Developmental aspects of biomineralisation in the Polynesian pearl oyster Pinctada margaritifera var. cumingii

Abstract

The shell biomineralisation with special reference to the nacreous region is observed during the development of the Polynesian pearl oyster. Ultrastructural changes were studied and timed for the first time from planktonic larval shells to two-year-old adult shells. During the first two weeks following fertilization, the prodissoconch-I shell structure is undifferentiated and uniformly granular. The prodissoconch-II stage which develops during the next two weeks acquires a columnar organization. Metamorphosis is characterized by the formation of the dissoconch shell with all the elements of an adult shell and marks the onset of the development of the nacre. Nucleation starts within an organic matrix from point sources forming 'crystal germs' which expand circularly until they fuse. The orientation of the contour lines of scalariform growth margins indicates the direction of shell growth. Five zones of growth (Z) were characterized. One-month-old shells show a homogenous zone, without particular figures (Z0). The contour lines are initially parallel to the growing shell edge (Z1), later becoming labyrinthic (finger prints, Z2) or perpendicular to the edge (Z3). A fourth zone (Z4) characterized by spiral growth and associated with the shell thickening is observed later in the umbo region. This development results from a gradual lowering of the rate of mineralisation over time and from changes in growth pattern from a predominant increase in size, shifting toward an increase in the thickness of the shell. The mineralisation patterns of the shells from larvae reared in hatchery conditions and larvae from the field appear similar. The use of quantified information on shape, density, and distribution of nucleation sites as an indicator of growth conditions is discussed. © 2001 Ifremer/CNRS/IRD/Éditions scientifiques et médicales Elsevier SAS Résumé − Biominéralisation de l'huître perlière polynésienne Pinctada margaritifera var. cumingii au cours de son développement. La biominéralisation de la coquille de l'huître perlière de Polynésie française, en particulier celle de sa partie nacrée, est étudiée au cours du développement du bivalve. Les changements de l'ultrastructure de sa coquille sont suivis, pour la première fois, du stade larvaire planctonique (prodissoconque I et II) au stade adulte de deux ans (dissoconque). Durant les deux premières semaines qui suivent la ponte, la prodissoconque I présente une structure granuleuse, sans aucune organisation apparente. La prodissoconque II qui se développe durant les deux semaines suivantes a une organisation en colonne. Au moment de la métamorphose, une dissoconque s'ajoute à la coquille larvaire. La fixation des larves planctoniques marque ainsi la mise en place de la couche nacrée. La taille, la densité et la *Correspondence and reprints.

Different secretory repertoires control the biomineralization processes of prism and nacre deposition of the pearl oyster shell

“…Ontological and paleontological data give congruent pictures. On one hand, the ontogenic data obtained on the modern pearl oysters (42,43) or other pteriomorphid bivalves (44) indicate that the first shell produced is organic (periostracum-like); this shell is then mineralized and made of aragonite granules (prodissoconch I). This step precedes the deposition of calcitic prisms (prodissoconch II), followed by the deposition of the nacreous layer after metamorphosis.…”

Sheet nacre growth mechanism: a Voronoi model

“…Its role is fundamental : nacre is growing simultaneously everywhere in the inner surface of the shell. That is the case for the development stages observed starting at the hatching stage, after the metamorphosis of the oyster (i.e., 1 month) until the adult stage (Mao Che et al, 2001). These authors have clearly shown (on the shell-side) that stairs-like growing was seen to be a constant factor of the mineralization, whatever the zone in the shell, i.e., nacre growing parallel to the edge, the Wnger-print-like nacre in the middle of the shell or in the thickening zones with the "ascending spirals" observed by Wada (1961).…”

Dual Gene Repertoires for Larval and Adult Shells Reveal Molecules Essential for Molluscan Shell Formation

“…Immediately after prodissoconch I, prodissoconch II is secreted from mantle tissue of the veliger larva. After settlement and metamorphosis of veliger larvae to become juveniles, adult shells, or dissoconch, are formed ( Waller 1981 ; Mao Che et al 2001 ). Adult molluscan shells are comprised mainly of calcite or aragonite, or both, which are assembled in highly variable microstructures ( Kobayashi 1969 ; Taylor 1973 ; Carter 1990 ).…”