External Form and Function
Shell
The mantle, lining the shell to the extent of its outer shell edge, secretes the growing edges of the shell. The edge of the mantle can be divided into three zones: the outer- most marginal zone, a medial pallial zone and a central zone, which extends inward across the surface of the mantle (Garcia-Gasca et al. 1994)
The shell consists of three parallel layers, which grow concentrically from the umbo. The outer, thin, horny coat of the periostracum, the middle prismatic layer, or ostracum, consists of polygonal prisms of calcite, and the inner nacre, which consists of the layers of conchiolin, interspersed with thin sheets of aragonite. The aragonite forms as thin platelets overlapping each other, parallel to the edge of the shell and has zigzag edges. The combination of the shape of the edges and the film-like layers creates the characteristic pearl luster (Farn 1986). The inner layer, hypostracum, is composed of nacre. The nacre has high malleable strength and plasticity compared with other mollusk shells, making it highly resistant to crushing forces and therefore providing good defense against a number of predators (Curry and Brear 1984). Nacre consists of flat polygonal tablets of aragonite with compound extracrystalline and intracrystalline organic matrices in a “bricks and mortar” format. Nacre is set down in layers in a intricate succession of actions involving progressive mineralization within the extrapallial fluid.
Under normal conditions the periostracal layer is secreted from the mantle edge and does not increase in thickness once it’s formed. The prismatic layer is secreted from the outer epidermis of the peripheral region of the mantle and is also only laid down once. The nacreous layer is secreted by the entire mantle surface and is continuously laid down throughout the entire lifespan of the animal. However, the repair of damaged shell requires the secretion of all layers in the original sequence, regardless of which layer is damaged. The mantle must therefore change its secretory capacity in these conditions. In pearl formation, the three layers are similarly secreted in order by the inserted mantle tissue around the nucleus.
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