Potential ultraviolet vision in pre-settlement larvae and settled reef fish—A comparison across 23 families

Short-wavelength-sensitive 1 (SWS1) opsin gene duplications and parallel visual pigment tuning support ultraviolet communication in damselfishes (Pomacentridae)

Damselfishes (Pomacentridae) are one of the most behaviourally diverse, colourful and species-rich reef fish families. One remarkable characteristic of damselfishes is their communication in ultraviolet (UV) light. Not only are they sensitive to UV, they are also prone to have UV-reflective colours and patterns enabling social signalling. Using more than 50 species, we aimed to uncover the evolutionary history of UV colour and UV vision in damselfishes. All damselfishes had UV-transmitting lenses, expressed the UV-sensitive SWS1 opsin gene, and most displayed UV-reflective patterns and colours. We find evidence for several tuning events across the radiation, and while SWS1 gene duplications are generally very rare among teleosts, our phylogenetic reconstructions uncovered two independent duplication events: one close to the base of the most species-rich clade in the subfamily Pomacentrinae, and one in a single Chromis species. Using amino acid comparisons, we found that known spectral tuning sites were altered several times in parallel across the damselfish radiation (through sequence change and duplication followed by sequence change), causing repeated shifts in peak spectral absorbance of around 10 nm. Pomacentrinae damselfishes expressed either one or both copies of SWS1, likely to further finetune UV-signal detection and differentiation. This highly advanced and modified UV vision among damselfishes, in particular the duplication of SWS1 among Pomacentrinae, might be seen as a key evolutionary innovation that facilitated the evolution of the exuberant variety of UV-reflectance traits and the diversification of this coral reef fish lineage.

Developing and adult reef fish show rapid light-induced plasticity in their visual system

The visual capabilities of fish are optimized for their ecology and light environment over evolutionary time. Similarly, fish vision can adapt to regular changes in light conditions within their lifetime, e.g., ontogenetic or seasonal variation. However, we do not fully understand how vision responds to irregular short-term changes in the light environment, e.g., algal blooms and light pollution. In this study, we investigated the effect of short-term exposure to unnatural light conditions on opsin gene expression and retinal cell densities in juvenile and adult diurnal reef fish (convict surgeonfish; Acanthurus triostegus). Results revealed phenotypic plasticity in the retina across ontogeny, particularly during development. The most substantial differences at both molecular and cellular levels were found under constant dim light, while constant bright light and simulated artificial light at night had a lesser effect. Under dim light, juveniles and adults increased absolute expression of the cone opsin genes, sws2a, rh2c and lws, within a few days and juveniles also decreased densities of cones, inner nuclear layer cells and ganglion cells. These changes potentially enhanced vision under the altered light conditions. Thus, our study suggests that plasticity mainly comes into play when conditions are extremely different to the species' natural light environment, i.e., a diurnal fish in "constant night". Finally, in a rescue experiment on adults, shifts in opsin expression were reverted within 24 h. Overall, our study showed rapid, reversible light-induced changes in the retina of A. triostegus, demonstrating phenotypic plasticity in the visual system of a reef fish throughout life.

Genome-wide identification and expression patterns of opsin genes during larval development in Chinese perch (Siniperca chuatsi)

Vision is important for fish to forage food and fishes express opsin genes to receive visual signals. Chinese perch (Siniperca chuatsi) larvae prey on other fish species larvae at firstfeeding but donoteat any zooplankton, the expression of opsin genes in S. chuatsilarvae is unknown. In this study, we conducted a whole-genome analysis and demonstrated that S. chuatsihave5cone opsin genes (sws1, sws2Aα, sws2Aβ, rh2and lws)and 2 rod opsin genes (rh1and rh1-exorh). The syntenicanalysisshowedthe flanking genes ofall opsin genes were conserved during fish evolution, but the ancestorof S. chuatsimightlost some opsin gene copies duringtheevolution.The phylogeneticanalysisshowed sws1of S. chuatsiwas closest to those of Lates calcariferwhich had a truncated sws1gene; the sws2Aα, sws2Aβ,lws,rh2,rh1 andrh1-exorh of S. chuatsihad a closer relationship with those of Percomorpha fishes.Importantly, results of in situhybridization showed the sws1 opsingene,which is related to forage zooplankton,had extremely low levelexpression in retinaat early stages.Surprisingly, the rh2 opsin gene had a high level expression at firstfeeding stage. The sws2Aα, sws2Aβand lwshad a little expression at early stages but the lwsshowed a increasing trend with larval development, rh1 opsin gene expression appeared at15 dph. In thisstudy, we found a specialpattern of visual opsin genes expression in S. chuatsi, it might influence the larval first feeding and feeding habit.

