Anyone who has looked at the aerial world during an underwater dive will have marvelled at the distortions in apparent size, shape, and position of aerial objects. In a new study, researchers have shown that archer fish can learn to cope with these strongly viewpoint-dependent distortions, and this ability enables the fish to precisely judge from any underwater viewpoint the absolute size of their aerial prey. The work is reported by Stefan Schuster, now at the University of Erlangen-Nuremburg, and colleagues at the University of Freiburg.

Many animals are able to judge the absolute size of visual objects, a skill that becomes especially useful for tasks such as selecting prey. The ability to judge absolute size, however, requires that the visual system be able to perform a critical correction to account for distance between the eye and the viewed object: dramatic differences in the size of the actual retinal image occur as the distance between viewer and object changes. A situation in which this corrective requirement is especially challenging arises for animals that need to judge the size of aerial prey from an underwater vantage point. Here, as the study of Schuster and colleagues shows, the complex optical situation arising from distance and refraction requires precise understanding of the relationship between an object's apparent size and the fish's relative position to the object.

In the experiments reported by the researchers, archer fish viewed a set of eight disks at various heights above the water surface from various horizontal distances and selected one of the disks as a shooting target based only on its absolute size. Although nave fish often selected disks that would be too large to be swallowed, all could eventually learn to judge absolute size with great precision; in doing so, they perfectly accounted for the complex optical situation posed by their underwater viewpoint. In a series of experiments, the researchers showed that the f
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Contact: Heidi Hardman
hhardman@cell.com
617-397-2879
Cell Press
6-Sep-2004

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