The modelling of optimal visual pigments of dichromatic teleosts in green coastal waters

A fish can change its stripes: investigating the role of body colour and pattern in the bluelined goatfish

Bluelined goatfish (Upeneichthys lineatus) rapidly change their body colour from a white horizontally banded pattern to a seemingly more conspicuous vertically banded red pattern, often when foraging. Given the apparent conspicuousness of the pattern to a range of observers, it seems unlikely that this colour change is used for camouflage and instead may be used for communication/signalling. Goatfish often drive multispecies associations, and it is possible that goatfish use this colour change as a foraging success signal to facilitate cooperation, increase food acquisition, and reduce predation risk through a 'safety in numbers' strategy. Using a novel approach, we deployed 3D model goatfish in different colour morphs-white without bands, white with black vertical bands, and white with red vertical bands-to determine whether the red colouration is an important component of the signal or if it is only the vertical banding pattern, regardless of colour, that fish respond to as an indicator of foraging success. Use of remote underwater video allowed us to obtain information without the influence of human observers on the communities and behaviours of other fish in response to these different colours exhibited by goatfish. We found that conspecifics were more abundant around the black- and red-banded model fish when compared with the white models. Conspecifics were also more likely to forage around the models than to pass or show attraction, but this was unaffected by model colour. No difference in the abundance and behaviour of associated heterospecifics around the different models was observed, perhaps due to the static nature of the models. Some species did, however, spend more time around the red- and black-banded fish, which suggests the change in colour may indicate benefits in addition to food resources. Overall, the results suggest that the body colour/pattern of U. lineatus is likely a signalling tool but further work is required to explore the benefits to both conspecifics and heterospecifics and to further determine the behavioural functions of rapid colour change in U. lineatus.

Can the Dynamic Colouration and Patterning of Bluelined Goatfish (Mullidae; Upeneichthys lineatus) Be Perceived by Conspecifics?

Bluelined goatfish (Upeneichthys lineatus) exhibit dynamic body colour changes and transform rapidly from a pale, buff/white, horizontally banded pattern to a conspicuous, vertically striped, red pattern when foraging. This red pattern is potentially an important foraging signal for communication with conspecifics, provided that U. lineatus can detect and discriminate the pattern. Using both physiological and behavioural experiments, we first examined whether U. lineatus possess visual pigments with sensitivity to long ("red") wavelengths of light, and whether they can discriminate the colour red. Microspectrophotometric measurements of retinal photoreceptors showed that while U. lineatuslack visual pigments dedicated to the red part of the spectrum, their pigments likely confer some sensitivity in this spectral band. Behavioural colour discrimination experiments suggested that U. lineatuscan distinguish a red reward stimulus from a grey distractor stimulus of variable brightness. Furthermore, when presented with red stimuli of varying brightness they could mostly discriminate the darker and lighter reds from the grey distractor. We also obtained anatomical estimates of visual acuity, which suggest that U. lineatus can resolve the contrasting bands of conspecifics approximately 7 m away in clear waters. Finally, we measured the spectral reflectance of the red and white colouration on the goatfish body. Visual models suggest that U. lineatus can discriminate both chromatic and achromatic differences in body colouration where longer wavelength light is available. This study demonstrates that U. lineatus have the capacity for colour vision and can likely discriminate colours in the long-wavelength region of the spectrum where the red body pattern reflects light strongly. The ability to see red may therefore provide an advantage in recognising visual signals from conspecifics. This research furthers our understanding of how visual signals have co-evolved with visual abilities, and the role of visual communication in the marine environment.

Sensory Drive, Color, and Color Vision

Colors often appear to differ in arbitrary ways among related species. However, a fraction of color diversity may be explained because some signals are more easily perceived in one environment rather than another. Models show that not only signals but also the perception of signals should regularly evolve in response to different environments, whether these primarily involve detection of conspecifics or detection of predators and prey. Thus, a deeper understanding of how perception of color correlates with environmental attributes should help generate more predictive models of color divergence. Here, I briefly review our understanding of color vision in vertebrates. Then I focus on opsin spectral tuning and opsin expression, two traits involved in color perception that have become amenable to study. I ask how opsin tuning is correlated with ecological differences, notably the light environment, and how this potentially affects perception of conspecific colors. Although opsin tuning appears to evolve slowly, opsin expression levels are more evolutionarily labile but have been difficult to connect to color perception. The challenge going forward will be to identify how physiological differences involved in color vision, such as opsin expression levels, translate into perceptual differences, the selection pressures that have driven those differences, and ultimately how this may drive evolution of conspecific colors.