Lens and cornea limit UV vision of birds - a phylogenetic perspective

Most vertebrates have UV-sensitive vision, but the UV sensitivity of their eyes is limited by the transmittance of the ocular media, and the specific contribution of the different media (cornea, lens) has remained unclear. Here, we describe the transmittance of all ocular media (OMT), as well as that of lenses and corneas of birds. For 66 species belonging to 18 orders, the wavelength at which 50% of light is transmitted through the ocular media to the retina (λT0.5) ranges from 310 to 398 nm. Low λT0.5 corresponds to more UV light transmitted. Corneal λT0.5 varies only between 300 and 345 nm, whereas lens λT0.5 values are more variable (between 315 and 400 nm) and tend to be the limiting factor, determining OMT in the majority of species. OMT λT0.5 is positively correlated with eye size, but λT0.5 of corneas and lenses are not correlated with their thickness when controlled for phylogeny. Corneal and lens transmittances do not differ between birds with UV- and violet-sensitive SWS1 opsin when controlling for eye size and phylogeny. Phylogenetic relatedness is a strong predictor of OMT, and ancestral state reconstructions suggest that from ancestral intermediate OMT, highly UV-transparent ocular media (low λT0.5) evolved at least five times in our sample of birds. Some birds have evolved in the opposite direction towards a more UV-opaque lens, possibly owing to pigmentation, likely to mitigate UV damage or reduce chromatic aberration.

Molecular evolution of ultraviolet visual opsins and spectral tuning of photoreceptors in anemonefishes (Amphiprioninae)

Many animals including birds, reptiles, insects, and teleost fishes can see ultraviolet (UV) light (shorter than 400 nm), which has functional importance for foraging and communication. For coral reef fishes, shallow reef environments transmit a broad spectrum of light, rich in UV, driving the evolution of diverse spectral sensitivities. However, the identities and sites of the specific visual genes that underly vision in reef fishes remain elusive and are useful in determining how evolution has tuned vision to suit life on the reef. We investigated the visual systems of 11 anemonefish (Amphiprioninae) species, specifically probing for the molecular pathways that facilitate UV-sensitivity. Searching the genomes of anemonefishes, we identified a total of eight functional opsin genes from all five vertebrate visual opsin subfamilies. We found rare instances of teleost UV-sensitive SWS1 opsin gene duplications that produced two functionally coding paralogs (SWS1α and SWS1β) and a pseudogene. We also found separate green sensitive RH2A opsin gene duplicates not yet reported in the family Pomacentridae. Transcriptome analysis revealed false clown anemonefish (Amphiprion ocellaris) expressed one rod opsin (RH1) and six cone opsins (SWS1β, SWS2B, RH2B, RH2A-1, RH2A-2, LWS) in the retina. Fluorescent in situ hybridization highlighted the (co-)expression of SWS1β with SWS2B in single cones, and either RH2B, RH2A, or RH2A together with LWS in different members of double cone photoreceptors (two single cones fused together). Our study provides the first in-depth characterization of visual opsin genes found in anemonefishes and provides a useful basis for the further study of UV-vision in reef fishes.

The Visual Opsin Gene Repertoires of Teleost Fishes: Evolution, Ecology, and Function

Visual opsin genes expressed in the rod and cone photoreceptor cells of the retina are core components of the visual sensory system of vertebrates. Here, we provide an overview of the dynamic evolution of visual opsin genes in the most species-rich group of vertebrates, teleost fishes. The examination of the rich genomic resources now available for this group reveals that fish genomes contain more copies of visual opsin genes than are present in the genomes of amphibians, reptiles, birds, and mammals. The expansion of opsin genes in fishes is due primarily to a combination of ancestral and lineage-specific gene duplications. Following their duplication, the visual opsin genes of fishes repeatedly diversified at the same key spectral-tuning sites, generating arrays of visual pigments sensitive from the ultraviolet to the red spectrum of the light. Species-specific opsin gene repertoires correlate strongly with underwater light habitats, ecology, and color-based sexual selection. Expected final online publication date for the Annual Review of Cell and Developmental Biology, Volume 37 is October 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Visual system diversity in coral reef fishes

Coral reefs are one of the most species rich and colourful habitats on earth and for many coral reef teleosts, vision is central to their survival and reproduction. The diversity of reef fish visual systems arises from variations in ocular and retinal anatomy, neural processing and, perhaps most easily revealed by, the peak spectral absorbance of visual pigments. This review examines the interplay between retinal morphology and light environment across a number of reef fish species, but mainly focusses on visual adaptations at the molecular level (i.e. visual pigment structure). Generally, visual pigments tend to match the overall light environment or micro-habitat, with fish inhabiting greener, inshore waters possessing longer wavelength-shifted visual pigments than open water blue-shifted species. In marine fishes, particularly those that live on the reef, most species have between two (likely dichromatic) to four (possible tetrachromatic) cone spectral sensitivities and a single rod for crepuscular vision; however, most are trichromatic with three spectral sensitivities. In addition to variation in spectral sensitivity number, spectral placement of the absorbance maximum (λ_max) also has a surprising degree of variability. Variation in ocular and retinal anatomy is also observed at several levels in reef fishes but is best represented by differences in arrangement, density and distribution of neural cell types across the retina (i.e. retinal topography). Here, we focus on the seven reef fish families most comprehensively studied to date to examine and compare how behaviour, environment, activity period, ontogeny and phylogeny might interact to generate the exceptional diversity in visual system design that we observe.

