seashell

Seashell refers to the hard outer covering produced by many marine mollusks, most commonly the gastropods (snails) and bivalves (clams, oysters, mussels). The shell is secreted by the mantle and grows with the animal, enlarging at the opening as the mollusk ages. The material is primarily calcium carbonate, occurring in two crystalline forms—aragonite and calcite—and organized into distinct layers. An outer organic layer, the periostracum, is followed by a prismatic calcitic layer and an inner nacreous layer, or mother of pearl, which gives some shells a lustrous interior.

Shell shape, color, and texture vary widely among species and reflect genetics, growth conditions, and diet. Pigmentation and nacreous layering produce a range of patterns, from plain to iridescent. The shell serves as protection from predators and environmental hazards, while also providing surface area for muscle attachment and, in some species, camouflage.

When the animal dies, the shell often remains as a mineralized artifact in the environment; shells may accumulate on beaches, be reused as tools or ornaments, or, in some cases, host other organisms such as hermit crabs. In paleontology, seashells are important because their calcium carbonate remains fossilize well and document past marine life.

Human use includes decoration, jewelry, buttons, and currency in some cultures, as well as scientific and educational specimens. Conservation concerns arise from overharvesting, habitat loss, and pollution, which can impact mollusk populations and shell availability.