Which materials make crustacean exoskeletons hard?

The exoskeleton of a crustacean is primarily composed of a procuticle made of chitin fibers embedded in a matrix of proteins. To achieve the rigidity characteristic of many crustaceans, such as crabs and lobsters, this structure is hardened through two primary processes:123

• Calcification: The cuticle is impregnated with calcareous substances, specifically calcium carbonate (often in the form of calcite or aragonite). This mineral deposition provides significant structural rigidity to the exoskeleton.23
• Sclerotization: The protein chains within the cuticle are stabilized through cross-linking, a process known as tanning. The resulting material, sclerotin, is a natural plastic that is exceptionally hard. In fact, areas requiring maximum hardness, such as the tips of claws and mandibles, are composed primarily of sclerotin rather than calcified material.23

These materials work in concert to provide the necessary support, protection, and muscle attachment sites required for the crustacean's lifestyle.13