Seeing the rainbow: mechanisms underlying spectral sensitivity in teleost fishes

Among vertebrates, teleost eye diversity exceeds that found in all other groups. Their spectral sensitivities range from ultraviolet to red, and the number of visual pigments varies from 1 to over 40. This variation is correlated with the different ecologies and life histories of fish species, including their variable aquatic habitats: murky lakes, clear oceans, deep seas and turbulent rivers. These ecotopes often change with the season, but fish may also migrate between ecotopes diurnally, seasonally or ontogenetically. To survive in these variable light habitats, fish visual systems have evolved a suite of mechanisms that modulate spectral sensitivities on a range of timescales. These mechanisms include: (1) optical media that filter light, (2) variations in photoreceptor type and size to vary absorbance and sensitivity, and (3) changes in photoreceptor visual pigments to optimize peak sensitivity. The visual pigment changes can result from changes in chromophore or changes to the opsin. Opsin variation results from changes in opsin sequence, opsin expression or co-expression, and opsin gene duplications and losses. Here, we review visual diversity in a number of teleost groups where the structural and molecular mechanisms underlying their spectral sensitivities have been relatively well determined. Although we document considerable variability, this alone does not imply functional difference per se. We therefore highlight the need for more studies that examine species with known sensitivity differences, emphasizing behavioral experiments to test whether such differences actually matter in the execution of visual tasks that are relevant to the fish.

Costs and compensation in zooplankton pigmentation under countervailing threats of ultraviolet radiation and predation

Evolutionary responses to opposing directions of natural selection include trade-offs, where the phenotype balances selective forces, and compensation, where other traits reduce the impact of one selective force. Zooplankton pigmentation protects from ultraviolet radiation (UVR) but attracts visual predators. This trade-off is understudied in the ocean where planktonic larvae in surface waters face ubiquitous UVR and visual predation threats. We tested whether crab larvae can behaviorally reduce UVR risk through downward swimming or expansion of photoprotective chromatophores. Then we examined whether more pigmented larvae are more heavily predated by silverside fish under natural sunlight in the tropics in three UVR treatments (visible light, visible + UVA, visible + UVA + UVB). Lastly, we tested the behavioral chromatophore response of larvae to predation threats in two light treatments. Armases ricordi avoided surface waters after exposure to sunlight with UVR. Armases ricordi, Armases americanum, and Eurypanopeus sp. consistently expanded chromatophores in UVR or visible light, while Mithraculus sculptus and Mithraculus coryphe showed no response. Fish preferred pigmented larvae on sunnier days in visible light lacking UVR. Lastly, both M. coryphe and M. sculptus unexpectedly expanded chromatophores in fish cues, but responses were inconsistent over trials and across light treatments. The more consistent larval responses to UVR than to predator cues and the lack of predator preferences in natural light conditions suggest that UVR may have a stronger influence on pigmentation than predation. This study improves our understanding of planktonic adaptation to countervailing selection caused by visual predation and exposure to UVR.

Lens transmittance shapes ultraviolet sensitivity in the eyes of frogs from diverse ecological and phylogenetic backgrounds

The amount of short wavelength (ultraviolet (UV), violet and blue) light that reaches the retina depends on the transmittance properties of the ocular media, especially the lens, and varies greatly across species in all vertebrate groups studied previously. We measured the lens transmittance in 32 anuran amphibians with different habits, geographical distributions and phylogenetic positions and used them together with eye size and pupil shape to evaluate the relationship with diel activity pattern, elevation and latitude. We found an unusually high lens UV transmittance in the most basal species, and a cut-off range that extends into the visible spectrum for the rest of the sample, with lenses even absorbing violet light in some diurnal species. However, other diurnal frogs had lenses that transmit UV light like the nocturnal species. This unclear pattern in the segregation of ocular media transmittance and diel activity is shared with other vertebrates and is consistent with the absence of significant correlations in our statistical analyses. Although we did not detect a significant phylogenetic effect, closely related species tend to have similar transmittances, irrespective of whether they share the same diel pattern or not, suggesting that anuran ocular media transmittance properties might be related to phylogeny.

