the endocuticle is broken down, the produced calcium and other solutes are reabsorbed into the blood. The epidermal cells begin to form a new epicuticle.

Ecdysis

Following apolysis, the shrimp is still encased in the old cuticle. When in this state the shrimp is described as being pharate. The shrimp flexes its abdomen to put strain on the old cuticle. This causes it to split between the carapace and the first abdominal segment, allowing the shrimp to spring out or pull itself free. The discarded cuticle is termed the exuvia. An influx of water then causes the shrimp’s body to expand and the new cuticle to stretch.

Metecdysis

After undergoing ecdysis the new, soft cuticle is exposed, so the shrimp is vulnerable and described as being teneral. It is during this time that the shrimp rapidly grows as the cuticle can still stretch. After a few hours the exocuticle will have formed its cross-bonds of chitin-proteins and the procuticle will have fullied calcified, hardening the cuticle.

Intermoult

This is the stage between moults where the cuticle is in its normal state.

Caridina shrimp tend to shed their exoskeletons between every one to two months in maturity, but far more frequently as growing juveniles (especially when the life cycle involved metamorphosis). Adult shrimp that will no longer grow benefit from moulting in different ways. For example, moulting regenerates lost limbs and damaged cuticles, and gives a clean surface for brooding of eggs.

Sources:

Ruppert E. E., Fox R. S., Barnes R. D., (2004). Invertebrate Zoology. Chapter 19, pg 649. Brookes/Cole, Thomson Learning.

(2008). Encyclopaedia Britannica. Z-Systems, Inc.

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