Comparative visual ecophysiology of mid-Atlantic temperate reef fishes

The absolute light sensitivities, temporal properties, and spectral sensitivities of the visual systems of three mid-Atlantic temperate reef fishes (Atlantic spadefish [Ephippidae: Chaetodipterus faber], tautog [Labridae: Tautoga onitis], and black sea bass [Serranidae: Centropristis striata]) were studied via electroretinography (ERG). Pelagic Atlantic spadefish exhibited higher temporal resolution but a narrower dynamic range than the two more demersal foragers. The higher luminous sensitivities of tautog and black sea bass were similar to other benthic and demersal coastal mid-Atlantic fishes. Flicker fusion frequency experiments revealed significant interspecific differences at maximum intensities that correlated with lifestyle and habitat. Spectral responses of the three species spanned 400-610 nm, with high likelihood of cone dichromacy providing the basis for color and contrast discrimination. Significant day-night differences in spectral responses were evident in spadefish and black sea bass but not tautog, a labrid with characteristic structure-associated nocturnal torpor. Atlantic spadefish responded to a wider range of wavelengths than did deeper-dwelling tautog or black sea bass. Collectively, these results suggest that temperate reef-associated fishes are well-adapted to their gradient of brighter to dimmer photoclimates, representative of their unique ecologies and life histories. Continuing anthropogenic degradation of water quality in coastal environments, at a pace faster than the evolution of visual systems, may however impede visual foraging and reproductive signaling in temperate reef fishes.

Visual spectral sensitivity of photopic juvenile Pacific bluefin tuna (Thunnus orientalis)

Although Pacific bluefin tuna is a species that relies on vision, its photopic visual function is not well known; we therefore recorded electroretinograms to investigate photopic spectral sensitivity in juveniles of this species (49-81 days post-hatch; standard length 74-223 mm). The peak spectral sensitivity wavelength was 505 nm. We estimated that two (λ(max) = 512-515 nm and 423-436 nm) or three (λ(max) = 512-515 nm, 423-436 nm, and 473 nm) types of cone visual pigments contribute to photopic vision; these spectral sensitivities are adapted to surface water habitats in clear ocean and coastal water.

The effects of dietary carotenoid supplementation and retinal carotenoid accumulation on vision-mediated foraging in the house finch

BACKGROUND

For many bird species, vision is the primary sensory modality used to locate and assess food items. The health and spectral sensitivities of the avian visual system are influenced by diet-derived carotenoid pigments that accumulate in the retina. Among wild House Finches (Carpodacus mexicanus), we have found that retinal carotenoid accumulation varies significantly among individuals and is related to dietary carotenoid intake. If diet-induced changes in retinal carotenoid accumulation alter spectral sensitivity, then they have the potential to affect visually mediated foraging performance.

METHODOLOGY/PRINCIPAL FINDINGS

In two experiments, we measured foraging performance of house finches with dietarily manipulated retinal carotenoid levels. We tested each bird's ability to extract visually contrasting food items from a matrix of inedible distracters under high-contrast (full) and dimmer low-contrast (red-filtered) lighting conditions. In experiment one, zeaxanthin-supplemented birds had significantly increased retinal carotenoid levels, but declined in foraging performance in the high-contrast condition relative to astaxanthin-supplemented birds that showed no change in retinal carotenoid accumulation. In experiments one and two combined, we found that retinal carotenoid concentrations predicted relative foraging performance in the low- vs. high-contrast light conditions in a curvilinear pattern. Performance was positively correlated with retinal carotenoid accumulation among birds with low to medium levels of accumulation (∼0.5-1.5 µg/retina), but declined among birds with very high levels (>2.0 µg/retina).

CONCLUSION/SIGNIFICANCE

Our results suggest that carotenoid-mediated spectral filtering enhances color discrimination, but that this improvement is traded off against a reduction in sensitivity that can compromise visual discrimination. Thus, retinal carotenoid levels may be optimized to meet the visual demands of specific behavioral tasks and light environments.

