The Ebbinghaus illusion in a fish (Xenotoca eiseni).

The tendency of fish to perceive the Ebbinghaus illusion was investigated. Redtail splitfins (Xenotoca eiseni, family Goodeidae) were trained to discriminate between two disks of different sizes. Then, fish were presented with two disks of the same size surrounded by disks of large or small size (inducers) arranged to produce the impression (to a human observer) of two disks of different sizes (in the Ebbinghaus illusion, a central disk surrounded by small inducers appears bigger than an identical one surrounded by large inducers). Fish chose the stimulus that, on the basis of a perception of the Ebbinghaus illusion, appeared deceptively larger or smaller, consistent with the condition of training. These results demonstrate that redtail splitfins tend to perceive this particular illusion. The results are discussed with reference to other related illusions that have been recently observed to be experienced by fish (such as the Navon effect), and with regard to their possible evolutionary implications.