Several features of the evolution of eyes and photoreceptors are examined in an effort to explore the relative roles of adaptation and historical and developmental constraints. Optical design shows clear evidence of adaptation, which in some respects approaches optima predictable from physics. The primate fovea, on the other hand, illustrates how adaptation can be channeled by developmental heritage. The primary structures of opsins reveal multiple evolutionary lineages within both Drosophila and humans. The pigments of vertebrae rods comprise a subset of opsins whose evolutionary relationships map onto the phylogeny of the parent species. The evolutionary reasons for why most rod pigments absorb maximally at 500 +/- 10 nm are obscure, as there is no convincing explanation based on adaptation alone. Rods are appropriately distinguished from cones on the basis of which opsin gene is expressed. This criterion is likely to be in conflict with other definitions in phyletic lines (e.g., geckos, snakes) that have long diurnal or nocturnal histories accompanied by loss of one or more opsin genes, followed by a secondary adaptation to life in a different photic environment. Color vision--a generalizable perception associated with the spectral composition of light--is usefully distinguished from wavelength-specific behaviors. The latter are also based on multiple visual pigments and more than one spectral class of receptors but cannot be altered by learning. The distinction is particularly forceful in bees, which exhibit both kinds of behavior. The evolution of primate color vision has been shaped by historical factors involving an extensive period of early mammalian nocturnality. Birds, by contrast, have more elaborate cones and a richer set of visual pigments. Avian color space can be represented in a tetrahedron.
Photocurrents of cones in the retinas of a small fish, Danio aequipinnatus (Cyprinidae) were recorded with suction pipette electrodes. Spectral sensitivity was measured between 277 and 697 nm. Four spectral classes of cone were found, with λmax at 560, 480, 408, and 358 nm. For the latter, we provide the first complete characterization of spectral sensitivity of a vertebrate ultraviolet (UV) photoreceptor. All cones responded with similar kinetics, except for a subset of the 560-nm cones, which were distinctly faster. The a-bands of the three cones absorbing maximally in the visible have the same bandwidth when log sensitivity is plotted versus normalized frequency, and in this respect they are indistinguishable from primate cones (“Mansfield's rule’). An eighth-degree polynomial in λmax/λ based on this combined data set (fish, primate) is presented as a template that is likely to have predictive value in describing cone spectra from other vertebrates. The α−band of the UV cone, however, is somewhat narrower than predicted by this function, is similar to other UV visual pigments, and an eighth-degree polynomial that describes its shape is also presented. These measurements also provide information on the β−band (i.e. cis peak region), difficult to obtain by microspectrophotometry. The β−band of cone pigments is found at longer wavelengths as the α−band shifts toward the red. A secondary rise in cone sensitivity around 280 nm indicates that photons absorbed by aromatic amino acids in the opsin (γ−band) excite the transduction cascade, but the quantum efficiency is not as high as when absorption occurs in the retinal-protein chromophore.
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 peak absorption at around 360 nm in teleost cone photoreceptors indicates that many teleost fishes may be adapted for vision in the UV range. Considering the characteristic absorption curve for visual pigments, about 18% of the downwelling light that illuminates objects at 30-m depth would be available to UV-sensitive cones. Strong scattering of UV radiation should produce unique imaging conditions as a very bright UV background in the horizontal view 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 cones contain a visual pigment based on a unique opsin which is highly conserved between fish species. Several powerful methods exist for demonstration of UV vision, but all are rather demanding in terms of technique and equipment. Demonstration that the eye lacks UV-blocking compounds that are present in many fish eyes is a simpler method that can indicate the possibility of UV vision. The only experimental evidence for the use of UV vision by fishes is connected to planktivory: detection of UV-opaque objects at close range against a bright UV background is enhanced by the physical properties of UV light. Once present, perhaps for the function of detecting food, UV vision may well be co-opted through natural selection for other functions. Recent discovery that UV vision is critically important for mate choice in some birds and lizards is a strong object lesson for fish ecologists and behaviourists. Other possible functions amount to far more than merely adding a fourth dimension to the visible spectrum. Since UV is scattered so effectively in water, it may be useful for social signalling at short range and reduce the possibility of detection by other, illegitimate, receivers. Since humans are blind to UV light, we may be significantly in error, in many cases, in our attempts to understand and evaluate visual aspects of fish behaviour. A survey of the reflectance properties of skin pigments in fishes reveals a rich array of pigments with reflectance peaks in the UV. For example, the same yellow to our eyes may comprise two perceptually different colours to fish, yellow and UV-yellow. It is clearly necessary for us to anticipate that many fishes may have some form of UV vision. 1999 The Fisheries Society of the British Isles 921 0022-1112/99/050921+23 $30.00/0 1999 The Fisheries Society of the British IslesF. 4. Immunolocalization of the zebrafish UV opsin to the short single cone outer segments. Immunopurified polyclonal antisera generated against either the amino terminus of the UV opsin (a) and (b) or the rod opsin (c) were incubated with frozen retinal tissue sections and detected with a Cy3-conjugated goat anti-rabbit second...
Budgerigars, Melopsittacus undulatus, were trained to discriminate monochromatic lights from mixtures of two comparison lights. The addition of small amounts of UV (365 nm) to blue or yellow lights dramatically changed the color for the birds. Hue matches showed the birds to be dichromatic both at long wavelengths (only P565 and P508 active) and at short wavelengths (only P370 and P445 active because of screening of P508 and P565 by cone oil droplets). In mid-spectrum (only P445 and P508 active), a hue match was achieved, but the results were more complicated because two opponent neural processes were activated. All observed hue matches were in quantitative agreement with calculations of relative quantum catch in the pairs of participating single cones and point to the presence of a minimum of three opponent neural processes. For the hue matches at mid- and short wavelengths, the calculations also predict peak values of absorbance of the cone oil droplets associated with P508 and P445. Relative intensity of the training light affected difficult matches at long but not short wavelengths, likely due to achromatic signals from the double cones. With suitable training, birds could make intensity discriminations at short wavelengths, where the double cones have diminished sensitivity.
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