Microtexture and Chitin/Calcite Orientation Relationship in the Mineralized Exoskeleton of the American Lobster

Abstract

The exoskeleton of the American lobster Homarus americanus is a hierarchical nanocomposite consisting of chitin–protein fibers, reinforced with amorphous calcium carbonate (ACC) and a small amount of crystalline calcite. Crystallographic polefigure analysis reveals two texture components of the crystalline a-chitin in the exoskeleton. One component represents the wellknown twisted plywood structure of chitin–protein fibers within the cuticle plane, and the second component represents fibers oriented roughly perpendicular to the cuticle surface. These perpendicular fibers interpenetrate the open canals of the planar honeycomblike structure originating from the well-developed pore-canal system present in this material. The calcite crystallites reveal fiber texture with the crystallographic c-axis oriented perpendicular to the cuticle surface, suggesting an orientation relationship between calcite and the organic chitin–protein fibers. Local orientation analysis using X-ray microdiffraction reveals that the crystalline calcium carbonate fraction is associated with the chitin–protein fibers oriented perpendicular to the surface. Calcite is exclusively found in the exocuticle and is mostly restricted to a thin layer in the outermost region, while the major part of the exocuticle and the whole endocuticle contain ACC exclusively. It is therefore speculated that the most likely function of calcite in the exoskeleton of the American lobster is related to impact- and wear-resistance.