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

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.