A seashell or sea shell, also known simply as a shell, is a hard, protective outer layer created by an animal that lives in the sea. The shell is part of the body of the animal. 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 been eaten by another animal or have rotted out.
The term seashell usually refers to the exoskeleton of an invertebrate (an animal without a backbone). Most shells that are found on beaches are the shells of marine mollusks ("molluscs" in British English), partly because many of these shells endure better than other seashells.
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 made of calcareous tubes cemented onto other surfaces. The shells of sea urchins are called ''tests'', and the moulted shells of crabs and lobsters are called ''exuviae''. While most seashells are external, some cephalopods have internal shells.
Seashells have been admired, studied and used by humans for many different purposes throughout history and pre-history. But seashells are not the only kind of shells. In a variety of habitats it is possible to find shells from freshwater animals such as freshwater mussels and freshwater snails, and it is also possible to find shells from land snails.
Terminology
When the word "seashells" is used to refer only to the shells of marine mollusks 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.
Molluscan seashells
The word "seashells" is often used to mean only the shells of marine mollusks. 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 however, there are numerous land and freshwater mollusks, see for example snail and freshwater bivalves. And 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.
Shell Beach, Western Australia is a beach which is entirely made up of the shells of the cockle ''Fragum erugatum'' as shown here.
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.
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.
By other 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.
By other 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''.
Human uses
Collecting shells as a hobby and a study
There are numerous popular books and field guides on the subject of shell-collecting. Although there are a number of books about land and freshwater mollusks, the majority of popular books emphasize, or focus exclusively on, the shells of marine mollusks. Both the science of studying mollusk shells and the hobby of collecting and classifying them are known as conchology. The line between professionals and amateur enthusiasts is often not well defined in this subject, because many amateurs have contributed to, and continue to contribute to, conchology and the larger science of malacology. Many shell collectors belong to "shell clubs" where they can meet others who share their interests. A large number of amateurs collect the shells of marine mollusks, and this is partly because many shells wash up empty on beaches, or live in the intertidal or sub-tidal zones, and are therefore easily found and preserved without much in the way of specialized equipment or expensive supplies. Some shell collectors find their own material and keep careful records, or buy only "specimen shells", which means shells which have full collecting data: information including how, when, where, in what habitat, and by whom, the shells were collected. On the other hand, some collectors buy the more widely available commercially-imported exotic shells, the majority of which have very little data, or none at all. To museum scientists, having full collecting data (when, where, and by whom it was collected) with a specimen is far more important than having the shell correctly identified. Some owners of shell collections hope to be able to donate their collection to a major natural history or zoology museum at some point, however, shells with little or no collecting data are usually of no value to science, and are likely not to be accepted by a major museum. Apart from any damage to the shell that may have happened ''before'' it was collected, shells can also suffer damage when they are stored or displayed. For an example of one rather serious kind of damage see Byne's disease.
Shell clubs
There are a number of clubs or societies which consist of people who are united by a shared interest in shells. In the USA these clubs are more common in southerly coastal areas, such as
Florida and
California, where the marine fauna is rich in species.
Identification
Seashells are usually identified by consulting general or regional shell-collecting
field guides, and specific scientific books on different taxa of shell-bearing mollusks (
monographs) or "iconographies" (limited text - mainly photographs or other illustrations). (For a few titles on this subject in the
USA, see the list of books at the foot of this article.) Identifications to the species level are generally achieved by examining illustrations and written descriptions, rather than by the use of
Identification keys, as is often the case in identifying plants and other phyla of invertebrates. The construction of functional keys for the identification of the shells of marine mollusks to the species level can be very difficult, because of the great variability within many species and families. The identification of certain individual species is often very difficult, even for a specialist in that particular family. Some species cannot be differentiated on the basis of shell character alone.