A detailed investigation of the visual system and visual ecology of the Barrier Reef anemonefish, Amphiprion akindynos

Vision plays a major role in the life of most teleosts, and is assumingly well adapted to each species ecology and behaviour. Using a multidisciplinary approach, we scrutinised several aspects of the visual system and ecology of the Great Barrier Reef anemonefish, Amphiprion akindynos, including its orange with white patterning, retinal anatomy and molecular biology, its symbiosis with anemones and sequential hermaphroditism. Amphiprion akindynos possesses spectrally distinct visual pigments and opsins: one rod opsin, RH1 (498 nm), and five cone opsins, SWS1 (370 nm), SWS2B (408 nm), RH2B (498 nm), RH2A (520 nm), and LWS (554 nm). Cones were arranged in a regular mosaic with each single cone surrounded by four double cones. Double cones mainly expressed RH2B (53%) in one member and RH2A (46%) in the other, matching the prevailing light. Single cones expressed SWS1 (89%), which may serve to detect zooplankton, conspecifics and the host anemone. Moreover, a segregated small fraction of single cones coexpressed SWS1 with SWS2B (11%). This novel visual specialisation falls within the region of highest acuity and is suggested to increase the chromatic contrast of Amphiprion akindynos colour patterns, which might improve detection of conspecifics.

Colours and colour vision in reef fishes: Past, present and future research directions

Many fishes, both freshwater or marine, have colour vision that may outperform humans. As a result, to understand the behavioural tasks that vision enables; including mate choice, feeding, agonistic behaviour and camouflage, we need to see the world through a fish's eye. This includes quantifying the variable light environment underwater and its various influences on vision. As well as rapid loss of light with depth, light attenuation underwater limits visual interaction to metres at most and in many instances, less than a metre. We also need to characterize visual sensitivities, fish colours and behaviours relative to both these factors. An increasingly large set of techniques over the past few years, including improved photography, submersible spectrophotometers and genetic sequencing, have taken us from intelligent guesswork to something closer to sensible hypotheses. This contribution to the special edition on the Ecology of Fish Senses under a shifting environment first reviews our knowledge of fish colour vision and visual ecology, past, present and very recent, and then goes on to examine how climate change may impinge on fish visual capability. The review is limited to mostly colour vision and to mostly reef fishes. This ignores a large body of work, both from other marine environments and freshwater systems, but the reef contains examples of many of the challenges to vision from the aquatic environment. It is also a concentrate of life, perhaps the most specious and complex on earth, suffering now catastrophically from the consequences of our lack of action on climate change. A clear course of action to prevent destruction of this habitat is the need to spend more time in it, in the study of it and sharing it with those not fortunate enough to see coral reefs first-hand. Sir David Attenborough on The Great Barrier Reef: "Do we really care so little about the Earth upon which we live that we don't wish to protect one of its greatest wonders from the consequences of our behaviours?"

Differences in ocular media transmittance among classical frog model species and its impact on visual sensitivity

The transmittance properties of the cornea, lens and humours of vertebrates determine how much light across the visible spectrum reaches the retina, influencing sensitivity to visual stimuli. Amphibians are the only vertebrate class in which the light transmittance of these ocular media have not been thoroughly characterised, preventing large-scale comparative studies and precise quantification of visual stimuli in physiological and behavioural experiments. We measured the ocular media transmittance in some commonly used species of amphibians (the bufonids Bufo bufo and Rhinella ornata and the ranids Lithobates catesbeianus and Rana temporaria) and found low transmittance of short wavelength light, with ranids having less transmissive ocular media than bufonids. Our analyses also show that these transmittance properties have a considerable impact on photoreceptor spectral sensitivity, highlighting the need to incorporate this type of measurements into the design of stimuli for experiments on visual function.

Visual system development of the spotted unicornfish, Naso brevirostris (Acanthuridae)

Ontogenetic changes of the visual system are often correlated to shifts in habitat and feeding behaviour of animals. Coral reef fishes begin their lives in the pelagic zone and then migrate to the reef. This habitat transition frequently involves a change in diet and light environment as well as major morphological modifications. The spotted unicornfish, Naso brevirostris, is known to shift diet from zooplankton to algae and back to mainly zooplankton when transitioning from larval to juvenile and then to adult stages. Concurrently, N. brevirostris also moves from an open pelagic to a coral-associated habitat before migrating up in the water column when reaching adulthood. Using retinal mapping techniques, we discovered that the distribution and density of ganglion and photoreceptor cells in N. brevirostris mostly changes during the transition from the larval to the juvenile stage, with only minor modifications thereafter. Similarly, visual gene (opsin) expression based on RNA sequencing, although qualitatively similar between stages (all fishes mainly expressed the same three cone opsins; SWS2B, RH2B, RH2A), also showed the biggest quantitative difference when transitioning from larvae to juveniles. The juvenile stage in particular seems mismatched with its reef-associated ecology, which may be due to this stage only lasting a fraction of the lifespan of these fishes. Hence, the visual ontogeny found in N. brevirostris is very different from the progressive changes found in other reef fishes calling for a thorough analysis of visual system development of the reef fish community.

