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Non-visual photoreceptors with diverse photopigments allow organisms to adapt to changing light conditions. Whereas visual photoreceptors are involved in image formation, non-visual photoreceptors mainly undertake various non-image-forming tasks. They form specialised photosensory systems that measure the quality and quantity of light and enable appropriate behavioural and physiological responses. Chromatophores are dermal non-visual photoreceptors directly exposed to light and they not only receive ambient photic input but also respond to it. These specialised photosensitive pigment cells enable animals to adjust body coloration to fit environments, and play an important role in mate choice, camouflage and ultraviolet (UV) protection. However, the signalling pathway underlying chromatophore photoresponses and the physiological importance of chromatophore colour change remain under-investigated. Here, we characterised the intrinsic photosensitive system of red chromatophores (erythrophores) in tilapia. Like some non-visual photoreceptors, tilapia erythrophores showed wavelength-dependent photoresponses in two spectral regions: aggregations of inner pigment granules under UV and short-wavelengths and dispersions under middle- and long-wavelengths. The action spectra curve suggested that two primary photopigments exert opposite effects on these light-driven processes: SWS1 (short-wavelength sensitive 1) for aggregations and RH2b (rhodopsin-like) for dispersions. Both western blot and immunohistochemistry showed SWS1 expression in integumentary tissues and erythrophores. The membrane potential of erythrophores depolarised under UV illumination, suggesting that changes in membrane potential are required for photoresponses. These results suggest that SWS1 and RH2b play key roles in mediating intrinsic erythrophore photoresponses in different spectral ranges and this chromatically dependent antagonistic photosensitive mechanism may provide an advantage to detect subtle environmental photic change.

Migratory species experience morphological and physiological changes during transitions between different life stages. In particular, modification of sensory systems is critical for animals to adapt to new environments. For example, to prepare for entry into seawater, salmonids undergo smoltification with dramatic changes in ultraviolet photoreceptors and polarized vision, which are important for orientation and foraging behaviours. Extraretinal organs are also involved in photoreception; however, the ontogenetic development of extraretinal photoreceptors is not well known, especially in migratory species. Here, we investigated whether rainbow trout dermal photoreceptors, melanophores, undergo change in spectral sensitivity during smoltification and which candidate molecules may account for this ontogenetic alteration. Our results showed that, contrary to parr melanophores which are insensitive to light, smolt melanophores displayed chromatic photoresponses with the emergence of cryptochrome and melanopsin expression. We suggest that these modifications may benefit the active foraging behaviour of smolts and enable adaptation to variable environments.

SUMMARY 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.

To examine the effect of wave-induced light fluctuations on the appearance of objects to fish, we recorded the spatial and temporal fluctuations of light reflected from a diffusely reflecting target that served as a simplified proxy for the body of a fish, and of light from the water background that a fish might be viewed against. Measurements were repeated at diverse depths, viewing azimuths, distances to the substrate, and sun conditions. Two conditions that are necessary for wave-induced light fluctuations to make objects more apparent to fish were satisfied. The contrast of light fluctuations reflected from either the object or water background was higher than the minimum contrast value that is detected by fish, or, alternatively, the contrast of light fluctuations reflected from both the object and water background was higher than the minimum contrast value detected by fish, but differed from one another. Furthermore, the frequency range where most of the power of wave-induced radiance fluctuations matched the frequency range of maximum contrast sensitivity in fish. Thus, light stimuli having spatial and temporal characteristics similar to those of wave-induced light fluctuations may make objects more apparent to fish. We suggest that the frequency characteristics of the visual systems of fish were likely shaped by wave-induced light fluctuations in aquatic ecosystems. Variability is one of the most distinctive and often ignored features of the underwater light field. This variability in photon flux, directionality, spectra, and polarization diverges in magnitude and timescale, ranging from seasonal, through diurnal, to short-term fluctuations lasting less than a second. In shallow water, surface waves are the principal cause for short-term variation in the underwater light (Snyder and Dera 1970; Dera and Olszewski 1978; Zaneveld et al. 2001). This variation arises as a result of focusing and defocusing of sunlight rays refracted at the water surface, producing spatial and temporal light fluctuations (Schenck 1957; Snyder and Dera 1970; Stramski and Dera 1988). These fluctuations in underwater light were suggested to influence the visibility of submerged objects (McFarland and Loew 1983; Loew and McFarland 1990). However, this possibility has not yet received adequate attention. Contrast sensitivity (CS) is the relative detection efficiency of contrast between light stimuli as a function of their frequency of modulation. The contrast between light stimuli is commonly estimated as the Michelson contrast (Lmax 2 Lmin)/(Lmax + Lmin), where Lmax and Lmin are the maximum and minimum radiance in the stimulus, respectively (Michelson 1920). Visual systems have a characteristic frequency at which spatially or temporally modulated light stimuli are best detected (Kelly 1972; McFarland and Loew 1983; Douglas and Hawryshyn 1990). This is the frequency at which spatial or temporal CS is highest. The frequency of maximum CS depends on the adaptation state of the eye (i.e., adjustment of the eye sensitivity to the background light), the mean intensity of the modulated light stimulus, and the visual angle

