What niche does a crayfish occupy?

Crayfish, often referred to as crawfish or crawdad, are freshwater crustaceans resembling small lobsters, to which they are closely related. They are found in bodies of fresh water that do not freeze to the bottom, 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 (10 feet) underground.

The study of crayfish is called astacology .

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 250 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.

Madagascar has a single (endemic) crayfish species, Astacopsis madagascarensis.
Astacus astacus
Astacus astacus

Europe is home to seven species of crayfish in the genera Astacus and Austropotamobius.

Cambaroides is native to Japan and eastern mainland Asia.


Crayfish also need to moult as they grow because their hard exoskeletons do not allow much room for expansion. Baby crayfish can moult on a daily basis but as they grow older, the regularity of moults decreases to a period of weeks or even months. The first few days after a moult, a crayfish's skin is very soft and it is very vulnerable to attacks from other animals and crayfish.

The early signs of moulting include lack of appetite and a slow down in activity. During this period the crayfish ingests calcium into an internal organ, not into the exoskeleton.

When the crayfish is ready to moult, it will try to find a hiding spot. Then it will move onto its back and begin fanning its pincers, legs and swimmerets (under the tail) in order to get as much oxygen as possible. The carapace will begin to crack behind the head; the new appendages then pierce the old shell; and then after about five minutes, a sudden, violent movement will detach the old shell from the crayfish.

The freshly moulted crayfish will invariably be larger as part of the growing process, but is vulnerable on several fronts. Firstly, the shell is very soft and vulnerable to predators, including other crayfish and fish. The crayfish needs to eat the old shell to replace the lost calcium and strengthen the weakened carapace.