As its name suggests, this small marine mammal is the only porpoise that lacks a dorsal fin (4), and instead has a ridge that runs down the middle of the back (2). The finless porpoise can also be distinguished by its rounded head, lacking an apparent beak (2), and relatively slender body (4), which is dark to pale grey and lighter on the underside (2). A scattering of horny tubercles (small, raised bumps) are found on the ridge; this is thought to create an anti-slip surface when mothers carry their calves on their back or, more likely, used as sensory organs, as numerous nerve endings are found in the tubercles (2). Three subspecies of the finless porpoise are recognised. Neophocaena phocaenoides asiaorientalis (Yangtze finless porpoise) and Neophocaena phocaenoides sunameri both have narrow and prominent ridges, while the ridge of Neophocaena phocaenoides phocaenoides is wider, and almost flat towards the head. N. p. phocaenoides is also a lighter colour when born, and darkens with age, resulting in almost black adults (2).