How Shells Are Made
Snail shells show a spiral pattern of growth

Mollusks are delicate creatures and these soft-bodied invertebrate sea and land creatures form one of the largest groups of animals on the planet. It includes varieties of snails, slugs, clams, oysters, squid, octopus, nautilus and chiton (a group with eight-piece "armored" shells). Smaller mollusks make tasty morsels for birds, fish and humans and so most mollusks have developed an "exoskeleton" as a protective living space. Like other skeletons, the exoskeleton grows with the organism it supports. The system works, too--mollusks are some of the most successful living organisms on earth, many types having survived relatively unchanged for millions of years. "Bivalves" like clams, oysters and mussels, form two distinct shells joined by a strong muscle but "univalves" such as snails, conchs and abalone grow one shell.

Using the Sea

A bivalve grows two identical shells held together by a large muscle

The life's work of a mollusk is to build its shell, a process that begins when the creature is in its infancy. Larval mollusks swim free, consuming plankton and simple organisms until they develop enough mass to latch onto rocks or other hard surfaces with their muscular little feet. Diatoms and protozoa they ingest, along with plankton, are secreted along their backs to form a "mantle" or "soft shell." As they age, calcium carbonate and other minerals from these foods migrates to the outer surface to form the outer layers of the shell. Shells grow in thickness and overall size as the mollusk consumes more calcium-rich plankton, diatoms and protozoa.

Adaptation and Pearls

This interior of this spiny shell held a rosy-colored occupant.

Heredity determines the shape, size and appearance of an exoskeleton just as it establishes the biology of the creature inside. Shells grow in ever-widening spirals and sprout spines of calcium carbonate to defend occupants against nosy predators. Many mollusks expel chemicals along with minerals to form colors and patterns in their shells that replicate their environments. As the shell grows in size and thickness, certain heavier minerals stay close to the creature's mantle on the interior of the shell to form a smooth surface for its soft-skinned occupant. In bivalves like oysters, this luminescent substance is called "Mother of Pearl" because it also coats any foreign matter that gets into the shell, forming a natural gem. Other mollusks create interiors that match their mantles in colorful shades of pink and brown. Shells of mollusks are often adopted after their deaths by other creatures but no further growth will take place once the creature that provided the building chemistry of the shell is gone.