Abstract
The “nacro-prismatic” shells are the most studied mollusks, and they are often said to be “the” model to unravel the biomineralization mechanisms. Nevertheless, the nacro-prismatic structure is not unique, despite most data are provided by only three genera. The aragonitic nacre is taxon dependent: in cephalopods and gastropods, nacre is columnar, whereas bivalves have a spiral or sheet nacre. The inner structure of gastropod and cephalopod columnar nacre differs. The shape of the tablets is specific of the taxa. Calcitic and aragonitic prisms exist. The composition of the organic matrices extracted from calcitic prisms with a similar shape and mineralogy strongly differs. The inner structure of aragonite prisms is complex, with a central zone and divergent elongated crystallites at the periphery. Additionally, the relationships between nacre and prisms are also taxonomically related. From these data, whatever the scale at which they are studied, every component of the “nacro-prismatic” model – nacre, prisms, and prism–nacre topographic relations – is highly variable, so that this “model” does not exist; it is a structure.
Highlights
The most common structure in mollusk shells is the aragonite crossed-lamellar layer, but the most studied is the “nacro-prismatic” arrangement
Almost all data about mollusks are from three bivalve genera with flat large shells: Atrina, Pinna, and Pinctada
It is impossible to enter into the detailed description of all mollusk shells
Summary
The most common structure in mollusk shells is the aragonite crossed-lamellar layer, but the most studied is the “nacro-prismatic” arrangement. Almost all data about mollusks are from three bivalve genera with flat large shells: Atrina, Pinna, and Pinctada. These genera are taxonomically related (Pteriomorphia), with. Cuif large polygonal prismatic units and an inner nacreous layer, so that separating the two layers for detailed analyses is not difficult. They are often used as “the” model to understand the biomineralization processes. The examination of the structure and composition of the layers and of the prismatic and nacreous units, as well as their relationships, demonstrates that the nacro-prismatic arrangement is not unique. The present article will concentrate largely on the differences between the nacro-prismatic shells
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