Calcareous otoliths in the inner ears of fishes are necessary for proper hearing and vestibular function. Sagittal otoliths are usually composed of the calcium carbonate polymorph aragonite but may contain the polymorph vaterite, a phenomenon called otolith crystallization. The causes of otolith crystallization are poorly understood. Thyroid hormone (TH) can influence the chemical microenvironment and structure of the inner ear, suggesting that TH may influence otolith crystallization. The present study examined the effect of exogenous TH treatment on sagittal otolith crystallization and growth in larval and juvenile rainbow trout, Oncorhynchus mykiss. In the first experiment, 110–179 day-old fish raised from TH-treated oocytes had significantly fewer sagittal otoliths containing the crystalline calcium carbonate polymorph vaterite as compared to untreated fish. Vaterite-containing otoliths were significantly longer than those containing the typical polymorph aragonite, although there was no effect of TH treatment on otolith length. In the second experiment, juveniles immersed in an exogenous solution of TH for 6 weeks had slightly longer otoliths (relative to fish length) than age-matched controls, but this effect was not significant. This juvenile population had a very high percentage (88.3 %) of vaterite sagittae overall and this percentage did not change significantly with treatment, suggesting the switch from aragonite to vaterite occurred prior to inclusion of the fish in the study. These results suggest that early manipulation of TH levels may affect calcium carbonate deposition on the otolith but that later TH exposure is unable to restore typical otolith composition.

Thyroid hormones (THs) play a vital role in vertebrate neural development, and, together with the beta isoform of the thyroid hormone receptor (TRβ), the development and differentiation of cone photoreceptors in the vertebrate retina. Rainbow trout undergo a natural process of cone cell degeneration during development and this change in photoreceptor distribution is regulated by thyroxine (T4; a thyroid hormone). In an effort to further understand the role of T4 in photoreceptor ontogeny and later developmental changes in photoreceptor subtype distribution, the influence of enhanced in ovo T4 content on the onset of opsin expression and cone development was examined. Juvenile trout reared from the initial in ovo -treated embryos were challenged with exogenous T4 at the parr stage of development to determine if altered embryonic exposure to yolk THs would affect later T4-induced short-wavelength-sensitive ( SWS1 ) opsin transcript downregulation and ultraviolet-sensitive (UVS) cone loss. In ovo TH manipulation led to upregulation of both SWS1 and long-wavelength-sensitive (LWS) opsin transcripts in the pre-hatch rainbow trout retina and to changes in the relative expression of TR β. After 7 days of exposure to T4, juveniles that were also treated with T4 in ovo had greatly reduced SWS1 expression levels and premature loss of UVS cones relative to T4-treated juveniles raised from untreated eggs. These results suggest that changes in egg TH levels can have significant consequences much later in development, particularly in the retina. * D : degree days D2 : deiodinase 2 LWS : long wavelength sensitive PL : photoreceptor layer qPCR : quantitative real-time RT-PCR RH : rhodopsin RPE : retinal pigment epithelium SWS : short wavelength sensitive T3 : triiodothyronine T4 : thyroxine TH : thyroid hormone TR : thyroid hormone receptor U : control UVS : ultraviolet sensitive

– The aquatic environment is being perturbed globally through increases in turbidity, which can have detrimental effects for the maintenance of fish diversity, especially in species dependent on visual cues for reproduction and species recognition. We performed a short-term manipulation of the visual environment in Lake Malawi to test for an immediate behavioural response to increased turbidity in territorial rock-dwelling cichlid fishes that use colourful visual cues to maintain territories near the substrate and attract mates. We found a significant movement of fish away from the substrate, with a concomitant shift from displaying territorial and courting behaviours to foraging behaviours, during the five minutes following the release of a turbidity plume over the area. This study is the first to test for and demonstrate an immediate behavioural response of a natural fish population to a short-term increase in turbidity that might mimic the initial (i.e., immediate) stage of a run-off event after rainfall in a deforested area.

In this review, we will discuss the recent literature on fish polarization vision and we will present a model on how the retina processes polarization signals. The model is based on a general retinal-processing scheme and will be compared with the available electrophysiological data on polarization processing in the retina. The results of this model will help illustrate the functional significance of polarization vision for both feeding behaviour and navigation. First, we examine the linkage between structure and function in polarization vision in general.

