Crayfish are small freshwater crustaceans that look like miniature lobsters — a hard-shelled body, two pincered claws and a curling, fan-tipped tail. The prize is the tail meat: sweet, firm and faintly briny, with a clean lake-water freshness. They turn brilliant red when boiled and are eaten by the heaping pound, the heart of a Louisiana crawfish boil and of the Nordic late-summer crayfish party.