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 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.
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 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.
The head has a sharp snout, and the eyes are on movable stalks. 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.
Crayfish are omnivorous, eating just about anything they can find or catch, dead or alive. Large food is held and torn to pieces in the large pincers and conveyed to the mouth by the smaller specialized legs near the head. That's what crayfish mostly do: loaf all day and look for food all night
Crayfish, common in streams and lakes, often conceal themselves under rocks or logs. They are most active at night, when they feed largely on snails, algae, insect larvae, worms, and tadpoles; some eat vegetation (various water plants). A dead fish worms, corn, and salmon eggs are also favourites of the crayfish. Studies show that adults (one year old) become most active at dusk and continue heavy feeding activity until daybreak. Young crayfish are more likely to be the ones out during bright sunny days, while the older crayfish are more active on cloudy days and during the night. General movement is always a slow walk, but if startled, crayfish use rapid flips of their tail to swim backwards and escape danger.
Most crayfish live short lives, usually less than two years. Therefore, rapid, high-volume reproduction is important for the continuation of the species. Many crayfish become sexually mature and mate in the October or November after they're born, but fertilisation and egg laying usually occur the following spring. The fertilised eggs are attached to the female' swimmerets on the underside of her jointed abdomen. There the 10 to 800 eggs change from dark to translucent as they develop. The egg-carrying female is said to be ‘in berry,’ because the egg mass looks something like a berry. Females are often seen "in berry" during May or June. The eggs hatch in 2 to 20 weeks, depending on water temperature. The newly-hatched crayfish stay attached to their mother until shortly after their second molt.
The natural predators of the Crayfish include alligators, burbots (a type of cod), chicken turtle, painted turtle, desman a type of otter, grackle a type of a bird.
In the open circulatory system blood flows from the heart through the arteries and returns into open sinuses. The digestive system has a stomach for grinding food and a gland for chemical processing. The antennal gland is the main excretory organ.