The blind Indus dolphin, Platanista indi

The Indus dolphin is an endemic, relict species, well adapted to the turbid water of the river. It is a constant swimmer with extremely short sleep phases, besides being a continuous emitter of sound. Loss of sight is compensated by an efficient sonar system. Alone among cetaceans, the Indus dolphin swims on its side, feeling its way along the bottom with the right or left flipper. These are secondary adaptations in a species whose Miocene ancestors inhabited clear coastal waters.