Sequence and localization of an ultraviolet (sws1) opsin in the retina of the Japanese sardine Sardinops melanostictus (Teleostei: Clupeiformes)

A full-length complementary (c)DNA encoding ultraviolet (UV)-sensitive opsin (sws1) was isolated from the retina of the Japanese sardine Sardinops melanostictus. The sws1 phylogenetic tree showed a sister group relationship with the Cypriniformes, following the ray-finned fish phylogeny. By expressing reconstituted opsin in vitro, it was determined that the maximum absorbance spectrum (λ_max ) of sws1 is around 382 nm, being intermediate in position between two subtypes of sws1 pigment that are UV sensitive (λ_max  = 355-380 nm) and violet sensitive (λ_max  = 388-455 nm), which have been reported to date. The ocular media transmitted >20% transmittance of light in the range of 360-600 nm. In situ hybridization analyses revealed that sws1 messenger (m)RNA is localized in a central single cone surrounded by four double cones in a square mosaic. The square mosaic occupies the ventro-temporal quadrant of the retina and the in situ hybridization signals were dominant in this area suggesting that the fish may use UV vision when looking upward. Based on these results, considerable significances of potential UV sensitivity, in relation to characteristic habits of S. melanostictus, are discussed.

Why UV vision and red vision are important for damselfish (Pomacentridae): structural and expression variation in opsin genes

Coral reefs belong to the most diverse ecosystems on our planet. The diversity in coloration and lifestyles of coral reef fishes makes them a particularly promising system to study the role of visual communication and adaptation. Here, we investigated the evolution of visual pigment genes (opsins) in damselfish (Pomacentridae) and examined whether structural and expression variation of opsins can be linked to ecology. Using DNA sequence data of a phylogenetically representative set of 31 damselfish species, we show that all but one visual opsin are evolving under positive selection. In addition, selection on opsin tuning sites, including cases of divergent, parallel, convergent and reversed evolution, has been strong throughout the radiation of damselfish, emphasizing the importance of visual tuning for this group. The highest functional variation in opsin protein sequences was observed in the short- followed by the long-wavelength end of the visual spectrum. Comparative gene expression analyses of a subset of the same species revealed that with SWS1, RH2B and RH2A always being expressed, damselfish use an overall short-wavelength shifted expression profile. Interestingly, not only did all species express SWS1 - a UV-sensitive opsin - and possess UV-transmitting lenses, most species also feature UV-reflective body parts. This suggests that damsels might benefit from a close-range UV-based 'private' communication channel, which is likely to be hidden from 'UV-blind' predators. Finally, we found that LWS expression is highly correlated to feeding strategy in damsels with herbivorous feeders having an increased LWS expression, possibly enhancing the detection of benthic algae.

Diminished foraging performance of a mutant zebrafish with reduced population of ultraviolet cones

Ultraviolet (UV) cones are photoreceptors that sense light in the range 300-450 nm and are found in the retinas of non-mammalian vertebrates and small mammals. Despite their widespread presence across taxa, the functions that these cones exert in the lives of animals remain largely unknown. In this study, I used the zebrafish lor (lots of rods) mutant, characterized by a diminished UV cone population compared to that of wild-type zebrafish, to test whether its foraging performance differed from that of the wild-type (control). The mean location distance and angle (variables that are reliable indicators of foraging performance) at which control fish detected zooplankton prey were, on average, 24 and 90% greater than corresponding measures for lor fish. Such inferior foraging performance of the mutant could be explained by reduced contrast perception of the prey, resulting from the diminished population of UV cones and associated sensitivity. Thus, UV cones enhance the foraging performance of zebrafish, a crucial ecological function that may explain why small zooplanktivorous fishes retain UV cones throughout their lives.

Change of ultraviolet light transmittance in growing chicken and quail eyes

The ocular media transmittance (OMT), the amount of light transmitted by the ocular media (the cornea, aqueous humour, lens and vitreous humour) to the retina, determines the sensitivity of vertebrate eyes to short-wavelength light, such as ultraviolet (UV). Earlier, we have measured the OMT of adult birds from a range of species and found that smaller eyes transmitted more UV-light to the retina than larger eyes. In the current study we measured OMT during post-hatch development in Japanese quails and domestic chickens. We show that in both species, OMT decreases as the eye size increases similarly to that what was found across various species, but that quails have lower OMT than expected from eye size. In both species, lens transmittance decreases linearly with lens thickness suggesting that UV-transmittance through the lenses is not actively controlled, but instead determined by UV-absorbance and scattering that occur in all biological tissues. Contrary to earlier assumptions of high cornea transmittance, we found that cornea transmittance is more variable, substantially influencing whole eye transmittance in all age groups of quail and in young chickens. It seems that additional absorbing pigments are used to more actively control cornea transmittance and thereby also overall OMT.