Numerous smaller and more obscure mollusk species (see micromollusk) are yet to be discovered and named. In other words, they have not yet been differentiated from similar species and assigned scientific (binomial) names in articles in journals recognized by the International Commission on Zoological Nomenclature (ICZN). Large numbers of new species are published in the scientific literature each year. There are currently an estimated 100,000 species of mollusks worldwide.
Non-marine "seashells"
The term "seashell" is also applied loosely to mollusk shells that are not of marine origin, for example by people walking the shores of lakes and rivers using the term for the freshwater mollusk shells they encounter. Seashells purchased from tourist shops or dealers may include various freshwater and terrestrial shells as well. Non-marine items offered may include large and colorful tropical land snail shells, freshwater apple snail shells, and pearly freshwater unionid mussel shells. This can be confusing to collectors, as non-marine shells are often not included in their reference books.
Significance of molluscan seashells in human culture
As currency
Seashells have been used as a medium of exchange in various places, including many Indian Ocean and Pacific Ocean islands, also in North America, Africa and the Caribbean.
The most common species of shells to be used as currency have been ''Cypraea moneta'', the “money cowry”, and certain tusk shells or dentalium, such as those used in North Western North America for many centuries.
Some tribes of the
indigenous peoples of the Americas used shells for
wampum and
hair pipes. The Native American ''
wampum belts'' were made of the shell of the
quahog clam.
It is of historic interest that the Dutch East India Company, a major force in the colonization of the Indian and Pacific Oceans, amassed a large portion of its vast fortune via trading shell money of the species ''Cypraea moneta'' and ''Cypraea annulus'', in exchange for commodities such as spices, exotic animals, and gemstones, all of which were considered valuable in Europe at the time.
As tools
Seashells have often been used as
tools, because of their strength and the variety of their shapes.
Giant clams (Family Tridacnidae) have been used as bowls, and when big enough, even as bathtubs and baptismal fonts.
''Melo melo'', the "bailer volute", is so named because Native Australians used it to bail out their canoes.
Many different species of bivalves have been used as scrapers, blades, clasps, and other such tools, due to their shape.
Some marine gastropods have been used for oil lamps, the oil being poured in the aperture of the shell, and the siphonal canal serving as a holder for the wick.
In horticulture
Because seashells are in some areas a readily available bulk source of calcium carbonate, shells such as
oyster shells are sometimes used as soil conditioners in
horticulture. The shells are broken or ground into small pieces in order to have the desired effect of raising the
pH and increasing the
calcium content in the soil.
In religion and spirituality
Seashells have played a part in religion and spirituality, sometimes even as ritual objects.
In Christianity, the scallop shell is considered to be the symbol of Saint James the Great, see ''Pecten jacobaeus''.
In Hinduism left-handed shells of ''Turbinella pyrum'' (the sacred shankha) are considered to be sacred to the god Vishnu. The person who finds a left-handed chank shell (one that coils to the left) is sacred to Vishnu, as well. The chank shell also plays an important role in Buddhism.
Cowries have often been considered to be symbols of female fertility. They were often treated as actual fertility charms. The dorsum of the shell resembles a pregnant belly, and the underside of the shell resembles a vulva. In the South Indian state of Kerala, cowries are used for making astrological predictions.
In Santeria, shells are used for divination purposes.
The
Moche culture of ancient
Peru worshipped animals and the sea, and often depicted shells in their art.
As musical instruments
Seashells have been used as musical instruments, wind instruments for many hundreds if not thousands of years. Most often the shells of large sea snails are used, as trumpets, by cutting a hole in the spire of the shell, or cutting off the tip of the spire altogether. Various different kinds of large marine gastropod shells can be turned into "blowing shells", however the most commonly encountered species used as "conch" trumpets are:
The sacred chank, ''Turbinella pyrum'', known in India as the shankha. In Tibet it is known as "dung-dkar".
The Triton shell also known as "Triton's trumpet" ''Charonia tritonis'' which is used as a trumpet in Melanesian and Polynesian culture and also in Korea and Japan. In Japan this kind of trumpet is known as the horagai. In Korea it is known as the nagak. In some Polynesian islands it is known as "''pu''".