Microspectrophotometric evidence for cone monochromacy in sharks

Sharks are apex predators, and their evolutionary success is in part due to an impressive array of sensory systems, including vision. The eyes of sharks are well developed and function over a wide range of light levels. However, whilst close relatives of the sharks-the rays and chimaeras-are known to have the potential for colour vision, an evolutionary trait thought to provide distinct survival advantages, evidence for colour vision in sharks remains equivocal. Using single-receptor microspectrophotometry, we measured the absorbance spectra of visual pigments located in the retinal photoreceptors of 17 species of shark. We show that, while the spectral tuning of the rod (wavelength of maximum absorbance, λ(max) 484-518 nm) and cone (λ(max) 532-561 nm) visual pigments varies between species, each shark has only a single long-wavelength-sensitive cone type. This suggests that sharks may be cone monochromats and, therefore, potentially colour blind. Whilst cone monochromacy on land is rare, it may be a common strategy in the marine environment: many aquatic mammals (whales, dolphins and seals) also possess only a single, green-sensitive cone type. It appears that both sharks and marine mammals may have arrived at the same visual design by convergent evolution. The spectral tuning of the rod and cone pigments of sharks is also discussed in relation to their visual ecology.

Comparative visual function in four piscivorous fishes inhabiting Chesapeake Bay

Maintaining optimal visual performance is a difficult task in photodynamic coastal and estuarine waters because of the unavoidable tradeoffs between luminous sensitivity and spatial and temporal resolution, yet the visual systems of coastal piscivores remain understudied despite differences in their ecomorphology and microhabitat use. We therefore used electroretinographic techniques to describe the light sensitivities, temporal properties and spectral sensitivities of the visual systems of four piscivorous fishes common to coastal and estuarine waters of the western North Atlantic: striped bass (Morone saxatilis), bluefish (Pomatomus saltatrix), summer flounder (Paralichthys dentatus) and cobia (Rachycentron canadum). Benthic summer flounder exhibited higher luminous sensitivity and broader dynamic range than the three pelagic foragers. The former were at the more sensitive end of an emerging continuum for coastal fishes. By contrast, pelagic species were comparatively less sensitive, but showed larger day-night differences, consistent with their use of diel light-variant photic habitats. Flicker fusion frequency experiments revealed significant interspecific differences at maximum intensities that correlated with lifestyle and habitat. Spectral responses of most species spanned 400-610 nm, with significant day-night differences in striped bass and bluefish. Anadromous striped bass additionally responded to longer wavelengths, similar to many freshwater fishes. Collectively, these results suggest that pelagic piscivores are well adapted to bright photoclimates, which may be at odds with the modern state of eutrified coastal and estuarine waters that they utilize. Recent anthropogenic degradation of water quality in coastal environments, at a pace faster than the evolution of visual systems, may impede visually foraging piscivores, change selected prey, and eventually restructure ecosystems.

Genetic diversification, vicariance, and selection in a polytypic frog

Spatial patterns of heritable phenotypic diversity reflect the relative roles of gene flow and selection in determining geographic variation within a species. We quantified color differentiation and genetic divergence among 20 populations of the red-eyed tree frog (Agalychnis callidryas) in lower Central America. Phylogenetic analyses revealed 5 well-supported mitochondrial DNA clades, and we infer from our phylogeny that geographic barriers have played a large role in structuring populations. Two phenotypic characters varied independently among isolated population groups: Flank coloration distinguished Caribbean from Pacific individuals, whereas leg coloration exhibited a more complex geographic pattern. We detected 3 generalized spatial patterns of genetic and phenotypic diversity: 1) phenotypic differentiation in the presence of historical connectivity, 2) phenotypic uniformity across genetically differentiated regions, and 3) codistribution of genetic and phenotypic characters. These patterns indicate that phenotypic diversification is highly regionalized and can result from spatial variation in localized adaptations, geographic isolation, genetic drift, and/or evolutionary stasis. Although the mode of selection underlying color variation was not the focal objective of this study, we discuss the possible roles of natural and sexual selection in mediating population differentiation. Our study underscores the fact that selection gradients vary across relatively small spatial scales, even in species that occupy relatively homogeneous environments.

