Crustacean skeletal elements: variations in the constructional morphology at different hierarchical levels

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The crustacean cuticle is a hierarchically organized composite material that covers the whole animal and forms the continuous exoskeleton. The organic phase of the cuticle is organized in at least seven hierarchical levels. It consists of chitin molecules forming crystalline fibrils enveloped by proteins. These fibrils form planes that are stacked in a twisted plywood structure constituting three of the cuticle´s four principal layers. At least two of them, the exo- and endocuticle, contain a mineral phase of mainly Mg-calcite, amorphous calcium carbonate (ACC) and phosphate (ACP). The high number of hierarchical levels and the compositional diversity provide a high degree of freedom for varying the physical, in particular mechanical, properties of the material. This makes the cuticle a versatile material ideally suited to form a variety of skeletal elements that are adapted to different functions and the eco-physiological strains of individual species. In our comparative approach, we combine experimental and mathematical methods to investigate how, and at which hierarchical levels structure and composition are modified to achieve the required physical properties. Using the accumulated knowledge from these studies, we develop a theoretical model that systematically describes the structure-composition-property relations of cuticle composites from the molecular to the macro-scale. The goal of our multidisciplinary project is to facilitate the development of optimized biomimetic materials within a knowledge-based design approach.