The Queen Conch ''Strombus gigas'', is or was sometimes used as a trumpet in the Caribbean.
In personal adornment
Whole seashells or parts of sea shells have been used as jewelry or in other forms of adornment since prehistoric times.
Mother of pearl was historically primarily a seashell product, although more recently some mother of pearl comes from freshwater mussels. Also see
pearl.
Shell necklaces have been found in Stone Age graves as far inland as the Dordogne Valley in France.
Seashells are often used whole and drilled, so that they can be threaded like
beads, or cut into pieces of various shapes. Sometimes shells can be found that are already "drilled" by predatory snails of the family
Naticidae. Fine whole shell necklaces were made by
Tasmanian Aboriginal women for more than 2,600 years. The necklaces represent a significant cultural tradition which is still practised by
Palawa women elders. The shells used include pearly green and blue-green maireener (rainbow kelp) shells, brown and white rice shells, black cats' teeth shells and pink button shells.
Naturally-occurring, beachworn, cone shell "tops" (the broken-off spire of the shell, which often has a hole worn at the tip) can function as beads without any further modification. In Hawaii these natural beads were traditionally collected from the beach drift in order to make puka shell jewelry. Since it is hard to obtain large quantities of naturally-occurring beachworn cone tops, almost all modern puka shell jewelry uses cheaper imitations, cut from thin shells of other species of mollusk, or even made of plastic.
Shells historically have been and still are made into, or incorporated into, necklaces, pendants, beads, earrings, buttons, brooches, rings, hair combs, belt buckles and other uses.
The shell of the large "bullmouth helmet" sea snail, scientific name ''Cypraecassis rufa'', was historically, and still is, used to make valuable cameos.
Mother of pearl from many seashells including species in the family Trochidae, Turbinidae, Haliotidae, and various pearly bivalves, has often been used in jewelry, buttons, etc.
In London, Pearly Kings and Queens traditionally wear clothing covered in patterns made up of hundreds of "pearl buttons", in other words, buttons made of mother-of-pearl or nacre. In recent years however, the majority of "pearl buttons" are imitations that are made of pearlescent plastic.
In crafts
"Sailor's Valentines" were late 19th century decorative keepsakes which were made in the Caribbean, and which were often purchased by sailors to give to their loved ones back home for example in England. These valentines consisted of elaborate arrangements of small seashells glued into attractive symmetrical designs, which were encased on a wooden (usually octagonal) hinged box-frame. The patterns used often featured heart-shaped designs, or included a sentimental expression of love spelled out in small shells.
The making of shellwork artifacts is a practice of Aboriginal women from La Perouse in Sydney, New South Wales, dating back to the 19th century. Shellwork objects include baby shoes, jewelry boxes and replicas of famous landmarks, including the Sydney Harbour Bridge and the Sydney Opera House. The shellwork tradition began as an Aboriginal women's craft which was adapted and tailored to suit the tourist souvenir market, and which is now considered high art.
In architectural decoration
Small pieces of colored and iridescent shell have been used to create
mosaics and
inlays, which have been used to decorate walls, furniture and boxes. Large numbers of whole seashells, arranged to form patterns, have been used to decorate mirror frames, furniture and man-made
grottos.
In art
The pleasing designs of seashells have caused them to be featured in art in various ways, in paintings, in sculpture, and so on. A very large outdoor sculpture at Akkulam of a gastropod seashell is a reference to the sacred chank shell ''Turbinella pyrum'' of India. Maggi Hambling designed a striking 13 ft (4 m) high sculpture of a scallop shell which stands on the beach at Aldeburgh, in England. The goddess of love, Venus or Aphrodite is often traditionally depicted rising from the sea on a seashell. In ''the Birth of Venus (Botticelli)'', Botticelli depicted the goddess Venus rising from the ocean on a scallop shell.