SUMMARY Lake Malawi boasts the highest diversity of freshwater fishes in the world. Nearshore sites are categorized according to their bottom substrate, rock or sand, and these habitats host divergent assemblages of cichlid fishes. Sexual selection driven by mate choice in cichlids led to spectacular diversification in male nuptial coloration. This suggests that the spectral radiance contrast of fish, the main determinant of visibility under water, plays a crucial role in cichlid visual communication. This study provides the first detailed description of underwater irradiance, radiance and beam attenuation at selected sites representing two major habitats in Lake Malawi. These quantities are essential for estimating radiance contrast and, thus, the constraints imposed on fish body coloration. Irradiance spectra in the sand habitat were shifted to longer wavelengths compared with those in the rock habitat. Beam attenuation in the sand habitat was higher than in the rock habitat. The effects of water depth, bottom depth and proximity to the lake bottom on radiometric quantities are discussed. The radiance contrast of targets exhibiting diffused and spectrally uniform reflectance depended on habitat type in deep water but not in shallow water. In deep water, radiance contrast of such targets was maximal at long wavelengths in the sand habitat and at short wavelengths in the rock habitat. Thus, to achieve conspicuousness, color patterns of rock- and sand-dwelling cichlids would be restricted to short and long wavelengths, respectively. This study provides a useful platform for the examination of cichlid visual communication.

The effects of handling time, ambient light intensity, and anaesthetic method, on the collection of spectral re- flectance data were assessed in two species of Malawi cichlids (Melanochromis auratus and Metriaclima zebra). Using spectrometry, colour patches were measured over 10 min, under increasing ambient light levels, and using three anaes- thetic methods. As time elapsed, maximum percent reflectance (Rmax) decreased across all anaesthetic methods and spe- cies, suggesting that measurements should be taken immediately after capture and anaesthetization. With increasing light intensity, Rmax increased significantly, suggesting that measurements should be taken under natural ambient conditions when possible. Finally, we found that anaesthetizing fish using an ice bath produced significantly higher Rmax than using either MS-222 (tricaine methanesulphonate) or clove oil (eugenol). However, the highest proportion of ultraviolet (UV) colouration was recorded while fish were anaesthetized with clove oil. Our results highlight the variation involved in measuring fish colour patterns using two related species, and thus indicate the need for a standardized approach to collect- ing spectral reflectance data in fish. Resume´ : Nous avons evalueles effets du temps de manipulation, de l'intensitelumineuse ambiante et de la methode d'anesthesie sur la recolte de donnees de reflectance spectrale chez deux especes de cichlides du Malawi (Melanochromis auratus et Metriaclima zebra). Nous avons mesurepar spectrometrie des taches de couleur pendant 10 minutes sous des intensites croissantes de lumiere ambiante et en employant trois methodes d'anesthesie. Amesure que le temps passe, le pourcentage maximum de reflectance (Rmax )d ecrooˆt sous toutes les methodes d'anesthesie et pour les deux especes, ce qui indique que les mesures devraient etre faites immediatement apres la capture et l'anesthesie. Sous une intensitelumineuse croissante, Rmax augmente de maniere significative, ce qui laisse croire que les mesures devraient se faire, si possible, dans les conditions ambiantes naturelles. Enfin, l'anesthesie des poissons dans un bain de glace produit un Rmax significative- ment plus eleveque l'utilisation de MS-222 (methanesulfonate de tricao¨ne) ou de l'huile de clou (eugenol). Cependant, la plus grande proportion de coloration ultraviolette (UV) est enregistree lorsque les poissons sont anesthesies avec de l'huile de clou. Nos resultats soulignent la variation qui existe dans la mesure des patrons de coloration de deux poissons apparen- tese t demontrent donc la necessited'une methode standardisee pour la recolte de donnees de reflectance chez les poissons. (Traduit par la Redaction)

The spectral beam attenuation coefficient is an important optical property of natural water used to quantify light propagation and visibility in the aquatic media, and to study the concentration of the water constituents. Although beam attenuation in the ultraviolet spectral range may be particularly informative, to date, no transmissometer capable of measuring the beam attenuation in the ultraviolet is commercially available. The portable hyperspectral beam transmissometer developed in our lab is capable of measuring across a broad spectral range (300–750 nm) at 2 nm spectral resolution. The transmissometer exhibits a small acceptance angle (0.55 to 0.59° across the spectrum), a well collimated spectral light beam, and precision of ±0.012 m−1. The attenuation of diverse water samples measured with our transmissometer was found to be significantly similar to that measured with a commercially available transmissometer. Moreover, the attenuation of filtered samples, measured with our transmissometer, was significantly similar to their absorption, measured with a bench-top spectrometer. Testing the transmissometer in the field, the transmission of water samples collected in Lake Malawi, Africa, was measured on site. The magnitude and spectral shape of attenuation were in general agreement with previous reports. All assessment stages confirm the performance, accuracy, and applicability of our transmissometer. The extended spectral range and high spectral resolution of our portable transmissometer make it an excellent tool for studying the characteristics and distribution of dissolved and particulate matter in aquatic media and exploring the constraints imposed on the visibility and visual communication of aquatic organisms known to have ultraviolet photosensitivity.