Comparative visual function in five sciaenid fishes inhabiting Chesapeake Bay

Maintaining optimal visual performance is a difficult task in the photodynamic coastal and estuarine waters in which western North Atlantic sciaenid fishes support substantial commercial and recreational fisheries. Unavoidable tradeoffs exist between visual sensitivity and resolution, yet sciaenid visual systems have not been characterized despite strong species-specific ecomorphological and microhabitat differentiation. We therefore used electroretinographic techniques to describe the light sensitivities, temporal properties, and spectral characteristics of the visual systems of five sciaenids common to Chesapeake Bay, USA: weakfish (Cynoscion regalis), spotted seatrout (Cynoscion nebulosus), red drum (Sciaenops ocellatus), Atlantic croaker (Micropogonias undulatus) and spot (Leiostomus xanthurus). Benthic sciaenids exhibited higher sensitivities and broader dynamic ranges in white light V/logI experiments than more pelagic forms. Sensitivities of the former were at the lower (more sensitive) end of an emerging continuum for coastal fishes. Flicker fusion frequency experiments revealed significant interspecific differences at maximum intensities that correlated with lifestyle and habitat, but no specific differences at dimmer intensities. Spectral responses of most sciaenids spanned 400-610 nm, with significant diel differences in weakfish and Atlantic croaker. Weakfish, a crepuscular predator, also responded to ultraviolet wavelengths; this characteristic may be more useful under less turbid conditions. Collectively, these results suggest that sciaenids are well adapted to the dynamic photoclimate of the coastal and estuarine waters they inhabit. However, the recent anthropogenic degradation of water quality in coastal environments, at a pace faster than the evolution of visual systems, has amplified the importance of characterizing visual function in managed aquatic fauna.

Multifocal lenses in a monochromat: the harbour seal

Previous photorefractive results from harbour seals indicated the presence of a multifocal lens. This was surprising because the evolution of multifocal lenses has served to compensate for chromatic aberration in animals with colour vision, which harbour seals as monochromats should not be capable of. To gain insight into the lens optics of these animals, we extended our photorefractive measurements in live seals under water and in air and,additionally, analyzed eight extracted juvenile harbour seal lenses with schlieren photography and a laser scanning technique. The results from all methods applied support the presence of multifocal lenses in harbour seals. However, the functional significance of multiple focal lengths in harbour seal lenses remains speculative. Interestingly, the slit pupils of harbour seals cannot be considered to be an adaptation to the multifocal optical system of the eye.

Variation in response to artificial selection for light sensitivity in guppies (Poecilia reticulata)

We performed artificial selection on the visual system in guppies (Poecilia reticulata), using the optomotor reaction threshold as the selection criterion. Two lines were selected for increased sensitivity to blue light, two were selected for increased sensitivity to red light, and two were unselected controls. There was significant response to selection in all four selected lines and significant heritability for sensitivity. An examination of the spectral sensitivity function showed that the form of the response differed between the red and blue lines and among the red lines. Such divergence is likely because there are many different mechanisms allowing response to selection for spectral sensitivity. Diverse mechanisms allow a divergent response by different populations to the same selective pressures. Such a mechanism can promote diversity in vision and visual signals, and any multicomponent system where different components can respond to the same selective regime.

Spectral sensitivities of the seahorsesHippocampus subelongatusandHippocampus barbouriand the pipefishStigmatopora argus

The Syngnathidae are specialized diurnal feeders that are known to possess a retinal fovea and use independent eye movements to locate, track, and strike individual planktonic prey items. In this study, we have investigated the spectral sensitivities of three syngnathid species: a pipefish and two seahorses. We used spectrophotometry to measure the spectral transmission properties of ocular lenses and microspectrophotometry to measure the spectral absorption characteristics of visual pigments in the retinal photoreceptors. The pipefish,Stigmatopora argus, together with the seahorseHippocampus subelongatus, is found in “green-water” temperate coastal seagrass habitats, whereas the second seahorse,H. barbouri, originates from a “blue-water” tropical coral reef habitat. All species were found to possess short wavelength absorbing pigment(s) in their lenses, with the 50% cut-off point ofS. argusandH. subelongatusat 429 and 425 nm respectively, whereas that ofH. barbouriwas located at 409 nm. Microspectrophotometry of the photoreceptors revealed that the rods of all three species contained visual pigment with the wavelength of maximum absorption (λmax) at approximately 500 nm. The visual pigment complement of the cones varied between the species: all possessed single cones with a λmaxclose to 460 nm butH. barbourialso possessed an additional class of single cone with λmaxat 430 nm. Three classes of visual pigment were found in the double cones, the λmaxbeing approximately 520, 537, and 560 nm in the two seahorses and 520, 537, and 580 nm in the pipefish. The spectral sensitivities of the syngnathids investigated here do not appear to conform to generally accepted trends for fishes inhabiting different spectral environments. The influence of the specialized feeding regime of the syngnathids is discussed in relation to our findings that ultra-violet sensitivity is apparently not necessary for zooplanktivory in certain habitats.