As an additive to poultry feeds
Sea shells found in the creek and backwater of the coast of west India are used as an additive to poultry feed. They are crushed and mixed with jawar maaze and dry fish.
Shells of other marine invertebrates
Arthropods
Many arthropods have sclerites, or hardened body parts, which form a stiff exoskeleton made up mostly of chitin. In crustaceans, especially those of the class Malacostraca (crabs, shrimps and lobsters, for instance), the plates of the exoskeleton may be fused to form a more or less rigid carapace. Moulted carapaces of a variety of marine malacostraceans often wash up on beaches. The horseshoe crab is an arthropod of the family Limulidae. The shells or exuviae of these arachnids are common in beach drift in certain areas of the world.
Sea urchins
Some
echinoderms such as
sea urchins, including
heart urchins and
sand dollars, have a hard "test" or shell. After the animal dies, the flesh rots out and the spines fall off, and then fairly often the empty test washes up whole onto a beach, where it can be found by a beachcomber. These tests are fragile and easily broken into pieces.
Brachiopods
The
brachiopods, or lamp shells, superficially resemble clams, but the phylum is completely unrelated to mollusks. Most lines of brachiopods ended during the
Permian-Triassic extinction event, and their ecological niche was filled by bivalves. A few of the remaining species of brachiopods occur in the low
intertidal zone and thus can be found live by beachcombers.
Annelids
Some
polychaetes, marine annelid worms in the family
Serpulidae, secrete a hard tube made of calcium carbonate, adhering to stones or other shells. This tube resembles, and can be confused with, the shell of marine gastropod mollusks in the family
Vermetidae, the worm snails.
Other more atypical kinds
A few other categories of marine animals leave remains which might be considered "seashells" in the widest possible sense of the word.
Vertebrate "shells": chelonians
Sea turtles have a
carapace and
plastron of
bone and
cartilage which is developed from their
ribs. Infrequently a turtle "shell" will wash up on a beach.
Hard corals
Pieces of the hard skeleton of corals commonly wash up on beaches in areas where corals grow.
The construction of the shell-like structures of corals are aided by a symbiotic relationship with a class of algae, zooxanthellae. Typically a coral polyp will harbor particular species of algae, which will photosynthesise and thereby provide energy for the coral and aid in calcification, while living in a safe environment and using the carbon dioxide and nitrogenous waste produced by the polyp. Coral bleaching is a disruption of the balance between polyps and algae, and can lead to the breakdown and death of coral reefs.
Soft corals
The skeletons of soft corals such as
gorgonians, also known as
sea fans and
sea whips, commonly wash ashore in the
tropics after storms.
Plankton and protists
Plant-like
diatoms and animal-like
radiolarians are two forms of
plankton which form hard
silicate shells.
Foraminifera and
coccolithophore create shells known as "
tests" which are made of calcium carbonate. All these shells and tests are usually (but in the case of foraminifera not always) microscopic in size.
See also
Bailey-Matthews Shell Museum
Gastropod shell
Mollusk shell
Ocean acidification
Seashell resonance
Seashell surface, a mathematical construct
References
Books
Abbott R. Tucker & S. Peter Dance, 1982, ''Compendium of Seashells, A full color guide to more than 4,200 of the World’s Marine shells'', E.P. Dutton, Inc, New York, ISBN 0-525-93269-0
Abbott R. Tucker, 1985, ''Seashells of the World: a guide to the better-known species'', 1985, Golden Press, New York, ISBN 0-307-24410-5
Abbott, R. Tucker, 1986, ''Seashells of North America'', St. Martin's Press, New York, ISBN 1-58238-125-9
Abbott, R. Tucker, 1974, ''American Seashells'', Second edition, Van Nostrand Rheinhold, New York, ISBN 0-442-20228-8.
External links
110 Photos of various seashells
Conchologists of America
Freshwater shells
National Museum of Australia Shell necklaces made by Palawa Elder.
Category:Zoology
Category:Mollusc anatomy
Category:Molluscs
Category:Mollusc products
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