SUMMARY We were interested in comparing the characteristics of polarization sensitivity in Atlantic salmon to those in Pacific salmon. Here we show that the common ancestor to the clade containing Salmo salar, Oncorhynchus mykiss, O. nerka, O. clarkii and Salvelinus fontinalis has the trait of ultraviolet polarization sensitivity. We examined spectral and polarization sensitivity of juvenile Atlantic salmon (Salmo salar) using both optic nerve compound action potential (CAP) and electroretinogram (ERG) recordings. Our experiments employed photic manipulation to adjust the sensitivity of the four cone mechanisms of Atlantic salmon. A spectrally broad background was used to ensure a contribution of all cone mechanisms to both spectral and polarization sensitivity. Chromatic adaptation was used to isolate the sensitivity of each of the four cone mechanisms for both spectral and polarization sensitivity. Under spectrally broad conditions, UV sensitive (UVS), mid wavelength sensitive (MWS) and long wavelength sensitive (LWS) cone mechanisms contributed to polarization sensitivity. CAP recordings produced the typical ‘W’ shaped polarization sensitivity curve reflecting two active polarization detectors with peaks at e-vector orientations of 0 deg, 90 deg and 180 deg, and troughs at 30 deg and 150 deg. ERG recordings produced a four-peaked polarization sensitivity curve reflecting two active polarization detectors and negative feedback activity, with peaks at e-vectors 0 deg, 45 deg, 90 deg, 135 deg and 180 deg, and troughs at 30 deg, 60 deg, 120 deg and 150 deg. Polarization-sensitivity measurements of isolated cone mechanisms revealed two orthogonal polarization detector mechanisms in Atlantic salmon, identical to that found in rainbow trout and other Pacific salmonid fishes. Moreover, under spectrally broad background conditions, CAP and ERG polarization sensitivity of Atlantic salmon did not differ significantly from that reported in Pacific salmonids.

The UVS cone mechanism is known to light adapt at low intensities in comparison to other cones. We were interested in whether this property was related to higher sensitivity in UVS cones or to network adjustments in sensitivity. We investigated spectral sensitivity of 107 individual cone photoreceptors in rainbow trout (Oncorhynchus mykiss) using a whole-cell voltage clamp technique. Mean time-to-peak response was 339+/-90ms and flash sensitivity for a 100ms flash was 4.37x10(-3)+/-2.50x10(-3)pAphotons(-1)microm(2), with no significant differences between the UVS, SWS, MWS and LWS cone classes. The spectral sensitivity of the UVS, SWS and LWS cones conformed to the expression of SWS1, SWS2 and LWS opsin genes. The spectral sensitivity of MWS cones, however, showed clear evidence of co-expression of RH2a and RH2b opsin pigments. The fish used in this study bridged the ontogenetic stage where the MWS cones shift their expression from RH2b to RH2a.

SUMMARY The cichlid fish radiations of the African Great Lakes are an important model for evolutionary biology. Cichlids have diverse colour vision systems and predominately express three cone visual pigments. However, rare populations of spectrally distinct cones have been found in a number of species, but it is not known whether they contribute to spectral sensitivity. Adult Nile tilapia, Oreochromis niloticus, an ancestral outgroup to the cichlid radiations in the Great Lakes, have three cone types: short-wavelength sensitive (SWS), medium-wavelength sensitive (MWS) and long-wavelength sensitive (LWS) cones, but evidence from microspectrophotometry and cone opsin gene expression suggests they may also have violet-sensitive (VS) cones. We used electrophysiology to assess spectral sensitivity in this species and found evidence of four sensitivity peaks in the ranges 380–420, 440–480, 500–600 and 600–680 nm, with maximal sensitivity at longer wavelengths. The continued presence of a 380–420 nm peak under long-wavelength chromatic adapting backgrounds indicates that this is due to a VS cone mechanism not the β-band of the LWS cone mechanism. Differences in spectral sensitivity curves recorded at different times of year revealed evidence of A1/A2 shifts. The presence of notches in the sensitivity curves and a multiple-mechanisms model used to assess cone contributions indicated that the curves are the result of four cone mechanisms (VS, SWS, MWS and LWS cones) and that chromatically opponent processes occur between mechanisms. The spectral transmittance of the lens steeply declines between 410–380 nm, limiting the short-wavelength limb of the VS cone. As adults, Nile tilapia appear to possess the necessary retinal mechanisms for colour vision. While maximal sensitivity to longer wavelengths is an adaptation to the wavelengths of light predominantly available in their natural habitats, their broad sensitivity range suggests that Nile tilapia possess a flexible, generalised visual system able to adapt to changes in visual environment in their highly variable natural habitat.

