Seashell

A seashell is the hard, protective outer structure produced by a variety of marine organisms, most commonly molluscs such as snails, clams and oysters. These shells serve primarily as exoskeletons that protect and support the soft-bodied animal within. After the animal dies, its shell often washes ashore along beaches, where it is collected naturally by beachcombers. Because most marine molluscan shells are composed of durable calcium carbonate, they persist far longer than chitin-based shells and therefore dominate beach drift. Other coastal exoskeletons, including the calcareous tubes of serpulid worms, the tests of sea urchins and the moulted shells of crustaceans, can also be found in marine debris.
Seashells are not limited to marine environments. Freshwater mussels and snails, as well as terrestrial snails, also produce protective shells; however, the term seashell is usually reserved for the shells of marine molluscs. The scientific study of molluscs is known as malacology, while the narrower study of shells is termed conchology.

Occurrence and collection

Seashells are most commonly found in beach drift, the accumulation of natural materials deposited by waves and tides along the strandline. Most shells found in this environment are empty: the animal has died and its tissues have decayed or been consumed by other organisms. Shell collecting has long been a popular pastime, but commercial shell-gathering—often involving the collection of living animals—can place pressure on local ecosystems and reduce populations of rare species. Responsible collecting practices involve minimal disturbance to living communities.

Shell formation and composition

Molluscs produce shells through secretions from a specialised layer of tissue called the mantle. Cells in the mantle deposit a framework of proteins and minerals, most notably calcium carbonate in the form of aragonite or calcite. This material is laid down in layers, giving rise to the characteristic strength and patterning of shells. As the mollusc grows, its mantle extends the shell incrementally.
Although many molluscan shells are external, some cephalopods—such as cuttlefish and spirula—possess internal shells, and other groups like octopuses lack shells altogether.
Marine mollusc shells vary greatly in size, shape, sculpture and colour. Tropical and subtropical seas are especially rich in species with large, ornate and vividly coloured shells, whereas the colder temperate zones host fewer such forms. Tiny species known as micromolluscs greatly outnumber larger ones, although their shells are often overlooked.

Types of molluscan seashells

BivalvesBivalve shells, composed of two hinged valves, are often the most abundant on sandy or sheltered beaches. Examples include clams, mussels, oysters and scallops. There are over 15,000 species living in both marine and freshwater habitats. The shell is secreted in layers and held together by a ligament at the hinge. Most bivalves are filter feeders, drawing water through their gills where food particles are trapped. Bivalves produce pearls and are also widely used as food.
GastropodsGastropod seashells—produced by sea snails—are common on beaches, particularly where rocky habitats lie offshore. Their coiled shells vary widely in form, from smooth cones to heavily sculptured spirals. Many are brightly coloured and robust.
Polyplacophorans (chitons)Chitons possess shells composed of eight articulated plates surrounded by a leathery girdle. After death, these plates often separate, washing ashore as individual pieces known as chiton valves or “butterfly shells”.
CephalopodsOnly a small number of cephalopods produce shells that regularly appear on beaches. The cuttlefish (Sepia) has an internal shell known as the cuttlebone. Spirula spirula possesses a coiled internal shell that is buoyant and often washes ashore in tropical regions. The nautilus is the sole living cephalopod with a well-developed external shell, while female argonauts construct delicate papery egg cases known as paper nautiluses. Extinct ammonites are common in the fossil record.

Seashells used by other animals

Abandoned molluscan shells provide important resources for many organisms. Hermit crabs are the most familiar example: nearly all species inhabit empty gastropod shells to protect their soft abdomens, exchanging shells as they grow. Some hermit crabs live far from the sea, including terrestrial species within the genus Coenobita.
Certain gastropods, such as the carrier shells of the family Xenophoridae, attach pebbles or other shells to their own as they grow—possibly for camouflage or stability. Small octopuses sometimes adopt empty shells as temporary shelters.