Multiple Genetic Mechanisms Contribute to Visual Sensitivity Variation in the Labridae

Coral reefs are one of the most spectrally diverse environments, both in terms of habitat and animal color. Species identity, sex, and camouflage are drivers of the phenotypic diversity seen in coral reef fishes, but how the phenotypic diversity is reflected in the genotype remains to be answered. The labrids are a large, polyphyletic family of coral reef fishes that display a diverse range of colors, including developmental color morphs and extensive behavioral ecologies. Here, we assess the opsin sequence and expression diversity among labrids from the Great Barrier Reef, Australia. We found that labrids express a diverse palette of visual opsins, with gene duplications in both RH2 and LWS genes. The majority of opsins expressed were within the mid-to-long wavelength sensitive classes (RH2 and LWS). Three of the labrid species expressed SWS1 (ultra-violet sensitive) opsins with the majority expressing the violet-sensitive SWS2B gene and none expressing SWS2A. We used knowledge about spectral tuning sites to calculate approximate spectral sensitivities (λmax) for individual species' visual pigments, which corresponded well with previously published λmax values for closely related species (SWS1: 356-370 nm; SWS2B: 421-451 nm; RH2B: 452-492 nm; RH2A: 516-528 nm; LWS1: 554-555 nm; LWS2: 561-562 nm). In contrast to the phenotypic diversity displayed via color patterns and feeding ecology, there was little amino acid diversity within the known opsin sequence tuning sites. However, gene duplications and differential expression provide alternative mechanisms for tuning visual pigments, resulting in variable visual sensitivities among labrid species.

Facing the environment: onset and development of UV markings in young fish

Most colour patterns in animals represent an elegant compromise between conspicuousness to ensure effective communication with preferred receivers and camouflage to avoid attracting the attention of unwanted predators. Many species, including several coral reef fishes, overcome this conflict by using ultraviolet (UV) colouration and signalling, as these colours are visible only over short distances and are often invisible to their predators. Despite a great interest in their behavioural significance and ecological influence on survival, little is known about when these colours first develop on the bodies of free-living animals. Here we show for the first time that the UV facial patterns of a coral reef fish do not develop in captivity but only when juveniles experience the socio-behavioural conditions of their natural environment. Using field and laboratory experiments, we determined that the onset and early development of these UV facial markings did not occur at metamorphosis. Instead, juveniles developed the UV markings during their first two weeks on the reef. Exposure to different reef environments revealed significant plasticity in the development of these markings. The direct or indirect (through intraspecific interactions) exposure to predators is a likely candidate trigger for the plastic development of these UV markings in the wild.

Spectral and spatial selectivity of luminance vision in reef fish

Luminance vision has high spatial resolution and is used for form vision and texture discrimination. In humans, birds and bees luminance channel is spectrally selective-it depends on the signals of the long-wavelength sensitive photoreceptors (bees) or on the sum of long- and middle-wavelength sensitive cones (humans), but not on the signal of the short-wavelength sensitive (blue) photoreceptors. The reasons of such selectivity are not fully understood. The aim of this study is to reveal the inputs of cone signals to high resolution luminance vision in reef fish. Sixteen freshly caught damselfish, Pomacentrus amboinensis, were trained to discriminate stimuli differing either in their color or in their fine patterns (stripes vs. cheques). Three colors ("bright green", "dark green" and "blue") were used to create two sets of color and two sets of pattern stimuli. The "bright green" and "dark green" were similar in their chromatic properties for fish, but differed in their lightness; the "dark green" differed from "blue" in the signal for the blue cone, but yielded similar signals in the long-wavelength and middle-wavelength cones. Fish easily learned to discriminate "bright green" from "dark green" and "dark green" from "blue" stimuli. Fish also could discriminate the fine patterns created from "dark green" and "bright green". However, fish failed to discriminate fine patterns created from "blue" and "dark green" colors, i.e., the colors that provided contrast for the blue-sensitive photoreceptor, but not for the long-wavelength sensitive one. High resolution luminance vision in damselfish, Pomacentrus amboinensis, does not have input from the blue-sensitive cone, which may indicate that the spectral selectivity of luminance channel is a general feature of visual processing in both aquatic and terrestrial animals.