Information on the anatomy of the eye and the topography of cone photoreceptor cells in the retina is presented for the Nile Tilapia (Oreochromis niloticus). In adults, the shape and proportions of the ocular components of the prominent eye conform to the general form of fish eyes, as determined using cryo-sectioned eyes. The lens is approximately spherical and there is little variation in the distance from the centre of the lens to the border between the choroid and retina at a range of angles about the optical axis. The average ratio of the distance from the centre of the lens to the retina: lens radius (Matthiessen’s ratio) is 2.44:1. In retinal wholemounts, single and double (twin) cone photoreceptors, forming a square mosaic, are present. Peak photoreceptor densities for both morphological cone types are found in the temporal retina. Using peak cone densities and estimates of focal length from cryo-sectioned eyes, visual acuity is calculated to be 5.44 cycles per deg. The lack of apparent specific ocular or retinal specializations and the relatively low visual acuity reflect the lifestyle of the Nile Tilapia and may allow it to adapt to changes in visual environment in its highly variable natural habitat as well as contributing to the ‘ecological flexibility’ of this species.

SUMMARY Rainbow trout possess ultraviolet-sensitive (UVS) cones in their retina that degenerate naturally during development. This phenomenon can be induced with exogenous thyroxine [T4, a thyroid hormone (TH)] treatment. However, the previous T4 exposure experiments employed static water immersion; a method that could introduce confounding stress effects on the fish. Because of this, it was uncertain if T4 alone was sufficient to induce retinal changes or if stress-related hormones were also necessary to initiate this process. Furthermore, it was unclear whether endogenous T4 was the factor responsible for initiating natural UVS cone loss during development. The current study examined the role of systemic T4 on the juvenile rainbow trout retina using a slow-release implant. Exogenous T4 treatment resulted in SWS1 opsin downregulation and UVS cone loss after four weeks of exposure, signifying that T4 is sufficient to induce this process. Blocking endogenous T4 production with propylthiouracil (PTU, an anti-thyroid agent) attenuated SWS1 downregulation and UVS cone loss in the retina of naturally developing rainbow trout, suggesting that endogenous T4 is necessary to initiate retinal remodelling during development. Quantitative real-time RT-PCR analysis demonstrated that several TH-regulating components are expressed in the trout retina, and that expression levels of the TH receptor isoform TRβ and the type 2 deiodinase (D2) change with T4 treatment. This suggests that T4 may act directly on the retina to induce UVS cone loss. Taken together, these results demonstrate that systemic TH is necessary and sufficient to induce SWS1 opsin downregulation and UVS cone loss in the retina of juvenile rainbow trout.

SUMMARY Rainbow trout undergo natural cone degeneration and thus are interesting models for examining mechanisms of neural degeneration. They have ultraviolet-sensitive (UVS) cones that are lost over most of the retina during development; only a small functional population remains in the dorsal retina. How this spatial distribution of UVS cones is maintained is unclear. Thyroxine(T4) induces UVS cone loss, and local thyroid hormone regulation was hypothesized to control UVS cone distribution. Thyroid hormone receptor alpha(TRα), thyroid hormone receptor beta (TRβ) and Type 2 deiodinase(D2) regulate thyroid hormone exposure to target cells. Regional retinal expression of these genes was investigated during exogenous T4 treatment and natural smoltification of rainbow trout. Each retina from dark-adapted parr,T4-treated parr and natural smolts was divided into four quadrants, and total RNA was isolated. Quantitative real-time RT-PCR analysis demonstrated that all retinal quadrants had increased accumulation of TRβ transcripts 2 days post-T4 treatment, corresponding to initiation of SWS1 opsin downregulation. Smolts exhibited decreased accumulation of TRαand TRβ transcripts in all quadrants, but this effect was most pronounced in the dorso-temporal (DT) retinal quadrant where UVS cones persist. By contrast, in 2 day T4-treated parr, the DT quadrant showed increased expression of TRα and TRβ. Furthermore, D2 transcripts decreased in the DT quadrant of T4-treated parr but increased in the DT quadrant of smolts. These results suggest that T4 upregulates TRβ expression to initiate SWS1 opsin downregulation, while TRα and TRβ downregulation occurs to prevent natural loss of UVS cones from the DT retina.

