Isopod eyes consist of functional units, the omatidia. Each omatidium contains a dioptric apparatus that is composed of a cornea and a crystal cone. The cornea consists of mineral embedded in an organic matrix, hard calcite being present distally, soft ACC proximally. The cuticle of the cornea has a bimodal function: it serves as a protective exoskeleton but forms also transparent micro-lenses. Thus, for the cuticle of the cornea, a balance has to be obtained between mechanical stability for protection and optical properties for vision.
In carbonate biological hard tissues microstructure is one of the keys for material optimization. We studied with high-resolution EBSD calcite organization in corneal cuticle of four isopod species native in distinct habitats: one marine (Sphaeroma serratum) and three terrestrial (Porcellio scaber, Tylos europaeus, Ligia oceanica). We find significant variations in calcite organization: The species with the thickest cuticle, marine (S. serratum) and terrestrial (T. europaeus), incorporates the thickest band of calcite into the cornea: calcite is in a graded arrangement. The species with a thin cuticle (L. oceanica, P. scaber, terrestrial) incorporate a very thin calcite band into their cornea. Here, calcite is almost single crystalline. For the terrestrial species calcite c-axis orientation is horizontal within the cuticle, in contrast to that of the marine species. For the terrestrial species we find a 3D arrangement of calcite that, for T. europaeus, this is linked to a graded organization of the mineral. For all four species calcite in the cornea is thinner relative to that in the head capsule. With the exception of P. scaber, for all other investigated species calcite arrangement in the cornea differs from that in the head capsule. Our results suggest that adaptations of calcite organization cover both: an increase in corneal cuticle transparency as well as maintenance of its mechanical strength (1, 2).
(1) Alagboso F I, Ziegler A (2014) Ultrastructure and mineral composition of the cornea cuticle in the compound eyes of a supralittoral and a marine isopod. Journal of Structural Biology 187, 158-173 DOI: 10.1016/j.jsb.2014.06.00257
(2) Griesshaber E, Yin X, Ziegler A, Kelm K, Checa A, Eisenhauer A, Schmahl WW (2017) Patterns of mineral organization in carbonate biological hard materials. Highlights in Applied Mineralogy, De Gruyter, ISBN 978-3-11-049122-7.