Hawaiian Shells and Formation

Appreciation of a shell’s beauty can only be enhanced by understanding a shell’s formation. The blood of a mollusk is rich in a liquid form of calcium. A soft, outer organ called the ‘mantle’ concentrates the calcium in areas where it can separate out from the blood, forming calcium carbonate crystals. The mantle continues depositing sheets of the crystal in varying thickness, shape, and orientation. Adjacent layers are often deposited with their crystal planes at right angles to each other greatly increasing the strength of the shell. The shell grows as the animal inside adds its building material to the leading edge near the opening. This causes the shell to become longer, wider, and stronger to better accommodate the growing animal inside.

The mantle orchestrates the designs and colors of the shell. Production of new shell material is influenced by several factors: sexual hormones, intrinsic rhythms, diet, acidity and temperature of water. Colors in shells are derived from organic pigments found in their food. Glandular cells collect these pigments, mix them with fluid calcite, and set this substance into the outer shell before it hardens. There are four main pigments that produce the many colors seen in shells: yellow carotenoids, black melanins, green porphyrins, and blue and red indigoids. Most color cells are located along the front edge of the mantle where new shell material is added. A straight color line or ray is formed when the color cells remain in the same position as the shell grows out. If color pigment production continually starts and stops, a pattern of dots or dashes is drawn on the shell. If the color cells actually migrate to one side, a slanting trail of color is produced. Other kinds of behavior by color cells can produce circles, triangles, and other shapes. Mollusks within a particular species have basic colors and patterns that are genetically inherited, but natural variation, like different hair color among people, gives each shell a character of its own.