Crayfish evolved from marine ancestors dating back some 280 million years. There are more than 300 species of crayfish worldwide, which are classified into three families: the Astacidae, the Cambridae (found only in the Northern Hemisphere), and the Parastacidae (indigenous
A crayfish at the Fish Point State Wildlife Area, Michigan. Photograph by Robert J. Huffman. Field Mark Publications. Reproduced by permission.
to the Southern Hemisphere). A few species have adapted to tropical habitats, but most live in temperate regions. None occur in Africa or the Indian subcontinent, although one species is found in Madagascar. Crayfish live in water, hiding beneath rocks, logs, sand, mud, and vegetation. Some species dig burrows, constructing little chimneys from moist soil excavated from their tunnel and carried to the surface. Some terrestrial species spend their whole life below ground in burrows, emerging only to find a mate. Other species live both in their tunnels as well as venturing into open water. Many species live mostly in open water, retreating to their burrows during pregnancy and for protection from predators and cold weather.