Saturday, August 8, 2009

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Palaeontological significance

Palaeontological significance

Borings in exoskeletons can provide evidence of animal behaviour. In this case, boringsponges attacked this hard clamshell after the death of the clam

Exoskeletons, as hard parts of organisms, are greatly useful in assisting preservation of organisms, whose soft parts usually rot before they can be fossilised. Mineralised exoskeletons can be preserved "as is", as shell fragments, for example. The possession of an exoskeleton also permits a couple of other routes to fossilisation. For instance, the tough layer can resist compaction, allowing a mould of the organism to be formed underneath the skeleton, which may later decay.[3] Alternatively, exceptional preservation may result in chitin being mineralised, as in the Burgess shale,[4] or transformed to the resistant polymerkeratin, which can resist decay and be recovered.

However our dependence on fossilised skeletons also significantly limits our understanding of evolution. Only the parts of organisms that were already mineralised are usually preserved, such as the shells of molluscs. It helps that exoskeletons often contain "muscle scars", marks where muscles have been attached to the exoskeleton, which may allow the reconstruction of much of an organism's internal parts from its exoskeleton alone.[3] The most significant limitation is that, although there are 30-plus phyla of living animals, two-thirds of these phyla have never been found as fossils, because most animal species are soft-bodied and decay before they can become fossilised.[5]

Mineralised skeletons first appear in the fossil record shortly before the base of the Cambrian period, 550 million years ago. The evolution of a mineralised exoskeleton is seen by some as a possible driving force of the Cambrian explosion of animal life, resulting in a diversification of predatory and defensive tactics. However, some Precambrian (Ediacaran) organisms produced tough outer shells,[3] while others, such asCloudina, had a calcified exoskeleton.[6] Some Cloudina shells even show evidence of predation, in the form of borings.[6]


Evolution

On the whole, the fossil record only contains mineralised exoskeletons, since these are by far the most durable. Since most lineages with exoskeletons are thought to have started out with a non-mineralised exoskeleton which they later mineralised, this makes it difficult to comment on the very early evolution of each lineage's exoskeleton. We do know that in a very short course of time just before the Cambrianperiod exoskeletons made of various materials — silica, calcium phosphate, calcite, aragonite, and even glued-together mineral flakes — sprang up in a range of different environments.[7]

Some Precambrian (Ediacaran) organisms produced tough but non-mineralised outer shells,[3] while others, such as Cloudina, had a calcifiedexoskeleton,[6] but mineralised skeletons did not become common until the beginning of the Cambrian period, with the rise of the "small shelly fauna". Just after the base of the Cambrian, these miniature fossils become diverse and abundant - this abruptness may be an illusion, since the chemical conditions which preserved the small shellies appeared at the the same time.[8] Most other shell forming organisms appear during the Cambrian period, with the Bryozoans being the only calclfying phylum to appear later, in the Ordovician. The sudden appearance of shells has been linked to a change in ocean chemistry which made the calcium compounds of which the shells are constructed stable enough to be precipitated into a shell. However this is unlikely to be a sufficient cause, as the main construction cost of shells is in creating the proteins andpolysaccharides required for the shell's composite structure, not in the precipitiation of the mineral components.[1] Skeletonisation also appeared at almost exactly the same time that animals started burrowing to avoid predation, and one of the earliest exoskeletons was made of glued-together mineral flakes, suggesting that skeletonisation was likewise a response to increased pressure from predators.[7]

Ocean chemistry may also control which mineral shells are constructed of. Calcium carbonate has two forms, the stable calcite, and themetastable aragonite, which is stable within a reasonable range of chemical environments but rapidly becomes unstable outsite this range. When the oceans contain a relatively high proportion of magnesium compared to calcium, aragonite is more stable, but as the magnesium concentration drops, it becomes less stable, hence harder to incorporate into an exoskeleton, as it will tend to dissolve.

With the exception of the molluscs, whose shells often comprise both forms, most lineages use just one form of the mineral. The form used appears to reflect the seawater chemistry - thus which form was more easily precipitated - at the time that the lineage first evolved a calcified skeleton, and does not change thereafter.[2] However, the relative abundance of calcite- and aragonite-using lineages does not reflect subsequent seawater chemistry - the magnesium/calcium ratio of the oceans appears to have a negligible impact on organisms' success, which is instead controlled mainly by how well they recover from mass extinctions.[9] A recently-discovered modern gastropod that lives near deep-sea hydrothermal vents illustrates the influence of both ancient and modern local chemical environments: its shell is made of aragonite, which is found in some of the earliest fossil molluscs; but it also has armor plates on the sides of its foot, and these are mineralised with the iron sulfides pyrite and greigite, which had never previously been found in any metazoan but whose ingredients are emitted in large quantities by the vents.[1]


Artificial "exoskeletons"

Humans have long used armour as an artificial exoskeleton for protection, especially in combat. Exoskeletal machines (also called powered exoskeletons) are also starting to be used for medical and industrial purposes, while powered human exoskeletons are a feature of science fiction writing, but are currently moving into prototype stage. Orthoses are a limited, medical form of exoskeleton.

An orthosis (plural orthoses) is a device which attaches to a limb, or the torso, to support the function or correct the shape of that limb or the spine. Orthotics is the field dealing with orthoses, their use, and their manufacture. An orthotist is a person who designs and fits orthoses. Aprosthesis (plural prostheses) is a device that substitutes for a missing part of a limb. If the prosthesis is a hollow shell and self-carrying, it is exoskeletal. If internal tubes are used in the device and the cover (cosmesis) to create the outside shape is made of a soft, non-carrying material, it is endoskeletal. Prosthetics is the field that deals with prostheses, use, and their manufacture. A prosthetist is a person who designs and fits prostheses.