A genetically explicit model of speciation by sensory drive within a continuous population in aquatic environments

BACKGROUND

The sensory drive hypothesis predicts that divergent sensory adaptation in different habitats may lead to premating isolation upon secondary contact of populations. Speciation by sensory drive has traditionally been treated as a special case of speciation as a byproduct of adaptation to divergent environments in geographically isolated populations. However, if habitats are heterogeneous, local adaptation in the sensory systems may cause the emergence of reproductively isolated species from a single unstructured population. In polychromatic fishes, visual sensitivity might become adapted to local ambient light regimes and the sensitivity might influence female preferences for male nuptial color. In this paper, we investigate the possibility of speciation by sensory drive as a byproduct of divergent visual adaptation within a single initially unstructured population. We use models based on explicit genetic mechanisms for color vision and nuptial coloration.

RESULTS

We show that in simulations in which the adaptive evolution of visual pigments and color perception are explicitly modeled, sensory drive can promote speciation along a short selection gradient within a continuous habitat and population. We assumed that color perception evolves to adapt to the modal light environment that individuals experience and that females prefer to mate with males whose nuptial color they are most sensitive to. In our simulations color perception depends on the absorption spectra of an individual's visual pigments. Speciation occurred most frequently when the steepness of the environmental light gradient was intermediate and dispersal distance of offspring was relatively small. In addition, our results predict that mutations that cause large shifts in the wavelength of peak absorption promote speciation, whereas we did not observe speciation when peak absorption evolved by stepwise mutations with small effect.

CONCLUSION

The results suggest that speciation can occur where environmental gradients create divergent selection on sensory modalities that are used in mate choice. Evidence for such gradients exists from several animal groups, and from freshwater and marine fishes in particular. The probability of speciation in a continuous population under such conditions may then critically depend on the genetic architecture of perceptual adaptation and female mate choice.

Sensory ecology and perceptual allocation: new prospects for neural networks

Sensory ecology provides a conceptual framework for considering how animals ought to design sensory systems to capture meaningful information from their environments. The framework has been particularly successful at describing how one should allocate sensory receptors to maximize performance on a given task. Neural networks, in contrast, have made unique contributions to understanding how 'hidden preferences' can emerge as a by-product of sensory design. The two frameworks comprise complementary techniques for understanding the design and the evolution of sensation. This article reviews empirical literature from multiple modalities and levels of sensory processing, considering vision, audition and touch from the viewpoints of sensory ecology and neuroethology. In the process, it presents modifications of extant neural network algorithms that would allow a more effective integration of these diverse approaches. Together, the reviewed literature suggests important advances that can be made by explicitly formulating neural network models in terms of sensory ecology, by incorporating neural costs into models of perceptual evolution and by exploring how such demands interact with historical forces.

Naturalistic color discriminations in polymorphic platyrrhine monkeys: Effects of stimulus luminance and duration examined with functional substitution

X-linked photopigment polymorphism produces six different color vision phenotypes in most species of New World monkey. In the subfamily Callitrichinae, the three M/L alleles underlying these different phenotypes are present at unequal frequencies suggesting that selective pressures other than heterozygous-advantage operate on these alleles. Earlier we investigated this hypothesis with functional substitution, a technique using a computer monitor to simulate colors as they would appear to humans with monkey visual pigments (Visual Neuroscience21:217–222, 2004). The stimuli were derived from measurements of ecologically relevant fruit and foliage. We found that discrimination performance depended on the relative spectral positioning of the substituted M and L pigment pair. Here we have undertaken a systematic examination of two simulation parameters—test field luminance and stimulus duration. Discriminability of the fruit colors depended on which phenotype was simulated but only at short stimulus durations and/or low luminances. Under such conditions, phenotypes with the larger pigment peak separations performed better. At longer durations and higher luminances, differences in performance across different substitutions tended to disappear. The stimuli used in this experiment were analyzed with several color discrimination models. There was limited agreement among the predictions made by these models regarding the capabilities of animals with different pigment pairs and none predicted the dependence of discrimination on changes in luminance and stimulus duration.

SENSORY TRADE-OFFS PREDICT SIGNAL DIVERGENCE IN SURFPERCH

Unidirectional elaboration of male trait evolution (e.g., larger, brighter males) has been predicted by receiver bias models of sexual selection and empirically tested in a number of different taxa. This study identifies a bidirectional pattern of male trait evolution and suggests that a sensory constraint is driving this divergence. In this system, the inherent trade-off in dichromatic visual detection places limits on the direction that sensory biases may take and thus provides a quantitative test of the sensory drive model. Here I show that sensory systems with trade-offs in detection abilities produce bidirectional biases and that signal design properties match these biases. I combine species-specific measurements and ancestral estimates with visual detection modeling to examine biases in sensory and signaling traits across five fish species occupying optically diverse habitats in the Californian kelp forest. Species-specific divergence in visual pigments correlates with changes in environment and produces different sensory biases--favoring luminance (brightness) detection for some species and chromatic (color) detection for others. Divergence in male signals (spectral reflectance of orange, blue, and silver color elements) is predicted by each species' sensory bias: color divergence favors chromatic detection for species with chromatically biased visual systems, whereas species with luminance sensory biases have signals favoring luminance detection. This quantitative example of coevolution of communication traits varying in a bidirectional pattern governed by the environment is the first demonstration of sensory trade-offs driving signal evolution.

