The UV visual world of fishes: a review

Ultraviolet-A radiation (320-400 nm) is scattered rapidly in water. Despite this fact, UV is present in biologically useful amounts to at least 100 m deep in clear aquatic environments. Discovery of UV visual pigments with pe ak absorption at around 360 nm in teleost cone photoreceptors indicates tha t many teleost fishes may be adapted for vision in the UV range. Considerin g the characteristic absorption curve for visual pigments, about 18% of the downwelling light that illuminates objects at 30-m depth would be availabl e to W-sensitive cones. Strong scattering of UV radiation should produce un ique imaging conditions as a very bright UV background in the horizontal vi ew and a marked veiling effect that, with distance, obscures an image. Many teleosts have three, or even four, classes of cone cells mediating colour vision in their retina and one can be sensitive to UV. These UV-sensitive c ones contain a visual pigment based on a unique opsin which is highly conse rved between fish species. Several powerful methods exist for demonstration of UV vision, but all are rather demanding in terms of technique and equip ment. Demonstration that the eye lacks W-blocking compounds that are presen t in many fish eyes is a simpler method that can indicate the possibility o f UV vision. The only experimental evidence for the use of UV vision by fis hes is connected to planktivory: detection of UV-opaque objects at close ra nge against a bright UV background is enhanced by the physical properties o f UV light. Once present, perhaps for the function of detecting food, UV vi sion may well be co-opted through natural selection for other functions. Re cent discovery that UV vision is critically important for mate choice in so me birds and lizards is a strong object lesson for fish ecologists and beha viourists. Other possible functions amount to far more than merely adding a fourth dimension to the visible spectrum. Since UV is scattered so effecti vely in water, it may be useful for social signalling at short range and re duce the possibility of detection by other, illegitimate, receivers. Since humans are blind to UV light, we may be significantly in error, in many cas es, in our attempts to understand and evaluate Visual aspects of fish behav iour. A survey of the reflectance properties of skin pigments in fishes rev eals a rich array of pigments with reflectance peaks in the UV. For example , the same yellow to our eyes may comprise two perceptually different colou rs to fish, yellow and UV-yellow. It is clearly necessary for us to anticip ate that many fishes may have some form of UV vision. (C) 1999 The Fisherie s Society of the British Isles.