Crayfish, crawfish, or crawdads – members of the superfamilies Astacoidea and Parastacoidea – are freshwater crustaceans resembling small lobster, to which they are related. They breathe through feather-like 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. Crayfish feed on living and dead animals and plants.
In Australia, New Zealand, 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.The name "crayfish" comes from the Old French. Some kinds of crayfish are known locally as lobster, 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. Some crayfish suffer from a disease called crayfish plague. This is caused by the water mould. Species of the genus Astacus are particularly susceptible to infection, allowing the more resistant signal crayfish to invade parts of Europe. Crayfish plague is not indigenous to Europe, rather it was brought there when North American species of crayfish were introduced.