Mate choice and visibility in the expression of a sexually dimorphic trait in a goodeid fish (Xenotoca variatus)

Male Xenotoca variatus (Bean, 1887) have shiny scales ("speckles") on their flanks, the number of which varies within and among populations. Using fish from two localities with turbid water and two with transparent water, we tested whether differences in the number of speckles were associated with differences either in water turbidity or with the expression of female mate choice. We also tested whether female mate choice was influenced by water turbidity. In our sample the number of speckles and water turbidity were not associated. A test including all the populations in a combined (factorial) analysis showed that in clear water females exhibit a preference for visiting the male with the largest number of speckles of a pair, though no population differences were detected. When tested in clear water, females spent more time close to a male with more speckles; in this instance, males were from a clear-water locality and possessed many speckles. Our findings suggest that female mate choice might not contribute to the geographical variation in speckle number, but may instead be constrained by the transmissibility of the signal.

Modelling divergence in luminance and chromatic detection performance across measured divergence in surfperch (Embiotocidae) habitats

This study predicts target detection performance in species-specific habitats for six surfperch (Embiotocidae) living in optically variable California kelp forests. Using species-specific measurements of habitat irradiance and photoreceptor absorbance in a simple dichromatic model for luminance and chromatic detection, the estimated performance of species' measured photopigments was compared to the theoretical maximum for each habitat. Modelling results suggest that changes in peak photoreceptor absorbance (lambda(max)), photoreceptor optical density, and photic environment may affect detection performance. Estimated performances for luminance detection were consistently high, while chromatic detection varied by habitat and demonstrated substantial improvements with increasing optical density differences between cone classes.

Crowding under scotopic conditions

Under certain circumstances, a subject's ability to discriminate spatial features of a target may be hampered by neighbouring contours. This phenomenon is popularly known as the "crowding effect", and it has been intensely studied for photopic vision: little attention has been paid to the effect at lower light levels. The underlying basis of the crowding effect has recently provoked some conjecture, with Hess and colleagues claiming that a passive "physical" phenomenon may either wholly [Vis. Res. 40 (2000) 365], or partially [J. Opt. Soc. Am. A--Opt. Image Sci. Vis. 17 (2000) 1516], account for the effect. In order to investigate the crowding effect under scotopic conditions, we conducted scotopic frequency of seeing experiments for Landolt C targets presented both with, and without, flanking bars; the size of the targets was varied so that frequency of seeing curves could be derived for each stimulus condition. Our results suggest that the spatial extent of crowding is significantly less for scotopic vision than for photopic vision at the same eccentricity--furthermore the effect does not seem to scale in proportion to target size. We also compared the resulting empirical curves to those that would be predicted by the hypothesis of Hess and colleagues. Our results do not support the hypothesis that the scotopic crowding effect is caused by a passive physical process.

The influence of stimulus and background colour on signal visibility in the lizard Anolis cristatellus

Anoline lizards communicate with visual displays in which they open and close a colourful throat fan called the dewlap. We used a visual fixation reflex as an assay to test the effects of stimulus versus background chromatic and brightness contrast on the probability of detecting a moving coloured (i.e. dewlap-like) stimulus in Anolis cristatellus. The probability of stimulus detection depended on two additive visual-system channels, one responding to brightness contrast and one responding to chromatic contrast, independent of brightness. The brightness channel was influenced only by wavelengths longer than 450nm and probably received input only from middle- and/or long-wavelength photoreceptors. The chromatic contrast channel appeared to receive input from three, or possibly four, different classes of cone in the anoline retina, including one with peak sensitivity in the ultraviolet. We developed a multi-linear regression equation that described most of the results of this study to a reasonable degree of accuracy. In the future, this equation could be used to predict the relative visibility of different-coloured stimuli in different habitat light conditions, which should be very useful for testing hypotheses that attempt to relate habitat light conditions and visual-system response to the evolution of signal design.

