Invertebrates
Crayfish
Astacoidea and Parastacoidea · also called crawfish, crawdad, mudbug, yabby
Turn over a rock in almost any clean stream outside Africa and there is a fair chance a small armoured animal will back away fast with its claws raised. Crayfish are the freshwater world’s version of the lobster: bottom-dwelling crustaceans that scavenge, burrow, and, in a few invasive cases, take over entire river systems.
Overview
Crayfish are freshwater decapod crustaceans, close relatives of the clawed lobsters, with around 640 species. They have a shielded head and thorax carrying the legs and a pair of large pincers, and a muscular segmented tail. Most are 5 to 15 cm long. They live in streams, lakes, swamps, ditches, and burrows across most of the world, with the greatest variety in the south-eastern United States and in Australia and New Zealand, and they are naturally absent from most of Africa.
Scavenger, burrower, escape artist
Crayfish eat almost anything organic and are important recyclers in fresh water. Many dig burrows into banks and floodplains, sometimes topped with a chimney of mud pellets, shaping the habitat for other animals. Startled, a crayfish flips its tail fan to shoot backward out of danger. A breeding female carries her eggs, and then her hatchlings, glued beneath her tail, a state called being in berry.
The plague-carriers
A few North American species — the signal crayfish and red swamp crayfish — have been spread worldwide for food and bait. They out-compete native species and erode banks, and they carry crayfish plague, a water mould that is harmless to them but has wiped out native crayfish across much of Europe.
Status
As a group, crayfish are Not Evaluated. A few species are among the most abundant and invasive freshwater animals on Earth, while many narrow-range species are threatened — roughly a third of North American crayfish are considered imperilled.
Crayfish: common questions
What is the difference between crayfish, crawfish, and crawdad?
There is no biological difference at all; they are three names for the same group of animals, and which one a person uses mostly depends on where they grew up. Crayfish is the standard term in scientific writing and in much of the northern and western United States, the United Kingdom, and elsewhere in the English-speaking world. Crawfish is usual across the southern United States, especially in Louisiana, where the animal is a major food. Crawdad is common in the central and midwestern states. Mudbug is an informal southern nickname. In Australia the equivalent animals are called yabbies, marron, or crays, and in New Zealand, koura.
Is a crayfish the same as a lobster?
They are close relatives but not the same. Both belong to the group of clawed decapod crustaceans, both have the same basic layout of a shielded head and thorax, walking legs, large front pincers, and a muscular tail, and a big crayfish and a small lobster can look very similar. The key differences are that crayfish live in fresh water while true lobsters are marine, crayfish are generally much smaller, most only 5 to 15 cm long, and crayfish hatch as miniature adults that cling to the mother, whereas marine lobsters release tiny larvae that drift as plankton before settling. The spiny lobsters sold as rock lobster or langouste are a separate group again, with no large front claws.
How do crayfish reproduce and care for their young?
After mating, the female extrudes her eggs and cements them to the small paddle-like swimmerets on the underside of her abdomen, where she fans them with fresh water and keeps them clean for several weeks or months. When the eggs hatch, the young do not swim away; they remain hooked to the mother's swimmerets through their first one or two moults, so she carries a squirming cluster of miniature crayfish beneath her tail. Only once they are a few millimetres long and able to feed and hide on their own do they let go and disperse. This direct development, without a planktonic larva, is one reason crayfish are confined to fresh water and spread slowly on their own.
Why are introduced crayfish such a problem?
A handful of tough, fast-breeding species, above all the North American signal crayfish and red swamp crayfish, have been carried around the world for food, for the aquarium and pond trade, and as fishing bait, and where they escape they cause serious damage. They out-compete and eat native crayfish, fish eggs, amphibian larvae, and water plants, and their burrowing erodes banks and muddies the water. Worst of all, the North American species carry a microscopic water mould called crayfish plague which does not harm them but is almost always fatal to European and other native crayfish, and it has driven species such as the European noble crayfish out of most of their former range.