Calcite distribution and orientation in the tergite exocuticle of the isopods Porcellio scaber and Armadillidium vulgare (Oniscidea, Crustacea)

The crustacean cuticle is a bio-composite consisting of hierarchically organized chitin-protein fibres, reinforced with calcite, amorphous calcium carbonate and phosphates. Comparative studies revealed that the structure and composition of tergite cuticle of terrestrial isopods is adapted to the habitat of the animals, and to their behavioural patterns to avoid predation. In this contribution we use FE-SEM, polarized SCμ-RSI and EBSD to investigate micro- and nano-patterns of mineral phase distribution and crystal orientation within the tergite cuticle of the two terrestrial isopod species Armadillidium vulgare and Porcellio scaber. The results show that the proximal regions of the exocuticle contain both calcite and ACC, with ACC located within the pore canals. Calcite forms hierarchically organised mesocrystalline aggregates of similar crystallographic orientation. Surprisingly, c-axis orientation preference is horizontal in regard to the local cuticle surface for both species, in contrast to mollusc and brachiopod shell structures in which the c-axis is always perpendicular to the shell surface. The overall sharpness of calcite crystal orientation is weak compared to that of mollusc shells. However, there are considerable differences in texture sharpness between the two isopod species. In the thick cuticle of the slow-walking A. vulgare calcite is more randomly oriented resulting in more isotropic mechanical properties of the cuticle. In contrast, the rather thin and more flexible cuticle of the fast- running P. scaber texture sharpness is stronger with a preference of c-axis orientation being parallel to the bilateral symmetry-plane of the animal, leading to more anisotropic mechanical properties of the cuticle. These differences may represent adaptations to different external and/or internal mechanical loads the cuticle has to resist during predatory attempts.