A seashell, also known as a sea shell, or simply as a shell, is the common name for a hard, protective outer layer, a shell, or in some cases a "test", that was created by a sea creature, a marine organism. The shell is part of the body of a marine animal. In most cases a shell is an exoskeleton, usually that of an animal without a backbone, an invertebrate. Seashells are most often found on beaches.

The word seashell is most often used to mean the shells of marine mollusks, i.e. mollusk shells. It can however also be used to mean the shells of a wide variety of other marine animals from various different phyla. For helpful introductory articles, see marine invertebrates and marine biology.

As well as marine mollusks, many other kinds of sea animals have exoskeletons or even internal shells which sometimes, after death, wash up on the beach and may be picked up by beachcombers. These shells include remains from species in other invertebrate phyla, such as the moulted shells or exuviae of crabs and lobsters, the shells of barnacles, horseshoe crab shells, the tests (endoskeletons) of sea urchins, sand dollars and seastars, brachiopod shells, and the shells of marine annelid worms in the family Serpulidae, which create calcareous tubes cemented onto other surfaces.

Seashells have been admired, studied and used by humans for many different purposes throughout history and pre-history.

Terminology

When the word "seashells" is used to mean only the shells of marine molluscs, (spelled "mollusks" in the USA), then studying seashells is part of conchology. Conchologists or serious collectors who have a scientific bias are in general careful not to disturb living populations and habitats: even though they may collect a few live animals, most responsible collectors do not often over-collect or otherwise disturb ecosystems. When studying the whole molluscan animal is included as well as studying the shell, then the study is known as malacology; a person who studies mollusks is known as a malacologist.

Finding seashells

Seashells are commonly found in beach drift, which is natural detritus deposited along strandlines on beaches by the waves and the tides. Shells are very often washed up onto a beach empty and clean, the animal having already died, and the soft parts having rotted away or having been eaten by either predators or scavengers.

Empty seashells are often picked up by beachcombers, and collecting these shells is a harmless hobby or study. However, the majority of seashells which are offered for sale commercially have been collected alive (often in bulk) and then killed and cleaned, specifically for the commercial trade. This type of large-scale exploitation can sometimes have a strong negative impact on local ecosystems, and sometimes can significantly reduce the distribution of rare species.