A number of teleost fishes have photoreceptor mechanisms to detect linearly polarized light. We studied the neuronal mechanism underlying this ability. It was found that a polarized signal could be detected in rainbow trout (Oncorhynchus mykiss) both in the electroretinogram (ERG) and in the compound action potential (CAP) measured in the optic nerve, indicating a strong retinal contribution to the processing of polarized light. The CAP recordings showed a W-shaped sensitivity curve, with a peak at 0 degrees , 90 degrees and 180 degrees , consistent with processes for both vertical and horizontal orientation. By contrast, the ERG recordings reveal a more complex pattern. In addition to the peaks at 0 degrees , 90 degrees and 180 degrees , two additional peaks appeared at 45 degrees and 135 degrees . This result suggests a specialized contribution of the outer retina in the processing of polarized light. The spectral sensitivity of the mechanisms responsible for these intermediate peaks was studied using chromatic adaptation. Here we show that long wavelength-sensitive (LWS) cone mechanism adaptation shifted the intermediate peaks towards 90 degrees , whereas ultraviolet-sensitive (UVS) cone mechanism adaptation shifted the peaks away from 90 degrees towards either 0 degrees or 180 degrees . These results provide further confirmation that the 90 degrees peak is dominated by the LWS cone mechanism and the 0 degrees and 180 degrees peaks are dominated by the UVS cone mechanism. In addition, a pharmacological approach was used to examine the retinal neural mechanisms underlying polarization sensitivity. The effect of blocking negative feedback from horizontal cells to cones on the ERG was studied by making intraocular injections of low doses of cobalt, known to block this feedback pathway. It was found that the intermediate peaks seen in the ERG polarization sensitivity curves were eliminated after application of cobalt, suggesting that these peaks are due to outer retinal inhibition derived from feedback of horizontal cells onto cones. A simple computational model was developed to evaluate these results. The model consists of opponent and non-opponent processing elements for the two polarization detectors. This model provides a first approximation analysis suggesting that opponent processing occurs in the outer retina for polarization vision. Although it seems that polarization vision uses a slightly more complicated coding scheme than colour vision, the results presented in this paper suggest that opponent and non-opponent channels process polarization information.

Ultraviolet-sensitive (UVS) cones disappear from the retina of salmonid fishes during a metamorphosis that prepares them for deeper/marine waters. UVS cones subsequently reappear in the retina near sexual maturation and the return migration to natal streams. Cellular mechanisms of this UVS cone ontogeny were investigated using electroretinograms, in situ hybridization, and immunohistochemistry against opsins during and after thyroid hormone (TH) treatments of rainbow trout (Oncorhynchus mykiss). Increasing TH levels led to UVS cone degeneration. Labeling demonstrated that UVS cone degeneration occurs via programmed cell death and caspase inhibitors can inhibit this death. After the cessation of TH treatment, UVS cones regenerated in the retina. Bromodeoxyuridine (BrdU) was applied after the termination of TH treatment and was detected in the nuclei of cells expressing UVS opsin. BrdU was found in UVS cones but not other cone types. The most parsimonious explanation for the data is that UVS cones degenerated and UVS cones were regenerated from intrinsic retinal progenitor cells. Regenerating UVS cones were functionally integrated such that they were able to elicit electrical responses from second-order neurons. This is the first report of cones regenerating during natural development. Both the death and regeneration of cones in retinae represent novel mechanisms for tuning visual systems to new visual tasks or environments.

SUMMARYIn this study, we demonstrate the capacity for damselfish (green chromis, Chromis viridis) to discriminate between different e-vector orientations of ultraviolet polarized light. We examined the ability of green chromis to resolve small differences in e-vector orientation of ultraviolet polarized light. Fish were successfully trained to swim towards an e-vector orientation of polarized light using a behavioural chamber. C. viridis was able to discriminate between the horizontal and the vertical plane of ultraviolet polarized light independent of brightness content of the stimuli. However, e-vector discrimination capability disappeared when the ultraviolet portion of the light stimuli was removed, indicating that the presence of ultraviolet light was critical for e-vector discrimination. Fish could also distinguish between relatively small e-vector orientations of ultraviolet polarized light. Functional implications for high e-vector discriminative capabilities could be used in functional domains such as feeding and communication.

Wavelength-dependent properties of tectal evoked potentials (TEPs) in rainbow trout, Oncorhynchus mykiss (Walbaum, 1792), were examined. It was found that TEP waveforms show distinct variation as a function of wavelength. In addition, the data suggest that the On and Off channels of the tectum each possess different wavelength-dependent characteristics. Middle wavelength stimulation typically evoked a waveform similar to that reported for another anamniote vertebrate, the toad (genus Bufo Laurenti, 1768). For the On and Off responses, this waveform comprised two negative deflections, N1 and N2, which were interrupted by a positive deflection, P2. The N2 deflection was followed by a final positive deflection, P3. Principal component analysis revealed that the N2–P3 complex of the On response was significantly more pronounced at longer wavelengths. In contrast, the N2–P3 complex was most pronounced at middle wavelengths for the Off response. The N1 deflection was relatively invariant with respect to wavelength. Should colour-opponent tectal units provide a significant contribution to the TEP, it is probable that its waveform characteristics provide signs of underlying neural processes which facilitate colour discrimination.