The spectral transmission of non-salticid spider corneas

Although many salticid spiders have been shown to have corneas that transmit ultraviolet (UV) light, whether the corneas of non-salticid spiders transmit UV has not been previously investigated. In this study, we determined the spectral corneal transmission properties of 38 species belonging to 13 non-salticid families. We used these data to estimate the T50 transmission cut-off value, the wavelength corresponding to 50% maximal transmission for each species. The corneas of almost all species from the families Deinopidae, Lycosidae, Oxyopidae, Pisauridae, Sparassidae and Thomisidae, all of which have been reported to rely to a substantial extent on vision, transmitted short wavelength light below 400 nm, ranging from 306 to 381 nm. However, species from the families Atypidae and Ctenizidae are not known to rely substantially on vision, and the corneas of these species tended to absorb light of wavelengths below 380 nm, which may not allow UV sensitivity in these spiders. Liphistiidae, the family widely regarded as most basal among spiders, is of particular interest. The species in this family are not known to make substantial use of vision, and yet we found that liphistiid corneas transmitted UV light with a low T50 value (359 nm). T50 values of non-salticid spider corneas also varied with light habitat. Species living in dim environments tended to have UV-opaque corneas, but species inhabiting open areas had UV-transmitting corneas. However, there was no evidence of corneal transmission properties being related to whether a species is diurnal or nocturnal.

Ultraviolet vision in lacertid lizards: evidence from retinal structure, eye transmittance, SWS1 visual pigment genes, and behaviour

Ultraviolet (UV) vision and UV colour patches have been reported in a wide range of taxa and are increasingly appreciated as an integral part of vertebrate visual perception and communication systems. Previous studies with Lacertidae, a lizard family with diverse and complex coloration, have revealed the existence of UV-reflecting patches that may function as social signals. However, confirmation of the signalling role of UV coloration requires demonstrating that the lizards are capable of vision in the UV waveband. Here we use a multidisciplinary approach to characterize the visual sensitivity of a diverse sample of lacertid species. Spectral transmission measurements of the ocular media show that wavelengths down to 300 nm are transmitted in all the species sampled. Four retinal oil droplet types can be identified in the lacertid retina. Two types are pigmented and two are colourless. Fluorescence microscopy reveals that a type of colourless droplet is UV-transmitting and may thus be associated with UV-sensitive cones. DNA sequencing shows that lacertids have a functional SWS1 opsin, very similar at 13 critical sites to that in the presumed ancestral vertebrate (which was UV-sensitive) and other UV-sensitive lizards. Finally, males of Podarcis muralis are capable of discriminating between two views of the same stimulus that differ only in the presence/absence of UV radiance. Taken together, these results provide convergent evidence of UV vision in lacertids, very likely by means of an independent photopigment. Moreover, the presence of four oil droplet types suggests that lacertids have a four-cone colour vision system.

Ultraviolet vision in birds: the importance of transparent eye media

Ultraviolet (UV)-sensitive visual pigments are widespread in the animal kingdom but many animals, for example primates, block UV light from reaching their retina by pigmented lenses. Birds have UV-sensitive (UVS) visual pigments with sensitivity maxima around 360–373 nm (UVS) or 402–426 nm (violet-sensitive, VS). We describe how these pigments are matched by the ocular media transmittance in 38 bird species. Birds with UVS pigments have ocular media that transmit more UV light (wavelength of 50% transmittance, λ_T0.5, 323 nm) than birds with VS pigments (λ_T0.5, 358 nm). Yet, visual models predict that colour discrimination in bright light is mostly dependent on the visual pigment (UVS or VS) and little on the ocular media. We hypothesize that the precise spectral tuning of the ocular media is mostly relevant for detecting weak UV signals, e.g. in dim hollow-nests of passerines and parrots. The correlation between eye size and UV transparency of the ocular media suggests little or no lens pigmentation. Therefore, only small birds gain the full advantage from shifting pigment sensitivity from VS to UVS. On the other hand, some birds with VS pigments have unexpectedly low UV transmission of the ocular media, probably because of UV blocking lens pigmentation.

High complexity of aquatic irradiance may have driven the evolution of four-dimensional colour vision in shallow-water fish

Humans use three cone photoreceptor classes for colour vision, yet many birds, reptiles and shallow-water fish are tetrachromatic and use four cone classes. Screening pigments, that narrow the spectrum of photoreceptors in birds and diurnal reptiles, render visual systems with four cone classes more efficient. To date, however, the question of tetrachromacy in shallow-water fish, that, like humans, lack screening pigments, is still unsolved. We raise the possibility that tetrachromacy in fish has evolved in response to higher spectral complexity of underwater light. We compared the dimensionality of colour vision in humans and fish by examining the spectral complexity of the colour-signal reflected from objects into their eyes. Here we show that fish require four to six cone classes to reconstruct the colour-signal of aquatic objects at the accuracy level achieved by humans viewing terrestrial objects. This is because environmental light, which alters the colour-signals, is more complex and contains more spectral fluctuations underwater than on land. We further show that fish cones are better suited than human cones to detect these spectral fluctuations, suggesting that the capability of fish cones to detect high-frequency fluctuations in the colour-signal confers an advantage. Taken together, we propose that tetrachromacy in fish has evolved to enhance the reconstruction of complex colour-signals in shallow aquatic environments. Of course, shallow-water fish might possess less than four cone classes; however, this would come with the inevitable loss in accuracy of signal reconstruction.

