Formation of the Outer Shell Layer in Pinctada margaritifera: Structural and Biochemical Evidence for a Sequential Development of the Calcite Units

Abstract

Calcite prismatic units that form the outer layers of “nacro-prismatic” Pelecypod shells are often used as biomineralization models due to their individual size, simple shape, and spatial arrangement. However, these models do not take into account the developmental history of the shell. After metamorphosis, a series of structural changes predating production of the prisms is commonly missing. Consequently, this study focuses on the early stages of the calcite biomineralization area of the Pinctada margaritifera as it occurs in the outer mantle groove. It also includes the structural changes following the typical “simple prism” status. The interpretation takes advantage of an ancient result from genomic investigations: the localisation of Prisilkin-39, a protein associated with production of the calcite units. A revision of the initial interpretation concerning the position of this Prisilkin-39-producing area provides additional evidence of the role of two distinct mineralizing sectors in the formation of the calcite units in the Pinctada shell: the outer mantle groove and the anterior mineralizing area of the shell mantle.