A defining feature of the arthropods is their non-living external skeleton (exoskeleton / cuticle) that still allows for growth (albeit noncontinuous) and mobility. The exoskeleton serves a number of functions. Where it is hardened it provides protection and strength, and gives support to the soft tissue, acting as a site for muscle attachment. The cuticle also acts as a physical barrier to pathogens.
Biochemistry of the Crustacean exoskeleton
The crustacean exoskeleton is secreted by the immediately underlying epidermis- a single layer of cuboidal epithelial cells- and so it follows that there is exoskeleton above every epidermal layer. This includes invaginations of the ectoderm and internal structures derived from embryonic invaginations, such as the foregut and hindgut (often referred to as the endoskeleton).
The cuticle is largely composed of protein and α-chitin, covalently bonded to form polysaccharides of N-actylglucosamine monomeric units. It is important to understand that the physical properties of the cuticle can be dramatically altered by chemical processes. Whilst the cuticle is initially soft and flexible, these processes can be used to harden and strengthen it. The two processes relevant to crustacea are sclerotisation and mineralisation.
Sclerotisation is present in all arthropods and is commonly referred to as ‘tanning’, which is derived from how the process darkens the cuticle as a side effect. This process is simply the formation of covalent bond cross-links between protein molecules, which dramatically increases the overall strength and rigidity of the cuticle.
Mineralisation is another method by which arthropods strengthen their exoskeleton, but it is generally only exploited by crustacea and diplopods (millipedes). It is achieved by impregnating the cuticle with calcium salts, usually calcium carbonate.
Structure and composition of the Crustacean exoskeleton
The cuticle has two distinct layers; the epicuticle and the procuticle. The epicuticle is the thin outermost layer. The procuticle is much thicker and can be further divided into two sub-layers, the endocuticle and the exocuticle.
The epicuticle is a thin layer of protein, lipoprotein, and lipid; always lacking chitin. It often has a waxy layer that protects the organism from osmotic influxes- a notable hazard of living in freshwater. The gills (and other similar interfaces) have a much thinner epicuticle with no wax layer, increasing the permeability to gases and ammonia, allowing easy exchange with the environment.
The procuticle is much thicker than the epicuticle, and is generally composed of protein and α-chitin covalently bonded to each other to form complex glycoprotein. This major layer of the exoskeleton is the main contributor to the overall strength and rigidity, and can be further divided into two sublayers; the exocuticle and the endocuticle. The exocuticle is directly beneath the procuticle and may be hardened in areas by sclerotisation. The endocuticle differs in that it is thicker, has less protein, more chitin, and is often strengthened by mineralisation.
Moulting information coming soon…
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