SREL Reprint #2537




Geometrical and crystallographic constraints determine the self-organization of shell microstructures in Unionidae (Bivalvia: Mollusca)

Antonio G. Checa1 and Alejandro Rodríguez-Navarro2

1Departamento de Paleontologia y Estratigrafia, Universidad de Granada, Granada 18002, Spain
2Savannah River Ecology Laboratory, University of Georgia, Drawer E, Aiken, SC 29802, USA

Abstract: Unionid shells are characterized by an outer aragonitic prismatic layer and an inner nacreous layer. The prisms of the outer shell layer are composed of single-crystal fibres radiating from spheruliths. During prism development, fibres progressively recline to the growth front. There is competition between prisms leading to the selection of bigger, evenly sized prisms. A new model explains this competition process between prisms, using fibres as elementary units of competition. Scanning electron microscopy and X-ray texture analysis show that, during prism growth, fibres become progressively orientated with their three crystallographic axes aligned, which results from geometric constraints and space limitations. Interestingly, transition to the nacreous layer does not occur until a high degree of orientation of fibres is attained. There is no selection of crystal orientation in the nacreous layer and, as a result, the preferential orientation of crystals deteriorates. Deterioration of crystal orientation is most probably due to accumulation of errors as the epitaxial growth is suppressed by thick or continuous organic coats on some nacre crystals. In conclusion, the microstructural arrangement of the unionid shell is, to a large extent, self-organized with the main constraints being crystallographic and geometrical laws.

Keywords: biomineralization; bivalve shell; microstructure; nacre; composite prisms; self-organization

SREL Reprint #2537

Checa, A. G., and A. Rodríguez-Navarro. 2001. Geometrical and cyrstallographic constraints determine the self-organization of shell microstructures in Unionidae (Bivalvia, Mollusca). Proceeding R. Soc. London B 268:771-778.

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