How Seashells are Made
The seashell of a mollusk itself is generally made of a calcium carbonate, which is secreted by the outer surface of the mantle of the shell as the creature ages, creating "layers" of shell. The composition can be thickened, enlarged and repaired, but does not play a part in the metabolism of the animal. When the mollusk eventually dies, the shell is usually the only part that remains intact.
The shell of a mollusk grows periodically, not continuously, like a small child. Because the upper surface of the shell is formed by tissue at the mantle edge while inner layers are secreted by all parts of the mantle surface, growth takes place in two directions. The first is parallel to the edge, indicating its growth in size, and the second is at right angles of the surface, indicating its growth in thickness. This varying growth process ensures that no two species are ever exactly the same. A shell formation can produce new structural elements, like folds or tubercles, which are easy to spot on larger whelks and conchs. As growth continues, these irregularities can produce what is known as an ornament or sculpture on the upper surface of the shell.
Collectors have always been amazed by the intricate design and varying color patterns of shells. The color and design of the shell is mainly dependent on the diet of the animal. In warmer waters where varieties of food sources are abundant, shell seekers will find thousands of species in all shapes, sizes and colors. In colder waters where food sources are typically the same, seekers will find mostly darker shells in hues of white, gray, and beige, lacking any type of vibrant color.
The food consumed by the mollusk causes pigments to be produced within the mantle of the mollusk, and these pigments are deposited in the layers lying directly beneath the periostracum, or surface of the shell. As with shell growth process, which can produce a variety of different shells in all shapes and sizes, the color pattern may also be the result of periodic activity. If the pigment secretion is continuous, then spiral or radial lines or bands will be laid down. However, if pigment secretion is periodic, perhaps by lack of food or simply a varying diet or feeding habit, then spots or flecks will appear on the shell. If the whole mantle is secreting pigment at the same rate, the shell will have uniform color, but if the process is interrupted, then axial or concentric lines appear. If the pigment is secreted in certain zones of the shell, then wavy bands or angular markings occur.
Depending on the species, the resulting shell itself is used for a variety of functions. Most marine mollusks use the shell for protection from predators and their hostile environment, as many mollusks can withdraw completely or at least partially into the shell. Terrestrial mollusks use the shell to prevent water loss that can lead to desiccation. Some mollusks use the shell as a tool to move or open objects, and others use the shell for locomotion and buoyancy whether it is crawling, swimming or floating.
The remarkable end result is an infinite number of shells that may be similar, but are never quite the same. As strange as it sounds, much of the mollusk's life can be determined simply by examining the shell.