Parenthetically, the exoskeleton has been used as an architectural model. See the lighthouse at St. Martin Island.

Perhaps the first animals to use a naturally-occurring "artificial exoskeleton" were the hermit crabs, the majority of which are obliged constantly to "wear" an empty gastropod shell, in order to protect their soft abdomens.

Molluscan seashells

Molluscan seashells

Seashells hand picked from beach drift in North Wales at Shell Island near Harlech Castle, Wales,bivalves and gastropods, March/April 1985

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 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, seemicromollusks.

(Not all molluscs are marine however: there are numerous land and freshwater molluscs, see for example snail and freshwater bivalves. And not all mollusks have an external shell: some mollusks (such as some cephalopods (squid and octopi)) have an internal shell, and some have no shell, see for example slug and nudibranch.)


Bivalves

Single valves of the bivalveSenilia senilis, plus twogastropods, washed up on the beach at Fadiouth, Senegal

Bivalves are often the most common numerous 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.

Whole and broken valves of thecockle, Fragum erugatum on the beach in Shell Beach, Western Australia

Shell Beach, Western Australia is a beach which is entirely made up of the shells of the cockle Fragum erugatum as shown here.


Gastropods

Turritella gastropod shells washed up on a beach at Playa Grande on the Pacific coast ofCosta Rica

Certain species of gastropod seashells can sometimes be commonly washed up on sandy beaches, and on beaches that are surrounded by rocky marine habitat.


Polyplacophorans

Loose valves or plates fromChiton tuberculatus from the beachdrift on Nevis, West Indies

Chiton plates or valves often wash up on beaches in areas where chitons are common. Chiton shells usually come apart 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

Cuttlebone from a Sepia sp.
Two shells of Spirula spirula

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.

Shells of 3 species of Nautilus

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

A carrier shell with attached seashells

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 inhermit 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.


An ocellated (spotted) octopus nestled in a clamshell
Amphioctopus marginatus using seashells as protection
  • 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

Caribbean land hermit crab using a seashell for protection
  • 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.


Human uses


Collecting shells as a hobby and a study

A selection of hand-picked mollusk shells found on beaches all over the world, including a fossil clam, and also an echinoderm, asand dollar

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 molluscs, the majority of 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 asconchology. 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 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 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.

There are many sources available for those who want to collect shells that they would normally not find in their vicinity.


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 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 ofIdentification 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 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.


Confusing non-marine with marine shells
A group of purchased (mostly marine) shells includes the shell of a large tropical land snail (upper right), and a shiny freshwater apple snail shell (center)

Because virtually all rivers discharge into the sea, and because heavy rain can carry land snail shells into rivers, the shells of freshwater snails and clams, and shells of land snails can sometimes wash up onto saltwater beaches, mixed with the shells of marine species. This can be confusing to shell collectors who are attempting to identify what they have found.

When seashells are purchased from shops or dealers, it is common to encounter various non-marine shells for sale as well. Sometimes the non-marine shells are mixed in with the marine shells. The non-marine items often include solid and colorful shells such certain tropical land snail shells, apple snail shells, and pearly freshwater unionid mussel shells. This also can be confusing to collectors.


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.

1742 drawing of shells of the money cowry, Cypraea moneta


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 shells are sometimes 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

A sacred chank shell on the flag of Travancore, India
Moche Shell, 200 A.D. Larco Museum Collection Lima, Peru.

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

  • In Hinduism left-handed shells of Turbinella pyrum (the sacred chank) 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.[2]


As musical instruments

Hindu priest sounding a ritual trumpet made from Turbinella pyrum
Korean military procession withCharonia trumpets

Seashells have been used as musical instruments. Most often large marine gastropods are used as trumpets by breaking or cutting a hole in the spire of the shell.

Almost any species of really large marine gastropod shell can be turned into a "blowing shell" but the most prominent species used to make these "conch" trumpets are:


In personal adornment

Use of gastropod shells, specifically cowries, in traditional dress of the Kikuyu people ofKenya, Africa.
A Pearly King and Queen

Seashells have been used as jewelry or in other forms of adornment since prehistoric times. Mother of pearlwas 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.
  • 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.
  • The shell of the large "bullmouth helmet" sea snail, scientific name Cypraecassis rufa, was historically, and still is, used to make cameos.


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 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.


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

Enormous seashell sculpture atAkkulam, Trivandum, India
Large sculpture of a scallop on the beach at Aldeburgh, England

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 shellTurbinella pyrum of India.

Maggi Hambling designed a striking 4 meter high sculpture of a scallop shell which stands on the beach atAldeburgh, in Great Britain.

Aphrodite, 1st century BC, 13 cms, 5 in

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 scallopshell.

Illustration from an 18th-century book, edited by Albertus Seba. These decorative arrangements were a popular way to display seashells at the time


Shells of other marine invertebrates


Arthropods

The moulted carapace of a lady crab found on the beach at Long Beach, Long Island, New York State

Many arthropods have sclerites, or hardened body parts, which form a stiff exoskeleton made up mostly ofchitin. 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.

Shell of horseshoe crab on a beach

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 more atypical kinds of "seashells"

A few other categories of marine animals leave remains which might be considered "seashells" in the widest possible sense of the word.

[edit]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

Dish with beachworn coral pieces, marine gastropod shells, and echinoderm tests, mostly from the Mediterranean

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 ofalgae, zooxanthellae. Typically a coral polyp will harbour particular species of algae, which willphotosynthesise and thereby provide energy for the coral and aid in calcification,[3] 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

A x-ray photograph of agorgonian

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

Marine diatoms form hard silicate shells

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.