Introduction
The Brachyura or "true crabs" are by far the most diverse group of decapod Crustacea with over 7,000 extant species. True crabs can usually be recognised by the flattened, broadened carapace and the greatly reduced abdomen folded underneath the body. These features have also independently evolved in several "crab" families in the Anomura. Brachyuran crabs have been able to colonise and diversify in an extraordinary range of habitats from deep sea hydrothermal vents below 3,600 m to alpine streams as high as 4,000 m.
Diagnosis
Carapace typically enlarged compared to reduced abdomen; antennular flagella usually short; first pereiopods developed as chelipeds; pereiopods 2–5 usually well developed, pereiopod 5 rarely strongly reduced or vestigial, pereiopods 2–5 typically non-chelate, pereiopods 4–5 sometimes subchelate or chelate; abdomen without tail fan, often flattened and reduced, folded underneath body; first and second male pleopods modified as gonopods; eggs incubated underneath abdomen on pleopods.
