The differential optomotor response of the four-eyed fish Anableps anableps.

The perception of motion is important for the survival and reproduction of many animals, including fish. In the laboratory, support for this idea comes from the observation that many fish show a tendency to follow a series of stripes revolving around a circular aquarium. This response, known as the optomotor response (OMR), is recognized as an innate behavior in many species. The 'four-eyed' fishes of the genus Anableps are an unusual fish from Central and South America and actually have only two eyes. Each eye is divided into upper and lower halves internally and externally. This peculiar dual visual system allows Anableps to feed on creatures that swim or land near or on the water surface or to flee from flying predators attacking from above. It was hypothesized that Anableps should also possess the OMR. We used the OMR as a test to investigate potential differential visual processing in Anableps on normal and 'blinded' fish (the eyes are actually covered--not physically blinded). It was found that the OMR does exist in Anableps and that the strength of this response is dependent on the visual field being tested--a stronger OMR was seen as a result of visual stimulation from the aerial environment.