In the narrowest sense any of a clade of Terragen decapod crustaceans in the infraorder Brachyura, characterized by a short broad flattened cephalothorax and a reduced abdomen that is typically folded underneath it. Eight of the limbs are walking or swimming legs, and as in other decapods the front pair is specialized as a large set of chelae, or claws, sometimes of equal size or sometimes with one larger than the other. Crabs have a pair of stalked eyes. They are a numerous, widespread and successful group; though they are primarily marine, terrestrial and fresh water species are also known, and prior to the Great Dying there were over 6800 species of true crabs on Old Earth.
More broadly, a crab is any member of other decapod groups such as king crabs, hermit crabs, or porcelain crabs that have derived a similar morphotype through convergent evolution. More broadly still, very distantly related animals like horseshoe crabs, or completely unrelated non-Terragen life forms may also have a crab-like morphotype and be called crabs. For instance some strikingly crab-like organisms have been catalogued on Trees, Chorus, and To'ul'h Prime. Even artificial non-biological forms may have a similar design. Crab-bots of various sizes are widespread, and some vec clades like the Fabers also have the crab body plan.
Crabs are common subjects of provolution projects, because they are capable of an amphibious lifestyle and because the baseline form already has grasping appendages. Many species have been raised to presophonce and full sophont status, usually with the aid of symbiotic bionano and intelligence augmenting wetware but also sometimes entirely through radical gengineering. Some of these neocrabs have become successful and valued members of galactic society, especially as amphibious traders and as artisans and performers. Other species have proved unable to adjust to the wider society, and instead can be found in selected polities throughout the Utopia Sphere, and some isolated polities elsewhere.