The name "crayfish" comes from the Old French word escrevisse (Modern French écrevisse).The word has been modified to "crayfish" by association with "fish" (folk etymology). The largely American variant "crawfish" is similarly derived.
Some kinds of crayfish are known locally as lobsters, crawdads,mudbugs, and yabbies. In the Eastern United States, "crayfish" is more common in the north, while "crawdad" is heard more in central and western regions, and "crawfish" further south, although there are considerable overlaps.
Crayfish members of the superfamilies Astacoidea and Parastacoidea are freshwater crustaceans resembling small lobsters, to which they are related. They breathe through feather-like gills and are found in bodies of water that do not freeze to the bottom; they are also mostly found in brooks and streams where there is fresh water running, and which have shelter against predators. Most crayfish cannot tolerate polluted water, although some species such as the invasive Procambarus clarkii are more hardy. Crayfish feed on living and dead animals and plants.
The body of a decapod crustacean, such as a crab, lobster, or prawn, is made up of twenty body segments grouped into two main body parts, the cephalothorax and the abdomen. Each segment may possess one pair of appendages, although in various groups these may be reduced or missing. On average, crayfish grow to 17.5 centimetres (6.9 in) in length, but some grow larger.
Crayfish are eaten all over the world. Like other edible crustaceans, only a small portion of the body of a crayfish is edible. In most prepared dishes, such as soups, bisques and étouffées, only the tail portion is served. At crawfish boils or other meals where the entire body of the crayfish is presented, other portions, such as the claw meat, may be eaten.
Claws of larger boiled specimens are often pulled apart to access the meat inside. Another favourite is to suck the head of the crayfish, as seasoning and flavour can collect in the fat of the boiled interior. A popular double entendre laden phrase heard around crawfish season in Louisiana derives from this practice: "suck the head; pinch the tail".
A common myth is that a crawfish with a straight tail died before it was boiled and is not safe to eat. In reality, crawfish that died before boiling can have curled tails as well as straight, as can those that were alive, and may very well be fine to eat. Boiled crawfish which died before boiling are safe to eat if they were kept chilled before boiling and were not dead for a long time. (This does not mean that a sack of crawfish that are all dead should be boiled.) A much better test than the straight tail as to the edibility of any crawfish is the tail meat itself; if it is mushy, it is usually an indication that it should be avoided.
Like all crustaceans, crayfish are not kosher because they are aquatic animals that do not have both fins and scales.They are therefore not eaten by observant Jews.