Structure and composition of Unio pictorum shell: arguments for the diversity of the nacroprismatic arrangement in molluscs

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Mollusc shells are complex organomineral structures, the arrangement and composition depending on the species. Most studies are dedicated to shells composed of an aragonite nacreous and a calcite prismatic layer, so the nacreous prismatic model based on Pinctada and Atrina–Pinna. Here, we studied the micro- and nanostructure, the mineralogy and composition of a nacroprismatic bivalve species: Unio pictorum. The prismatic layer of Unio is aragonite, and the inner structure of the prismatic units strongly differs from those of the calcitic layers. The shape of the prisms varies depending on their growth stage. The first layers of nacre are similar to those of gastropods (columnar nacre), then evolve towards the typical bivalve arrangement (sheet nacre). Na, Sr, Mg, P and S are present in both prisms and nacre. The organic prismatic envelopes are rich in sulphur amino acids, whereas organic sulphate is present within the prisms and the nacreous tablets. P is present as phosphate, probably a mixture of organic and mineral complex. Chemical distribution maps confirm the absence of an organic membrane between the nacre and the prisms. The comparison of the structure, mineralogy and composition of Unio pictorum and different species show the diversity of nacroprismatic shells, and that these features are taxonomically dependent.

Lay description

Mollusc shells are hard structures made by a soft tissue called mantle. In bivalve shells, the growing edge of the mantle is the most active zone for the secretion of the shell. Studies of the structure, mineralogy and composition of the shells have shown that they are a mixture of mineral and organic components. The main minerals are calcite and aragonite. The main structures are prisms (aragonite or calcite), foliated (calcite), nacre (aragonite) and the crossed lamellar layer (aragonite). Most often, two layers are present in a single shell. The arrangement and mineralogy of the layers are taxonomically dependent. Looking the inner structure of a layer, it has been shown that every taxon has its own arrangement: the shapes of the tablets of the nacre of Pinna and Pinctada are clearly different. The chemical contents are also species specific.

Lay description

Mollusc shells are not pure minerals. They comprise proteins, sugars and lipids. Despite the few number of analysed shells, it appears that the compositions of the organic components are also taxonomically dependent. Because of their abundance and availability of analytical systems, the main studies are dedicated to proteins. Nevertheless, the presence of sugars and lipids has been evidenced using extracted components or in situ analyses.

Lay description

The formation of the shell and how the organism controls its structure and composition are not yet unravelled. Most studies are dedicated to shells composed of an aragonite nacreous and a calcite prismatic layer such as Pinctada and the closely related Atrina–Pinna. The nacroprismatic arrangement is considered as a model, but despite a superficial similarity, the structure and composition of both nacre and prisms of these taxa differ.

Lay description

In this paper, we have studied the structure and composition of another nacroprismatic shell of a bivalve mollusc: Unio pictorum. In situ techniques, such as thin sections, scanning electron microscope, atomic force microscope, Raman spectroscopy, energy dispersive X-ray spectroscopy, and X-ray absorption near edge structure spectroscopy were used. Although both layers are aragonite in Unionid shells, not only their structure, but their compositions also differ. The presence of S and P in the organic components is consistent with the presence of sulphated sugars and phosphorylated proteins in mollusc shells. The comparison with other nacroprismatic aragonite shells demonstrates, if necessary, that the characteristics are taxonomically dependent.

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