Temporal range: Mesozoic–recent
|Northern kōura, Paranephrops planifrons (Parastacidae)|
Crayfish, also known as crawfish, crawdads, freshwater lobsters, mountain lobsters, mudbugs, or yabbies are freshwater crustaceans resembling small lobsters (to which they are related). Taxonomically, they are members of the superfamilies Astacoidea and Parastacoidea. They breathe through feather-like gills. Some species are found in brooks and streams where fresh water is running, while others thrive in swamps, ditches, and paddy fields. Most crayfish cannot tolerate polluted water, although some species such as Procambarus clarkii are hardier. Crayfish feed on animals and plants, either living or decomposing, and detritus. 
The name "crayfish" comes from the Old French word escrevisse ( Modern French écrevisse).   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 southwestern regions, and "crawfish" further south, although considerable overlaps exist. 
The study of crayfish is called astacology. 
In Australia (on the eastern seaboard), New Zealand and South Africa, the term crayfish or cray generally refers to a saltwater spiny lobster, of the genus Jasus that is indigenous to much of southern Oceania,  while the freshwater species are usually called yabby or kōura, from the indigenous Australian and Māori names for the animal respectively, or by other names specific to each species. Exceptions include western rock lobster (of the Palinuridae family) found on the west coast of Australia; the Tasmanian giant freshwater crayfish (from the Parastacidae family) found only in Tasmania; and the Murray crayfish found along Australia's Murray River.
In Singapore, the term crayfish typically refers to Thenus orientalis, a seawater crustacean from the slipper lobster family.    True crayfish are not native to Singapore, but are commonly found as pets, or as an invasive species ( Cherax quadricarinatus) in the many water catchment areas, and are alternatively known as freshwater lobsters.  
The body of a decapod crustacean, such as a crab, lobster, or prawn (shrimp), is made up of twenty 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. On average, crayfish grow to 17.5 centimetres (6.9 in) in length, but some grow larger. Walking legs have a small claw at the end.
There are three families of crayfish, two in the Northern Hemisphere and one in the Southern Hemisphere. The Southern Hemisphere ( Gondwana-distributed) family Parastacidae, with 14 extant genera and two extinct genera, live(d) in South America, Madagascar and Australasia. They are distinguished by the absence of the first pair of pleopods.  Of the other two families, the three genera of the Astacidae live in western Eurasia and western North America, while the 15 genera of the family Cambaridae live in eastern Asia and eastern North America.
The greatest diversity of crayfish species is found in southeastern 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. Many crayfish are also found in lowland areas where the water is abundant in calcium, and oxygen rises from underground springs. 
In 1983, Louisiana designated the crayfish, or crawfish as they are commonly referred, as their official state crustacean.  Louisiana produces 100 million pounds of crawfish per year with the red swamp and white river crawfish being the main species harvested.  Crawfish are a part of Cajun culture dating back hundreds of years.  A variety of cottage industries have developed as a result of commercialized crawfish iconology. Their products include crawfish attached to wooden plaques, T-shirts with crawfish logos, and crawfish pendants, earrings and necklaces made of gold or silver. 
Australia has over 100 species in a dozen genera. Australia is home to the world's three largest freshwater crayfish – the Tasmanian giant freshwater crayfish Astacopsis gouldi, which can achieve a mass of over 5 kilograms (11 lb) and is found in rivers of northern Tasmania, , the Murray crayfish Euastacus armatus, which can reach 2.5 kilograms (5.5 lb), although there have been reports of animals up to 3 kilograms (6.6 lb) and is found in much of the southern Murray-Darling basin.  and marron from Western Australia (now believed to be two species, Cherax tenuimanus and C. cainii) which may reach a weight of 2.2 kilograms (4.9 lb). Many of the better-known Australian crayfish are of the genus Cherax, and include the , common yabby (Cherax destructor), western yabby (Cherax preissii) and red-claw crayfish (Cherax quadricarinatus).  The marron species C. tenuimanus is critically endangered, while other large Australasian crayfish are threatened or endangered.
Fossil records of crayfish older than 30 million years are rare, but fossilised burrows have been found from strata as old as the late Palaeozoic or early Mesozoic.   The oldest records of the Parastacidae are in Australia, and are 115 million years old. 
Some crayfish suffer from a disease called crayfish plague, caused by the North American water mould Aphanomyces astaci which was transmitted to Europe when North American species of crayfish were introduced there.  Species of the genus Astacus are particularly susceptible to infection, allowing the plague-coevolved signal crayfish to invade parts of Europe.
Crayfish are eaten worldwide. Like other edible crustaceans, only a small portion of the body of a crayfish is eaten. In most prepared dishes, such as soups, bisques and étouffées, only the tail portion is served. At crawfish boils or other meals where the entire body of the crayfish is presented, other portions, such as the claw meat, may be eaten. Like all crustaceans, crayfish are not kosher because they are aquatic animals that do not have both fins and scales.  They are therefore not eaten by observant Jews. 
As of 2005, Louisiana supplies 95% of the crayfish harvested in the US.  In 1987, Louisiana produced 90% of the crayfish harvested in the world, 70% of which were consumed locally.  In 2007, the Louisiana crawfish harvest was about 54,800 tons, almost all of it from aquaculture.  About 70%–80% of crayfish produced in Louisiana are Procambarus clarkii (red swamp crawfish), with the remaining 20%–30% being Procambarus zonangulus (white river crawfish). 
Crayfish are preyed upon by a variety of ray-finned fishes,  and are commonly used as bait, either live or with only the tail meat. They are a popular bait for catching catfish,  largemouth bass, smallmouth bass, striped bass,  perch, pike  and muskie.  When using live crayfish as bait, anglers prefer to hook them between the eyes, piercing through their hard, pointed beak which causes them no harm; therefore, they remain more active. 
When using crayfish as bait, it is important to fish in the same environment where they were caught. An Illinois State University report that focused on studies conducted on the Fox River and Des Plaines River watershed stated that rusty crayfish, initially caught as bait in a different environment, were dumped into the water and "outcompeted the native clearwater crayfish."  Other studies confirmed that transporting crayfish to different environments have led to various ecological problems, including the elimination of native species.  Transporting crayfishes as live bait has also contributed to the spread of zebra mussels in various waterways throughout Europe and North America, as they are known to attach themselves to exoskeleton of crayfishes.   
Crayfish are kept as pets in freshwater aquariums, typically with bluegill or bass, rather than goldfish or tropical or subtropical fish. They prefer foods like shrimp pellets or various vegetables, but will also eat tropical fish food, regular fish food, algae wafers, and small fish that can be captured with their claws. A report by the National Park Service  as well as video and anecdotal reports by aquarium owners  indicate that crayfish will eat their molted exoskeleton "to recover the calcium and phosphates contained in it."  As omnivores, crayfish will eat almost anything; therefore, they may explore the edibility of aquarium plants in a fish tank. However, most species of dwarf crayfish, such as Cambarellus patzcuarensis, will not destructively dig or eat live aquarium plants.  They are also relatively non-aggressive and can be kept safely with dwarf shrimp.[ citation needed]
In some nations, such as the United Kingdom, 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 released into a different catchment. There is a potential for ecological damage when crayfish are introduced into non-native bodies of water: e.g., crayfish plague in Europe, or the introduction of the common yabby (Cherax destructor) into drainages east of the Great Dividing Range in Australia. 
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