A seashell is the hard, protective casing created by an invertebrate animal that lives in the ocean. These structures are technically the exoskeleton of the creature, built to shield its soft body from the harsh marine environment. When found on the beach, the shell represents the discarded skeletal remains of a once-living organism, resulting from a continuous, lifelong process of growth and reinforcement.
The Creatures That Build Them
The organisms responsible for creating most seashells belong to the phylum Mollusca, including familiar animals like clams, oysters, and sea snails. These animals possess a unique organ called the mantle, a specialized sheet of tissue that covers the soft body mass. The mantle is solely responsible for secreting the materials required to construct, maintain, and enlarge the shell throughout the animal’s life.
The primary function of the shell is physical protection from predators, which must either drill through or crush the shell to reach the soft body inside. For many species, the shell also provides structural support and a defense against desiccation, helping to seal the animal inside during low tide or when exposed to air. Because the shell is never shed, the mollusk must continuously add material to the edges of the structure to accommodate its increasing body size.
How Seashells Are Constructed
The construction of a seashell is a biological process known as biomineralization, where the mollusk uses dissolved minerals from the surrounding seawater. The main mineral component is calcium carbonate ($\text{CaCO}_3$), which is crystallized onto a supportive framework of organic material. This framework, composed of proteins and a carbohydrate called chitin, is secreted first and acts as a scaffold that directs the shape and growth of the shell.
The shell structure is made of three distinct layers, each with a specific composition and function. The outermost layer is the periostracum, which is thin and unmineralized, consisting mostly of organic proteins that help shield the shell from acidic erosion. Beneath this lies the prismatic layer, where the calcium carbonate is deposited in the form of calcite crystals, oriented perpendicularly to the shell surface to provide strength.
The innermost layer, which is in direct contact with the animal’s mantle, is often the nacre, commonly known as mother-of-pearl. This layer is constructed from microscopic, overlapping tiles of aragonite, a crystal form of calcium carbonate, cemented together by protein layers. The precise arrangement of these aragonite platelets creates the shell’s characteristic iridescence and gives the entire structure its high fracture resistance.
A Look at Shell Diversity
The diversity of shells found on beaches reflects the variety within the Mollusca phylum, categorized by the number and arrangement of their shell parts. The most common shells belong to the class Gastropoda, recognized by their single, coiled shell, often referred to as univalves. These include animals like whelks, conchs, and sea snails, whose shells grow in an ever-widening spiral.
The other major type are the Bivalves, characterized by a shell divided into two halves, or valves, joined by a flexible hinge. Species such as clams, oysters, and scallops fall into this category, with the two valves opening and closing to protect the soft animal inside. Less common are the chitons, which possess a shell composed of eight overlapping plates that provide flexibility while still offering protection.
Why Shells Wash Ashore
The shells that accumulate on the beach only begin their journey to the shore after the mollusk inside has died. Following the death of the animal, the soft tissue will either decompose or be consumed by scavengers and other marine life. Once the shell is empty, it loses the animal’s ability to anchor itself to the seabed or burrow into the sediment.
The empty shell becomes a passive object, susceptible to the physical forces of the ocean. The action of currents, tides, and wave energy, especially during storms, lifts the shells from the ocean floor and transports them toward the coastline. Over time, the constant tumbling in the surf smooths, chips, and polishes the shell before depositing it on the sand.
Even after washing ashore, empty shells maintain an important ecological role in the coastal environment. They provide shelter for small creatures, such as house-hunting hermit crabs, which require a discarded shell to survive. The shells eventually break down, slowly releasing their calcium carbonate back into the environment, where it helps to replenish beach sand and enrich the surrounding habitat.