Spectral transmission of the principal-eye corneas of jumping spiders: implications for ultraviolet vision

Ultraviolet (UV) vision plays an important role in interspecific and intraspecific communication in many animals. However, UV vision and its adaptive significance have been investigated in only approximately 1% of more than 5000 species of jumping spiders (Araneae: Salticidae), renowned for their unique, complex eyes that support exceptional spatial acuity and visually based behaviour. To appreciate the adaptive significance of UV vision, it is important to establish whether salticids can perceive UV and whether the perception of UV varies with ecological factors such as light environment. In this study, we measured the UV-transmission properties of the principal-eye corneas of 128 salticid species. We found that the corneas of all measured species were able to transmit UV light, making the perception of UV possible. Three classes of corneal spectral transmission curves were identified; the majority of species had a Class II curve with a less-steep slope and a gradual onset of the transmission cut-off; all the remaining species had a Class I curve with a very steep slope and a sharp cut-off except for one species that had a Class III curve with an intermediate step, which appeared as a shoulder on the descending part of the transmission curve. The T(50) cut-off transmission values (the wavelength at which 50% of the maximum transmission is reached) in salticid corneas vary with species and light habitat. The corneas of species inhabiting open bush had a higher relative transmission at short wavelengths in the UV than forest species. This is the first investigation of corneal transmission in spiders and suggests that UV perception is widespread in salticids.

Ontogeny in the visual system of Nile tilapia

Retinal neurogenesis in fish facilitates cellular rearrangement throughout ontogeny, potentially allowing for optimization of the visual system to shifts in habitat and behaviour. To test this possibility, we studied the developmental trajectory of the photopic visual process in the Nile tilapia. We examined ontogenetic changes in lens transmission, photoreceptor sensitivity and post-receptoral sensitivity, and used these to estimate changes in cone pigment frequency and retinal circuitry. We observed an ontogenetic decrease in ultraviolet (UV) photoreceptor sensitivity, which resulted from a reduction in the SWS1 cone pigment frequency, and was associated with a reduction in lens transmission at UV wavelengths. Additionally, post-receptoral sensitivity to both UV and long wavelengths decreased with age, probably reflecting changes in photoreceptor sensitivity and retinal circuitry. This novel remodelling of retinal circuitry occurred following maturation of the visual system but prior to reaching adulthood, and thus may facilitate optimization of the visual system to the changing sensory demands. Interestingly, the changes in post-receptoral sensitivity to long wavelengths could not be predicted by the changes observed in lens transmission, cone pigment frequency or photoreceptor sensitivity. This finding emphasizes the importance of considering knowledge of visual sensitivity and retinal processing when studying visual adaptations and attempting to relate visual function to the natural environment. This study advances our understanding of ontogeny in visual systems and demonstrates that the association between different elements of the visual process can be explored effectively by examining visual function throughout ontogeny.

The fish eye view: are cichlids conspicuous?

The extent of animal colouration is determined by an interplay between natural and sexual selection. Both forces probably shape colouration in the speciose, rock-dwelling cichlids of Lake Malawi. Sexual selection is thought to drive male colouration, overcoming natural selection to create conspicuous colour patterns via female mate choice and male-male competition. However, natural selection should make female cichlids cryptic because they mouthbrood their young. We hypothesize that as a result of both sexual and natural selection, males will have colours that are more conspicuous than female colours. Cichlid spectral sensitivity, especially in the ultraviolet, probably influences how colours appear to them. Here we use simple models of the trichromatic colour space of cichlid visual systems to compare the conspicuousness of male and female nuptial colours of nine species. Conspicuousness of colours was evaluated as their Euclidian distance in colour space from environmental backgrounds and from other colours on the same fish. We find in six of the nine species that breeding males have colours that are statistically more conspicuous than female colours. These colours contrast strongly with each other or with the backgrounds, and they fall within a range of spectra best transmitted in the habitat. Female colour distances were sometimes smaller, suggesting that females of some species are more cryptic than males. Therefore, selection can differentially act to generate male colours that are more conspicuous than those in females. However, in two species, females had colours that were more conspicuous than male colours, suggesting that other selective forces and possibly sexual conflicts are acting in this system.