We made multi-unit recordings from the optic tectum of Betta splendens while they viewed the agonistic display of conspecifics. Movement of displaying animals evoked brisk, arrhythmic bursting in tectal units. Particularly high frequency spike bursts coincided with the onset of full display, a behavioral element known to be of ethological significance. These results show that it is possible to obtain informative electrophysiological recordings from optic tectum using behaving conspecifics as stimuli. More frequent use of natural stimuli can be expected to contribute to a greater understanding of the link between the neural substrates of vision and visually guided behavior.

We have examined the vertebrate visual system from a liquid crystalline perspective, with the aim of understanding the mechanisms responsible for polarization vision in vertebrates. Using a technique called micro-spectrophotometry (MSP), we show that the different types of light-sensitive cells in the retina absorb polarized light differently. Based on these measured geometries of absorbance, analytic solutions to Maxwell's equations using a 4×4 matrix technique demonstrate the possibility of intrinsic linear dichroism under axial illumination in those photoreceptors sensitive to polarized light. This provides a new mechanism for axial polarization sensitivity in vertebrate photoreceptors.

Divergent mate choice patterns between populations may lead to premating isolation and speciation. However, similar or identical mate choice patterns may also constrain phenotypic divergence, although not necessarily genetic divergence, across populations. We examined spectral characteristics necessary for male mate choice of abstract female models in sockeye salmon, Oncorhynchus nerka , in experiments conducted on the breeding grounds and under controlled conditions in the wild. Males preferentially selected and spawned with models with a red hue, the predominant colour of females. Small changes in wavelength, saturation and brightness affected preference. Sexually naive males showed the same preference as experienced males, suggesting that a preference for red hue might be innate. We discuss these results in relation to female colour, the transmission environment and the visual physiology of salmon to evaluate further the possible role of a pre-existing bias for red in the evolution of red breeding colour in nonanadromous (kokanee) salmon. Our results suggest that a sensory bias can account for the re-emergence of red in kokanee, after its loss during the initial development of a freshwater morph.

We examined orientation responses of juvenile salmonids to a polarized-light stimulus under laboratory conditions. Juvenile rainbow trout, Oncorhynchus mykiss , steelhead (anadromous), O. mykiss , and brook char, Salvelinus fontinalis , were trained using an operant conditioning methodology to orient relative to the axis of a linear-polarized light field. On average, rainbow trout responded correctly relative to the orientation of the light stimulus approximately 70% of the time within five training sessions. The proportion of correct responses increased further under an intermittent reinforcement schedule. We released trained and untrained fish individually in a circular tank and quantified orientation responses using a digital image-tracking system. Experimentally naive rainbow trout had no directional tendency, in contrast to trained rainbow trout, steelhead and brook char, which oriented relative to the plane of polarized light. Trained fish showed no orientation response when a diffuser was used to depolarize the light source. Rainbow trout trained to orient parallel to a polarization axis in the laboratory were tested under natural skylight before sunset. These fish oriented parallel to the bearing of maximally polarized light in the celestial hemisphere. Copyright 2003 Published by Elsevier Science Ltd on behalf of The Association for the Study of Animal Behaviour.

SUMMARY Ultraviolet visual sensitivity appears to be reduced and, possibly, lost during smoltification in anadromous populations of salmonid fishes. Similar changes occur in non-anadromous salmonids over a mass range that is associated with smoltification in their anadromous conspecifics. However, in sexually mature adult salmonids, ultraviolet-sensitive cones are present in the dorso-temporal retina, suggesting that ultraviolet sensitivity (i) may be regained with sexual maturity or (ii) might never be completely lost. Both smoltification and the transition to sexual maturity are regulated, in part, by the hormone thyroxine. Thyroxine treatment of juvenile Oncorhynchus mykiss results in precocial developmental changes that mimic smoltification, including a reduction of ultraviolet sensitivity. However, whether loss of ultraviolet sensitivity in O. mykiss or in other species of salmonids is complete during normal development (or in response to thyroxine treatment) is unclear. In the present study, we have ‘mapped’ topographically ultraviolet photosensitivity during natural and hormone-induced smoltification. Thyroxine-treated O. mykiss juveniles and anadromous steelhead O. mykiss smolts were examined for ultraviolet visual sensitivity by recording compound action potentials from the optic nerve. By selectively illuminating either the dorsal or the ventral retina, we have shown that the reduction of ultraviolet sensitivity occurs primarily in the ventral retina in both groups of fish. Ultraviolet sensitivity remains intact in the dorsal retina.

