Crabs are decapodcrustaceans living in any of the world's oceans, in fresh water, or on land, generally covered with a thick exoskeleton and armed with a single pair of claws. True crabs belong to the infraorder Brachyura, which typically have a very short projecting "tail" (Greek: βραχύς / brachys = short, οὐρά / οura = tail), or where the reduced abdomen is entirely hidden under the thorax. Many other animals with similar names – such as hermit crabs, king crabs, porcelain crabs, horseshoe crabs and crab lice – are not true crabs.
Ocypode quadrata is a species of ghost crab, once described as an "occult, secretive alien from the ancient depths of the sea". It is a common species along the Atlantic coast of the United States, where it is the only species of ghost crab; its range of distribution extends from its northernmost reach on Rhode Island's beaches south along the coasts of the tropical Western Atlantic Ocean to the beach of Barra do Chui, in Rio Grande do Sul in Southern Brazil.