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Crayfish, often referred to as crawfish, or crawdads, are freshwater crustaceans resembling small lobsters, to which they are closely related. They breathe through gills and are found in bodies of water that do not freeze to the bottom; they are also mostly found in brooks and streams where there is fresh water running, and which have shelter against predators. Most crayfish cannot tolerate polluted water, although some species such as the invasive Procambarus clarkii are more hardy. Some crayfish have been found living as much as 3 m (10feet) underground.
In Australia and New Zealand, the name crayfish (or cray) generally refers to a saltwater spiny lobster, of the type Jasus that is indigenous to much of southern Oceania, whilst the freshwater species are usually considered a yabby, or a koura, from the Aboriginal, and Māori, names for the animal.
The study of crayfish is called astacology .
The name "crayfish" comes from the Old French word escrevisse (Modern French écrevisse) from Old Frankish *krebitja (cf. crab), from the same root as crawl. The word has been modified to "crayfish" by association with "fish" (folk etymology). The largely American variant "crawfish" is similarly derived.
Some kinds of crayfish are known locally as lobsters, crawdads , mudbugs  and yabbies. In the Eastern United States, "crayfish" is more common in the north, while "crawdad" is heard more in central and western regions, and "crawfish" further south, although there are considerable overlaps .
- Main article: Decapod anatomy
The body of a decapod crustacean, such as a crab, lobster, or prawn, is made up of nineteen body segments grouped into two main body parts, the cephalothorax and the abdomen. Each segment may possess one pair of appendages, although in various groups these may be reduced or missing. A crayfish is usually 7.5 centimeters long.
Geographical distribution and classificationEdit
There are three families of crayfish, two in the northern hemisphere and one in the southern hemisphere. The southern-hemisphere (Gondwana-distributed) family Parastacidae lives in South America, Madagascar and Australasia, and is distinguished by the lack of the first pair of pleopods . Of the other two families, members of the Astacidae live in western Eurasia and western North America and members of the family Cambaridae live in eastern Asia and eastern North America.
The greatest diversity of crayfish species is found in south-eastern North America, with over 330 species in nine genera, all in the family Cambaridae. A further genus of astacid crayfish is found in the Pacific Northwest and the headwaters of some rivers east of the Continental Divide.
Australasia is another centre of crayfish diversity, with over 100 species in a dozen genera. Many of the better-known Australian crayfish are of the genus Cherax, and include the marron (Cherax tenuimanus), red-claw crayfish (Cherax quadricarinatus), yabby (Cherax destructor) and western yabby (Cherax preissii). The world's largest crayfish, Astacopsis gouldi, which can achieve a mass in excess of 3 kilograms, is found in the rivers of northern Tasmania.
Crayfish are kept in freshwater aquariums. They prefer foods like shrimp pellets or various vegetables but will also eat tropical fish food, algae wafers, and even small fish that can be captured by their claws, such as goldfish or minnows. Their disposition towards eating almost anything will also cause them to consume most aquarium plants in a fish tank; however, crayfish are fairly shy and may attempt to hide under leaves or rocks. When keeping a crayfish as a pet, one must provide a hiding space. At night, some fish become less energetic and settle to the bottom. The crayfish might see this as a chance for an easy meal, or a threat, and injure or kill the fish with its claws. Crayfish are effective scavengers and will consume fish carcasses. They sometimes will consume an exoskeleton after it is moulted. Since crayfish are accustomed to being around ponds or rivers they will have a tendency to shift gravel around on the bottom of your tank, creating mounds or trenches to emulate a burrow. Crayfish are great escape artists and will try to climb out of the tank, so any holes in the hood should be covered.
In some nations, such as England, United States, Australia, and New Zealand, imported alien crayfish are a danger to local rivers. The three species commonly imported to Europe from the Americas are Orconectes limosus, Pacifastacus leniusculus and Procambarus clarkii.  Crayfish may spread into different bodies of water because specimens captured for pets in one river are often flung back into a different one. There is a potential for ecological damage when crayfish are introduced into nonnative bodies of water. Crayfish kept as pets should never be released to the wild due to this potential hazard to the ecosystem.
- ↑ International Association of Astacology.
- ↑ 2.0 2.1 Pableaux Johnson. Mudbug Madness : Crawfish. Bayou Dog. URL accessed on 2006-08-28.
- ↑ Bert Vaux & Scott A. Golder. Dialect survey. Harvard University. URL accessed on 2006-09-30.
- ↑ Horton H. Hobbs, Jr. (1974). Synopsis of the families and genera of crayfishes (Crustacean: Decapoda). Smithsonian Contributions to Zoology 164: 1–32.
- ↑ Lee, James R. TED Case Studies Crayfish Plague #478 European Crayfish Dispute. URL accessed on 2008-01-20.
- Gilbertson, Lance; Zoology Lab Manual; McGraw Hill Companies, New York; ISBN 0-07-237716-X (fourth edition, 1999)
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