Crawdad

Crayfish, or crawdads, are crustaceans that live in freshwater environments throughout the world, except for India and Antarctica. These animals have five pairs of legs, or 10 legs total—hence the Latin name for the crayfish order, known as Decapoda (“10-footed”). The front two legs are modified into large claws, called chelae, used for defending themselves and snagging food.

There are nearly 600 known species of these creatures, with new varieties found every year. The southeastern United States and, to a lesser extent, Australia, are considered crayfish diversity hot spots.

They eat just about anything they can get those chelae on, including insects, algae, fish, invertebrates, carrion, and plant detritus. Though they don’t exactly sing, these little decapods do produce sounds both in and out of water. 

A thin appendage that draws water and air through the gills makes a noise called a “pulse train” that sounds a bit like Morse code. The noises made are likely used for communicating with other crawdads, and to alert others to the presence of predators.

A wide variety 

They come in a range of shapes and sizes. Some are flashy, such as the New Guinea’s thunderbolt crayfish, with dazzling pink, purple, and blue hues. Then there’s the Murray spiny freshwater crayfish of Australia, which sports punk-rock spikes. Others are simpler, such as the beige-and-brown calico crayfish of the midwestern and eastern U.S.

All crayfish have blue blood due to the copper-based pigment hemocyanin. (Related: Animal blood comes in a rainbow of colors. Here’s why.)

Crayfish also all wear the same costume, so to speak—their exoskeletons are made of a tough chemical called chitin. To grow bigger, they must shed their skins, or molt, and form a new exoskeleton.  

Crayfish have a long rostrum, a pointed structure in the front of their heads that looks like a nose but is thought to offer protection for their compound eyes. Under the main part of their tail are small appendages called swimmerets that help them move and assist with carrying eggs. Their tail fans can curl under the body, which help to protect eggs. And escape. A “tail-flip” is an instinctive reaction that propels the crayfish backward, away from a potential predator or disturbance.

Crayfish mostly live in lakes, rivers and streams, though some can spend a great deal of time on land, such as the upland burrowing crayfish. Some of these so-called "terrestrial" crayfish dig water-filled tunnels that they inhabit in cooler months. They use their mouth and claws to excavate, carrying mud pies to the surface, piling it up beside the burrow which allows them to stay close to safety if a predator comes along.

These mounds of mud pile up beside the burrow, creating towers, known as chimneys, that can be up to eight inches high and go down three feet deep. They are thought to help keep air flowing through the tunnels.

Reproduction 

Crayfish are solitary animals and mainly interact during mating season, which varies across species, like many other breeding behaviors that can be quite elaborate.

Female signal crayfish, for example, seem to do a bait and switch. They chemically signal their interest by spraying urine but then go into fight mode, aggressively attacking males who respond.

They’re not being fickle. Courting a fight and a mate at the same time is likely the female’s way of figuring out the strongest males—and choosing them as partners. (Related: Crayfish females lure males with urine, then play hard to get.)

Rusty crayfish males turn aggressive during mating season, fighting and even attempting to break up copulating pairs.

When pairs do get together, the male grasps the female and turns her so their undersides are together, then uses his swimmerets to pass a spermatophore to either her external underside or into a sperm storage chamber. 

The female releases hundreds of eggs and when they are fertilized they will stick to her tail via an adhesive called glair. After a month or more they hatch, forgoing a larval stage and looking, instead, like tiny adults. These stay attached to their mother after hatching for three molts until they become free-swimming crayfish.

Conservation

Crayfish are a keystone species, meaning they are integral to the overall health of their environment. They are a food source for numerous animals in their ecosystems, including raccoons, foxes, snakes, turtles, fish and waterbirds. Humans love them, too. In Louisiana, people consume between 120 to 150 million pounds of them a year.

They have more than their fair share of nicknames, too. In the northeastern U.S. you’re more likely to hear “crayfish,” while “crawfish” is the moniker in the South. Elsewhere, from West Virginia to Wyoming and along the West Coast, "crawdad" is popular. Some also call them “mudbugs.” In Australia the species Cherax destructor, the common freshwater crayfish, is a known as a “yabby”—though other Cherax species are considered “yabbies” too.

Despite their importance, a 2015 study using International Union for Conservation of Species criteria found 32 percent of all crayfish species are threatened with extinction. Four are already extinct, and 21 percent are data deficient, meaning scientists don’t know enough about their populations or threats to assess them.

One-fifth of North American crayfish species are threatened with extinction, and only two percent of species occur in areas that are protected, according to the U.S. Forest Service. 

The reasons for their endangerment vary by geography. In the U.S., habitat loss, urban development, damming, and pollution affect crayfish. In Australia, 65 percent of species are under major threat from climate change, compared with only five percent in the U.S. Other threats to Australian species include overharvesting, competition from invasive species, and agricultural expansion.

Crayfish often serve as indicators of ecological quality, and can be affected by the presence of pollutants such as antidepressants.