Animal shell
From Wikipedia, the free encyclopedia
A shell is the hard, rigid outer covering, or integument, of certain animals. More specific scientific names include exoskeleton, carapace, and peltidium. A shell may be made of nacre (a combination of calcium and protein), chitin, bone and cartilage, or silica.
Contents |
[edit] Molluscs of the sea, traditional "sea shells"
While many sea animals produce exoskeletons, usually only those of molluscs (also spelt "mollusk") are normally considered to be "sea shells". The majority of shell-forming molluscs belong to the classes Gastropoda (univalves, or snails) or Bivalvia. Three other shell-bearing classes are Scaphopoda (tusk shells), Polyplacophora (segmented chitons) and Monoplacophora (single-shelled chiton-like animals, also called Tryblidia). Some species of Cephalopoda also build shells, including the primitive Nautilus order which produces the famous "chambered Nautilus" shell; although some taxa of cephalopods such as octopuses and squid only form small internal shells.
Malacology, the scientific study of molluscs as living organisms, has a branch devoted to shells, called conchology - although it should be noted that these terms are sometimes used interchangeably, even by scientists (this is more common in Europe).
The shell will grow over time as the animal inside adds its building material to the leading edge near the opening. This causes the shell to become longer and wider to better accommodate the growing animal inside. A mollusc shell is formed, repaired and maintained by a part of the mollusc called the mantle. Injuries to or abnormal conditions of the mantle are often reflected in the shell they form and tend. When the animal encounters harsh conditions which limit its food supply or otherwise cause it to become dormant for a while, the mantle often ceases to produce the shell substance. When conditions improve again and the mantle resumes its task, a "growth line" which extends the entire length of the shell is produced, and the pattern and even the colors on the shell after these dormant periods are sometimes quite different from previous colors and patterns. Interestingly, each species of mollusc animals will build the external shell in specific shape, pattern, ornamentation, and color.
Shells are composite materials of calcium carbonate, found either as calcite or aragonite and organic macromolecules, mainly proteins and polysaccharides. Shells can have enumerous ultrastructural motiffs, the most common being crossed-lamellar (aragonite), prismatic (aragonite or calcite), homogeneous (aragonite), foliated (aragonite) and nacre (aragonite). Although not the most common, the nacre is the most studied layer. Shells of the class Polyplacophora are made of aragonite
Nacre is secreted by the epithelial cells (formed by the germ layer ectoderm) of the mantle tissue of certain species of mollusk. Mollusk blood is rich in dissolved calcium. In these mollusks the calcium is concentrated out from the blood where it can crystallize as calcium carbonate. Nacre is continually deposited onto the inner surface of the animal's shell (the iridescent nacreous layer also known as mother of pearl), both as a means to smooth the shell itself and as a defense against parasitic organisms and damaging detritus.
When a mollusc is invaded by a parasite or is irritated by a foreign object that the animal cannot eject, a process known as encystation entombs the offending entity in successive, concentric layers of nacre. This process eventually forms what we call pearls and continues for as long as the mollusk lives. Almost any species of bivalve or gastropod is capable of producing pearls, but only a few, such as the famous pearl oysters, are highly prized.
Mollusc shells (especially those formed by marine species) are very durable and outlast the otherwise soft-bodied animals that produce them by a very long time (sometimes thousands of years). They fossilise easily, and fossil mollusc shells date all the way back to the Cambrian period. Large amounts of shells may form sediment and become compressed into limestone.
Shells of marine molluscs (some of which wash up on beaches or live in the intertidal or sub-tidal zones and are therefore easily found without specialized equipment) are called "seashells", and are collected by a large number of enthusiasts (who collect "specimen shells" - shells which come with information about them such as how, when, where and in what habitat they were collected), especially in the tropical and sub-tropical areas of our planet, where there are more species of colourful, large and intertidal seashells than in regions further north.
[edit] Other molluscs
There are of course fresh-water shell-bearing molluscs, and the class Gastropoda ("snails") contains many species which live on land, without any need of bodies of water: these are appropriately called "land snails" and are also highly prized by many collectors. It is a little known fact that there are actually more species of land snails than there are marine: they cannot disperse very quickly, so populations are frequently isolated from each other, resulting in situations where adjacent islands and even valleys separated by hills or mountains, contain closely-related but clearly separate species of land snails. There are all cinds of shells. Shells come in many deffrent shaps and sizes.
[edit] Shells in other animals
While not "shells" in the strict sense, a large variety of other animal taxa form exoskeletons of calcium carbonate, chiton or silica which are used for protection, locomotion, defence, structure or feeding.
[edit] Other sea creatures
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[1], while living in a safe environment and using the carbon dioxide and nitrogenous waste produced by the polyp. Due to the strain the algae can put on the polyp, stress on the coral (such as warmer sea temperatures due to global warming) often triggers ejection of the algae, known on a large scale as coral bleaching as it is the algae that gives coral colour. This allows the polyp to live longer during stressful periods, and to regain the algae at a later time; however if the conditions persist the polyps and corals die without the photosynthetic algae[2].
Some echinoderms (starfish, sea urchins, sand dollars) and some polychaetes (annelid worms) also have hard exoskeletons. Another group of sea creatures with a shell are the now-extinct ostracoderms ("shell-skins"), a type of armoured marine fish which flourished in North America and Europe during the Ordovician, Silurian and Devonian geological periods. Another animal phylum that produces shells are the brachiopods.
[edit] 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, shrimp and lobsters, for instance), the plates of the exoskeleton may be fused to form a more or less rigid carapace.
- The rigid part of an insect's exoskeleton is called the procuticle; when outgrown, this construct must be shed during moulting.
[edit] Chelonians
Turtles, tortoises and terrapins also form a hard carapace and plastron of bone and cartilage which is developed from their ribs.
[edit] Planktons and protists
Plant-like diatoms and animal-like radiolarians are two forms of plankton which form hard silicate shells. Foraminifera and coccolithophores create shells called "tests" of calcium carbonate.
[edit] See also
[edit] References
- ^ Madl, P. and Yip, M. (2000). Field Excursion to Milne Bay Province - Papua New Guinea. Retrieved on 2006-03-31.
- ^ W. W. Toller, R. Rowan and N. Knowlton (2001). "Repopulation of Zooxanthellae in the Caribbean Corals Montastraea annularis and M. faveolata following Experimental and Disease-Associated Bleaching". The Biological Bulletin 201: 360-373.
