The Mueller-Lyer illusion in the teleost fish Xenotoca eiseni

In the Mu¨ller-Lyer illusion, human subjects usually see a line with two inducers at its ends facing outwards as longer than an identical line with inducers at its ends facing inwards. We investigate the tendency for fish to perceive, in suitable conditions, line length according to the Mu¨ller-Lyer illusion. Redtail splitfins (Xenotoca eiseni, family Goodeidae) were trained to discriminate between two lines of different length. After reaching the learning criterion, the fish performed test trials in which they faced two lines (black or red) of identical length, differing only in the context in terms of arrangement of the inducers, which were positioned at the ends of the line, either inward, outward, or perpendicular. Fish chose the stimulus that appear to humans as either longer or shorter, in accordance with the prediction of the Mueller-Lyer illusion, consistently with the condition of the training. These results show that redtail splitfins tend to be subject to this particular illusion. The results of the study are discussed with reference to similar studies concerning the same illusion as recently observed in fish. Contrasting results are presented. The significance of the results in light of their possible evolutionary implications is also discussed.