Brachyura is an infraorder of decapod crustaceans comprising the true crabs. They typically have a very short projecting tail-like abdomen, usually hidden entirely under the thorax. Their exoskeleton is often thickened and hard. They generally have five pairs of legs, and the frontmost pair end in chelae, which function as pincers. They are present in all the world's oceans, in freshwater, and on land, often hiding themselves in small crevices or burrowing into sediment. True crabs are omnivores, feeding on a variety of food, including a significant proportion of algae, as well as detritus and other invertebrates. True crabs are widely consumed by humans as food, with over 1.5 million tonnes caught annually.