Abstract
Consistently classified among the references for calcite simple prisms, the microstructural units that form the outer layer of the Pinctada margaritifera have been investigated through a series of morphological, crystallographical and biochemical characterizations. It is often said that the polygonal transverse shape of the prisms result from the competition for space between adjacent crystals. In contrast to this classical scheme the Pinctada prisms appear to be composed of four successive developmental stages from the concentrically growing disks on the internal side of the periostracum to the morphological, structural and compositional changes in both envelopes and mineral components at the end of the prisms. These latest structural and compositional changes predate nacre deposition, so that the end of prism growth is not caused by occurrence of nacre, but by metabolic changes in the secretory epithelium. This sequence makes obvious the permanent biological control exerted by the outer cell layer of the mantle in both organic envelopes and mineralizing organic phases.
Highlights
Shells of Pinctada margaritifera, the bivalve species on which Polynesian pearl industry relies, exhibit simple organization and structure
This study shows that the calcite prisms of the Pinctada margaritifera are constructed employing a developmental sequence that can be incorporated into the overall scheme of shell formation as summarized by Taylor and Kennedy [9] (p. 275): “Evidence suggests that there is a generative zone in the inner part of the periostracal groove and cells produced there change their form and function during the growth of the animal”
What Taylor and Kennedy apply to the whole shell fits perfectly with the structural history of the prismatic layer itself, as formation and developmental timing of the prismatic units are directly linked to the mechanism of shell growth at the outer margin of the mantle
Summary
Shells of Pinctada margaritifera, the bivalve species on which Polynesian pearl industry relies, exhibit simple organization and structure. In addition to the nacreous material that has triggered extensive economic interest and continued investigation during the last century [1], the calcite prisms that form the outer shell layer in Pinctada (and members of Pteriidae family) have drawn attention owing to their visible structural patterns. These prisms were invariably classified as “simple prisms” in the main treatises describing structural organizations of Mollusk shells [2,3,4,5,6]. Question arises regarding the determinism of a crystallization process that enables formation of such regular crystal-like units without any known precursor
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