Seashell

Seashell

[see-shel]
A seashell, also known as a sea 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 the exoskeleton, usually that of an animal without a backbone, an invertebrate.

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

Seashells are commonly found in beach drift, 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 found 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 exploitation can sometimes have a strong negative impact on the distribution of rarer species, and on local ecosystems.

Many other kinds of sea animals have exoskeletons or shells which may, after death, wash up on the beach and may be picked up by beachcombers; these 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, sea urchin and sand dollar tests, brachiopod shells, and the shells of marine annelid worms in the family Serpulidae, which create calcareous tubes.

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. If studying the whole molluscan animal is included, then the study is known as malacology; a person who studies mollusks is known as a malacologist.

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

Molluscan seashells

The word "seashells" is often used to mean only the shells of marine mollusks: bivalves (or clams), gastropods (or snails), tusk shells or scaphopods and chitons or polyplacophorans. These are the kind of seashells which are perhaps the most familiar, and are very often the most commonly encountered, both in the wild and for sale as decorative objects. Not all molluscs are marine: there are numerous land and freshwater molluscs, see for example snail and freshwater bivalves. Not all mollusks have an external shell: some have an internal shell, and some have no shell, see slug.

Marine species of mollusc are more numerous than land and freshwater species, and 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 molluscs 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.

Use 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.

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,and very often shells of bivalves, or smaller gastropods are used, depending on what is available on the particular substrate where the animal lives.

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 books on land and freshwater molluscs, many popular books emphasize, or focus exclusively on, the shells of marine molluscs.

Both the science of studying mollusc 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.

A large number of amateurs collect the shells of marine molluscs, 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 special 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 on a specimen is far more important than having the shell correctly identified. Some owners of shell collections hope to be able to donate the collection to a major natural history or zoology museum at some point, however, a shell with little or no collecting data is very often of no value to science.

Identification of mollusk shells

Seashells are usually identified by consulting general or regional shell-collecting field guides, and specific scientific books on different taxa of shell-bearing molluscs (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 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 Committee 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.

Significance in human culture

As currency

Seashells have been used as a medium of exchange in various places, including many Indian and Pacific Ocean islands, 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 mollusc.
  • It is of historic interest that the Dutch East Indian 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 that were considered valuable in Europe at the time.

As tools

Seashells have often been used as tools due to their variety of shapes.

  • Giant clams (Family Tridacnidae) have been used as bowls, and when big enough, even as bathtubs and baptismal fonts.
  • 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 religion and spirituality

Seashells have played a part in religion and spirituality, sometimes even as ritual objects.

  • The scallop shell is also considered the symbol of Saint James the Great.
  • In Hinduism, the left-handed Chank shell is considered sacred to the god Vishnu. One 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 were often considered symbols of female fertility, as the shape of the underside of the shell has a resemblance to a vulva.
  • 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, usually trumpets; the most prominent examples are the Triton shell (Charonia tritonis L.), used as a trumpet in Melanesian and Polynesian culture, and the Queen Conch (Strombus gigas L.), sometimes used as a trumpet in the Caribbean.

In personal adornment

Seashells have been used as jewelry since prehistory.

  • 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.
  • Shells have been formed into, or incorporated into pendants, beads, buttons, brooches, rings, and hair combs, among other uses.
  • The shell of the Bullmouth Helmet snail was and is used to make cameos.
  • Mother of pearl from many species including top snails, abalone and various bivalves, has often been used in jewelry, buttons, etc.
  • Pearly Kings and Queens traditionally wear clothing covered in patterns made up of "pearl buttons", in other words, buttons made of mother-of-pearl or nacre.

In crafts

"Sailor's Valentines" were late nineteenth century decorative keepsakes which were made in the Caribbean, and which were often purchased by sailors to give to their loved ones back home. They consisted of elaborate arrangements of 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.

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, for example, in Botticelli’s Venus, the goddess Venus (goddess) is depicted as rising from the ocean on a scallop shell.

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 another arthropod which is not a crustacean but an arachnid. 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 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.

Brachiopods

The brachiopods, or lamp shells, superficially resemble clams, but the phylum is completely unrelated to molluscs. 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 "seashells"

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.

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 harbour 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.

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

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

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