Least vision in a dolphin

The dolphin displaying the least degree of vision are the South Asian river dolphins (genus Platanista), which comprise the Ganges river dolphin (P. gangetica and the Indus river dolphin (P. minor), two freshwater species inhabiting the Ganges and Indus rivers, respectively, of Asia. These are the only dolphins lacking a crystalline lens in their eyes. Indeed, the optic opening of each eye is so tiny, no more than a pinprick in diameter, that virtually no light can enter the eyes, thereby rendering their vision almost non-existent.

As these dolphin species inhabit muddy rivers, vision is of little or no use, so they navigate by echolocation instead, emitting trains of high-frequency clicks (15-150 kHz) and then listening to their aural reflections, thus building up 3D sound pictures, just like bats do.