Crayfish, also called crawfish or crawdad, are closely related to the lobster. More than half of the more than 500 species occur in North America, particularly Kentucky (Mammoth Cave) and Louisiana in the Mississippi basin. Crayfish also live in Europe, New Zealand, East Asia and throughout the world, including the Tristan da Cunha Islands. Nearly all live in freshwater, although a few survive in salt water. Crayfish are characterised by a joined head and thorax, or midsection, and a segmented body, which is sandy yellow, green, or dark brown in colour. The head has a sharp snout, and the eyes are on movable stalks. Crayfish are usually about 7.5 cm (3 inches) long.
The crayfish is typical of most shrimplike crustaceans and is characterised by a joined head and thorax, or midsection, and a segmented body, which is sandy yellow, green, white, pink or dark brown in colour.
Crayfish are usually about 7.5 cm (3 inches) long. Among the smallest is the 2.5-centimetre-long Cambarellus diminutus of the south-eastern United States. Among the largest is Astacopsis gouldi of Tasmania; its length may reach 40 cm and its weight about 3.5 kg (8 pounds).
The head has two pairs of sensory antennae and a pair of eyes on movable stalks. The appendages, or pereiopods, of the thorax include four pairs of walking legs which, as well as walking, are to probe cracks and crevices between rocks looking for food. Crayfish also own one pair of clawbearing chelipeds, which it extends in front of its body while moving. These strong pinchers are specialised for cutting, capturing food, attack, and defence. A pinch can hurt! The crayfish also has several pairs of specialised food handling "legs," bailers to cycle water over the gills, and five pairs of swimmerets which are under the abdomen. All of these "legs" can be regenerated if broken off.
Crayfish have a hard outside skeleton. This jointed exoskeleton provides protection and allows movement, but limits growth. As a result, the crayfish regularly gets too big for its skeleton, sheds it, and grows a new larger one. This is called molting. and occurs six to ten times during the first year of rapid growth, but less often during the second year. For a few days following each molt, crayfish have soft exoskeletons and are more vulnerable to predators.