How Is the Shell Created in Mollusks?

Mother of Pearl

• The lustrous substance that forms the inside of every mollusk shell is a mixture of argonite and calcite and is also known as mother of pearl. This inner layer is also known as the nacreous layer or the hypostracum. The mollusk creates this substance naturally from its own body and the minerals in the surrounding sea water. The iridescent glow of mother-of-pearl is highly prized for jewelry, furniture and a wide variety of other decorations and adornments.

Central Shell

• The middle layer of the mollusk shell is called the ostracum, or the prismatic layer. It is made up of octagonal calcite crystals that measure 1 micron each in size. In freshwater environments, this layer is made mostly of argonite. Despite the name, this part of the shell is often a dull brown color and is intended to stabilize the more delicate inner layer where the mollusk itself resides.

Mantle

• The outer coating of the shell is called the mantle, or periostracum. This is the first part of the shell that forms during the mollusk's immature stages after the umbo and eventually grows into the mollusk's final protective layer. This layer is organically formed from keratin proteins digested and secreted by the mollusk and can range in color from brown, black, dark blue or green.

Colors and Pigments

• Mollusk shells form with a wide variety of colors and patterns. These shell pigments depend greatly on the composition of the minerals in the surrounding area, since the mollusk takes these minerals in and then secretes them to form its shell. Colors range from shades of red to black and brown and take on various patterns and stripes that reflect the growth patterns of the mollusk the same way the rings of a tree trunk do.