Fruits, foliage and the evolution of primate colour vision

Primates are apparently unique amongst the mammals in possessing trichromatic colour vision. However, not all primates are trichromatic. Amongst the haplorhine (higher) primates, the catarrhines possess uniformly trichromatic colour vision, whereas most of the platyrrhine species exhibit polymorphic colour vision, with a variety of dichromatic and trichromatic phenotypes within the population. It has been suggested that trichromacy in primates and the reflectance functions of certain tropical fruits are aspects of a coevolved seed-dispersal system: primate colour vision has been shaped by the need to find coloured fruits amongst foliage, and the fruits themselves have evolved to be salient to primates and so secure dissemination of their seeds. We review the evidence for and against this hypothesis and we report an empirical test: we show that the spectral positioning of the cone pigments found in trichromatic South American primates is well matched to the task of detecting fruits against a background of leaves. We further report that particular trichromatic platyrrhine phenotypes may be better suited than others to foraging for particular fruits under particular conditions of illumination; and we discuss possible explanations for the maintenance of polymorphic colour vision amongst the platyrrhines.

Spectral tuning of dichromats to natural scenes

Multispectral images of natural scenes were collected from both forests and coral reefs. We varied the wavelength position of receptors in hypothetical dichromatic visual systems and, for each receptor pair estimated the percentage of discriminable points in natural scenes. The optimal spectral tuning predicted by this model results in photoreceptor pairs very like those of forest dwelling, dichromatic mammals and of coral reef fishes. Variations of the natural illuminants in forests have little or no effect on optimal spectral tuning, but variations of depth in coral reefs have moderate effects on the spectral placement of S and L cones. The ratio of S and L cones typically found in dichromatic mammals reduces the discriminability of forest scenes; in contrast, the typical ratio of S and L cones in coral reef fishes achieves nearly the optimal discrimination in coral reef scenes.

Stomatopod photoreceptor spectral tuning as an adaptation for colour constancy in water

Where colour is used in communication absolute judgement of signalling spectra is important, and failures of colour constancy may limit performance. Stomatopod crustaceans have unusual eyes in which the midband contains ten or more classes of photoreceptor. For constancy based on receptor adaptation to a fixed background, elementary theory predicts and we confirm by modelling, that stomatopods' narrow-band receptors outperform more broadly tuned receptors. Similar considerations could account for the small spectral separation of receptors in each midband row. Thus, stomatopods seem to trade-off sensitivity and signal-to-noise ratio for increased colour constancy.

Color vision in the manatee (Trichechus manatus)

Four manatees were trained to discriminate between a colored stimulus and a shade of gray in a two-fold simultaneous choice situation. The colors blue, green, red and blue-green were tested against shades of gray varying from low to high relative brightness. The animals distinguished both blue and green from a series of grays but failed to discriminate red and blue-green from certain steps of grays. The manatees could not discriminate between a UV-reflecting white target and an UV-absorbing white target. The results indicate that manatees possess color vision which is most likely dichromatic.

Specialization of retinal function in the compound eyes of mantis shrimps

Visual function and its specialization at the level of the retina were studied in 13 species of stomatopod crustaceans, representing three superfamilies: Gonodactyloidea, Lysiosquilloidea, and Squilloidea. We measured attenuation and irradiance spectra in the environment of each species, at the actual depths and times of activity where we observed individuals. We also characterized the intrahabdomal filters of all study species and determined the absolute spectral sensitivity functions and approximate photon capture rates of all photoreceptor classes below the level of the 8th retinular cell in seven of these species. Shallow-water gonodactyloid species have four distinct classes of intrarhabdomal filters, producing photoreceptors that are relatively insensitive but which have the broadest spectral coverage of all. Deep-water gonodactyloids and all lysiosquilloids have filters that are spectrally less diverse. These species often discard the proximal filter classes of one or more receptor types. As a result, their retinas are more sensitive but have reduced spectral range or diversity. The single squilloid species has the most sensitive photoreceptors of any we observed, due to the lack both of intrarhabdomal filters and tiered photoreceptors. Photon absorption rates, at the times of animal activity, were similar in most photoreceptor classes of all species, whether the receptors were tiered or untiered, or filtered or unfiltered. Thus, the retinas of stomatopods are specialized to operate at similar levels of stimulation at the times and depths of actual use, while evidently maintaining the greatest possible potential for spectral coverage and discrimination.

