A seashell (or sea shell), also known simply as a shell, is a hard, protective outer layer usually created by an animal or organism that lives in the sea. Most seashells are made by mollusks, such as snails, clams, and oysters to protect their soft insides. Empty seashells are often found washed up on beaches by beachcombers. The shells are empty because the animal has died and the soft parts have decomposed or been eaten by another organism.
A seashell is usually the exoskeleton of an invertebrate (an animal without a backbone), and is typically composed of calcium carbonate and partially chitin. Most shells that are found on beaches are the shells of marine mollusks, partly because these shells are usually made of calcium carbonate, and endure better than shells made of chitin.
Apart from mollusk shells, other shells that can be found on beaches are those of barnacles, horseshoe crabs and brachiopods. Marine annelid worms in the family Serpulidae create shells which are tubes made of calcium carbonate cemented onto other surfaces. The shells of sea urchins are called "tests", and the moulted shells of crabs and lobsters are exuviae. While most seashells are external, some cephalopods have internal shells.
Seashells have been used by humans for many different purposes throughout history and prehistory. However, seashells are not the only kind of shells; in various habitats, there are shells from freshwater animals such as freshwater mussels and freshwater snails, and shells of land snails.
Terminology When the word "seashells" refers only to the shells of marine mollusks, then studying seashells is part of conchology. Conchologists and amateur researchers are in general careful not to disturb living populations and habitats. Most responsible collectors do not over-collect or otherwise disturb ecosystems even if they capture few live specimens.
The study of mollusks (including their shells) is known as malacology; a person who studies mollusks is known as a malacologist.
Occurrence 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.
Empty seashells are often picked up by beachcombers. 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.
Shell synthesis Seashells are created by the mollusks that use them for protection. Mollusks have an outside layer of tissues on their bodies the mantle which creates the shell material and which connects the shell to the mollusk. The specialized cells in the mantle form the shell using different minerals and proteins. The proteins are then used to create the framework that supports the growing shell. Calcium carbonate is the main compound of shell structure, aiding in adhesion.
Molluscan seashells
The word seashell is often used to mean only the shell of a marine mollusk. Marine mollusk shells that are familiar to beachcombers and thus most likely to be called "seashells" are the shells of marine species of bivalves (or clams), gastropods (or snails), scaphopods (or tusk shells), polyplacophorans (or chitons), and cephalopods (such as nautilus and spirula). These shells are very often the most commonly encountered, both in the wild, and for sale as decorative objects.
Marine species of gastropods and bivalves are more numerous than land and freshwater species, and the shells are often larger and more robust. The shells of marine species also often have more sculpture and more color, although this is by no means always the case.
In the tropical and sub-tropical areas of the planet, there are far more species of colorful, large, shallow water shelled marine mollusks than there are in the temperate zones and the regions closer to the poles.
Although there are a number of species of shelled mollusks that are quite large, there are vast numbers of extremely small species too, see micromollusks.
Not all mollusks are marine. There are numerous land and freshwater mollusks, see for example snail and freshwater bivalves. In addition, not all mollusks have an external shell: some mollusks such as some cephalopods (squid and octopuses) have an internal shell, and many mollusks have no shell, see for example slug and nudibranch.
Bivalves
Bivalves are often the most common seashells that wash up on large sandy beaches or in sheltered lagoons. They can sometimes be extremely numerous. Very often the two valves become separated.
There are more than 15,000 species of bivalves that live in both marine and freshwater. Examples of bivalves are clams, scallops, mussels, and oysters. The majority of bivalves consist of two identical shells that are held together by a flexible hinge. The animal's body is held protectively inside these two shells. Bivalves that do not have two shells either have one shell or they lack a shell altogether. The shells are made of calcium carbonate and are formed in layers by secretions from the mantle. Bivalves, also known as pelecypods, are mostly filter feeders; through their gills, they draw in water, in which is trapped tiny food particles. Some bivalves have eyes and an open circulatory system. Bivalves are used all over the world as food and as a source of pearls. The larvae of some freshwater mussels can be dangerous to fish and can bore through wood.
Shell Beach, Western Australia, is a beach which is entirely made up of the shells of the cockle Fragum erugatum.
Gastropods
Certain species of gastropod seashells (the shells of sea snails) can sometimes be common, washed up on sandy beaches, and also on beaches that are surrounded by rocky marine habitat.
There are more than 65,000 species of gastropods that live in marine, freshwater, and terrestrial environments. Examples of gastropods include snails, slugs, whelks, limpets, and abalones. Most gastropods have a single coiled shell, which is made of calcium carbonate and is secreted in layers by the mantle, although some species have reduced shells or no shell at all. A key feature of gastropods is their muscular foot, which they use for crawling, burrowing, or swimming, and their head typically bears tentacles and eyes. Many gastropods feed using a radula, a ribbon-like structure with tiny teeth that scrape, cut, or drill food, though some species filter feed or prey on other animals. Gastropods have a wide variety of reproductive strategies and undergo a developmental process known as torsion, which twists the body and internal organs. They are used worldwide as a source of food, and some species produce valuable materials such as mother-of-pearl. Certain marine gastropods can be harmful to other organisms; for example, cone snails possess venomous harpoons capable of paralyzing prey.
Polyplacophorans
Chiton plates or valves often wash up on beaches in rocky areas where chitons are common. Chiton shells, which are composed of eight separate plates and a girdle, usually come apart not long after death, so they are almost always found as disarticulated plates. Plates from larger species of chitons are sometimes known as "butterfly shells" because of their shape.
Cephalopods
Only a few species of cephalopods have shells (either internal or external) that are sometimes found washed up on beaches.
Some cephalopods such as Sepia, the cuttlefish, have a large internal shell, the cuttlefish bone, and this often washes up on beaches in parts of the world where cuttlefish are common.
Spirula spirula is a deep water squid-like cephalopod. It has an internal shell which is small (about 1 in or 24 mm) but very light and buoyant. This chambered shell floats very well and therefore washes up easily and is familiar to beachcombers in the tropics.
Nautilus is the only genus of cephalopod that has a well-developed external shell. Females of the cephalopod genus Argonauta create a papery egg case which sometimes washes up on tropical beaches and is referred to as a "paper nautilus".
The largest group of shelled cephalopods, the ammonites, are extinct, but their shells are very common in certain areas as fossils.
Molluscan seashells used by other animals Empty molluscan seashells are a sturdy, and usually readily available, "free" resource which is often easily found on beaches, in the intertidal zone, and in the shallow subtidal zone. As such they are sometimes used second-hand by animals other than humans for various purposes, including for protection (as in hermit crabs) and for construction.
Mollusks Carrier shells in the family Xenophoridae are marine shelled gastropods, fairly large sea snails. Most species of xenophorids cement a series of objects to the rim of their shells as they grow. These objects are sometimes small pebbles or other hard detritus. Very often shells of bivalves or smaller gastropods are used, depending on what is available on the particular substrate where the snail itself lives. It is not clear whether these shell attachments serve as camouflage, or whether they are intended to help prevent the shell sinking into a soft substrate.
Small octopuses sometimes use an empty shell as a sort of cave to hide in, or hold seashells around themselves as a form of protection like a temporary fortress.
Invertebrates
Almost all genera of hermit crabs use or "wear" empty marine gastropod shells throughout their lifespan, in order to protect their soft abdomens, and in order to have a strong shell to withdraw into if attacked by a predator. Each individual hermit crab is forced to find another gastropod shell on a regular basis, whenever it grows too large for the one it is currently using.
Some hermit crab species live on land and may be found quite some distance from the sea, including those in the tropical genus Coenobita.
Conchology