Visual pigments and the photic environment: the cottoid fish of Lake Baikal

The endemic cottoid fish of Lake Baikal in Eastern Siberia offer a singular opportunity for examining within a number of closely related species, the relationships of visual pigments, photoreceptor complements and depth within a deep freshwater environment. The lake, the deepest (1600 m) and one of the largest and most ancient in the world, is unique in that the oxygen levels at the bottom are only reduced to about 80% of the surface levels. We have studied, by light microscopy, microspectrophotometry and visual pigment extraction, the retinas from 17 species of Baikal cottoids that live at different depths within the lake. Generally the retinas contain, in addition to rods, large green-sensitive double cones and small blue-sensitive single cones: surprisingly for freshwater fish, the visual pigments are based on Vitamin A1. The lambda max of both rods and cones are displaced to shorter wavelengths with increasing depth. Surface species have cones with lambda max at about 546, 525 and 450 nm and rods at 523 nm, deeper living species retain cones, but with lambda max shifting towards 500 and 425 nm and with rods at 480 nm, whereas the deepest living fish possess only rods (lambda max 480-500 nm). These data clearly show a correlation between photoreceptor complement, visual pigment lambda max and depth, but question the hypothesis that there is a correlation of pigment lambda max with water colour since, in contrast to oceanic waters, the maximum transmission of Baikal water is between 550 and 600 nm.

The distribution and nature of colour vision among the mammals

1. An oft-cited view, derived principally from the writings of Gordon L. Walls, is that relatively few mammalian species have a capacity for colour vision. This review has evaluated that proposition in the light of recent research on colour vision and its mechanisms in mammals. 2. To yield colour vision a retina must contain two or more spectrally discrete types of photopigment. While this is a necessary condition, it is not a sufficient one. This means, in particular, that inferences about the presence of colour vision drawn from studies of photopigments, the precursors of photopigments, or from nervous system signals must be accepted with due caution. 3. Conjoint signals from rods and cones may be exploited by mammalian nervous systems to yield behavioural discriminations consistent with the formal definition of colour vision. Many mammalian retinas are relatively cone-poor, and thus there are abundant opportunities for such rod/cone interactions. Several instances were cited in which animals having (apparently) only one type of cone photopigment succeed at colour discriminations using such a mechanism. it is suggested that the exploitation of such a mechanism may not be uncommon among mammals. 4. Based on ideas drawn from natural history, Walls (1942) proposed that the receptors and photopigments necessary to support colour vision were lost during the nocturnal phase of mammalian history and then re-acquired during the subsequent mammalian radiations. Contemporary examination of photopigment genes along with the utilization of better techniques for identifying rods and cones suggest a different view, that the earliest mammals had retinas containing some cones and two types of cone photopigment. Thus the baseline mammalian colour vision is argued to be dichromacy. 5. A consideration of the broad range of mammalian niches and activity cycles suggests that many mammals are active during photic periods that would make a colour vision capacity potentially useful. 6. A systematic survey was presented that summarized the evidence for colour vision in mammals. Indications of the presence and nature of colour vision were drawn both from direct studies of colour vision and from studies of those retinal mechanisms that are most closely associated with the possession of colour vision. Information about colour vision can be adduced for species drawn from nine mammalian orders.(ABSTRACT TRUNCATED AT 400 WORDS)

Vitamin A2-based visual pigments in fully terrestrial vertebrates

As part of a broad study of the ocular and extraocular photoreceptors of reptiles, we have used high performance liquid chromatography (HPLC) to identify the retinoids present in whole eye extracts of the arboreal lizard Anolis carolinensis and the non-arboreal ruin lizard Podarcis sicula. Unexpectedly, only vitamin A2-derived chromophore was detected in Anolis, while a mixture of vitamin A1- and vitamin A2-derived chromophores was detected in Podarcis. These are the first examples of fully terrestrial vertebrates using vitamin A2-derived chromophore for visual pigment generation. Furthermore, microspectrophotometric (MSP) data for Anolis show a class of photoreceptor having a visual pigment with maximum absorbance at about 625 nm, some 40 nm further into the red than has been found in any terrestrial vertebrate examined to date.

Variation in the appearance of guppy color patterns to guppies and their predators under different visual conditions

Color patterns of natural populations of guppies (Poecilia reticulata) are a compromise between sexual selection and predation avoidance. Field data on ambient light spectra, water transmission spectra, courtship and attack distances, and cone pigments of guppies and their predators were used to calculate measures of conspicuousness of guppies under various combinations of visual conditions and vision. The results suggest that color patterns are relatively more conspicuous to guppies at the times and places of courtship and relatively less conspicuous at the times and places of maximum predator risk. Some implications to the evolution of vision, visual communication and behavior are discussed.