Orientation responses of juvenile rainbow trout (Oncorhynchus mykiss) to two linearly polarized light patches were examined under controlled laboratory conditions. Fish were trained to swim the length of the training tank under a polarized light field created by two linearly polarized stimuli that were oriented either parallel or perpendicular to the length of the tank. Trained fish were released in a circular tank and their angular responses were recorded. For each testing paradigm, the E-vector (electric vector) orientation of one of the two linearly polarized light patches was varied by 15° between 0° and 90°. Each fish was therefore tested in seven different paradigms in which the two E-vector ori- entations differed by 0°, 15°, 30°, 45°, 60°, 75°, and 90°. Rainbow trout oriented in a bimodal distribution when the two E-vector orientations differed by 0°, 15°, 30°, 45°, and 90°. These results suggest that rainbow trout perceived the two stimuli as being the same when the two E-vector orientations differed by 45° or less. Conversely, rainbow trout did not significantly orient when the two E-vector orientations differed by 60° and 75°. Rainbow trout may be able to discriminate two E-vector orientations that differ between 60° and 75°, and therefore they do not significantly orient, since they perceive two distinct E-vectors to orient to instead of one. When rainbow trout were exposed to a depolar- ized light field, they did not exhibit significant orientation subsequent to the E-vector cue. Resume : L'orientation de Truites arc-en-ciel ( Oncorhynchus mykiss) en reponse a deux taches de lumiere polarisee rectiligne a ete examinee dans des conditions controlees de laboratoire. Les poissons ont ete entraines a nager le long de l'aquarium sous un champ de lumiere polarisee cree par deux stimulus polarises rectilignes, orientes parallelement ou perpendiculairement a la longueur de l'aquarium. Les poissons entraines ont ete relâches dans un aquarium circulaire et l'angle de leurs reactions a ete enregistre. Pour chacun des paradigmes a la base des tests, l'orientation du vecteur E (vecteur electrique) de l'une des taches de lumiere polarisee a ete modifiee de 15° entre 0° et 90°. Chaque poisson a donc ete teste d'apres sept paradigmes differents, dans lesquels l'orientation d'un vecteur E est decalee de 0° ,1 5°, 30° ,4 5° ,6 0° ,7 5° et 90° par rapport a celle de l'autre vecteur E. Les orientations des truites se repartissent selon une distribution bimodale lorsque les deux vecteurs E different de 0° ,1 5° ,3 0° ,4 5° et 90°. Ces resultats indiquent que les truites percoivent les deux stimulus comme s'il s'agissait d'un seul lorsque les orientations des vecteurs E different de moins de 45°. Inversement, les truites ne reagissent pas de facon particuliere lorsque les vecteurs E sont orientes avec une difference de 60° a7 5°. Les truites sont peut-etre capables de distinguer deux orientations avec une difference de 60° a7 5° et elles n'adoptent pas d'orientation particuliere parce qu'elles percoivent les deux vecteurs E plutot qu'un seul pour s'orienter. Lorsque les Truites arc-en-ciel sont par la suite exposees a un champ de lumiere depolarisee, elles ne changent pas le orientation pas de nouveau et gardent celle qu'elles ont adoptee lors de l'exposition aux vecteurs E. (Traduit par la Redaction) 415

Polarization vision in vertebrates has been marked with significant controversy over recent decades. In the last decade, however, models from two laboratories have indicated that the spatial arrangement of photoreceptors provides the basis for polarization sensitivity.Work in my laboratory, in collaboration with I. Novales Flamarique and F. I. Harosi, has shown that polarization sensitivity depends on a well–defined square cone mosaic pattern and that the biophysical properties of the square cone mosaic probably account for polarization vision in the ultraviolet spectrum. The biophysical mechanism appears to be based on the selective reflection of axial–polarized light by the partitioning membrane, formed along the contact zone between the members of the double cones, onto neighbouring ultraviolet–sensitive cones. In this short review, I discuss the historical development of this research problem.

Characterization of visual opsins can be achieved with the use of specific polyclonal or monoclonal antibodies. In this study, antiserum was raised against a conjugate protein including an N-terminal sequence of the short wavelength sensitive (SWS, commonly referred to as blue-sensitive) visual pigment opsin in goldfish (Carrasius auratus). The antiserum showed immunoreactivity to retinae of phylogenetically distant teleosts, including the goldfish, rainbow trout (Oncorhynchus mykiss), and bluehead wrasse (Thalassoma bisasciatum). Complementary DNA (cDNA) was synthesized from goldfish retinal messenger RNA. An N-terminal sequence of the SWS opsin was PCR-amplified from the cDNA and subsequently cloned into an expression vector. Expressed protein was purified by metal chelate affinity chromatography and used to immunize New Zealand white rabbits. Immune serum was collected and used in immunocytochemical assays, which revealed that the antiserum contains antibodies specific for an opsin that resides in single-cone photoreceptors of the goldfish, rainbow trout, and bluehead wrasse. The results confirm those from earlier studies of goldfish in which short-wavelength sensitivity was localized to single cones, and suggest that the association between short-wavelength sensitivity and single cones may be a feature common to many teleosts. The production of antiserum specific for SWS opsins provides a useful tool in the characterization of opsin expression within teleost retinae.

Although tropical coral reefs are one of the most spectrally complex habitats, there is relatively little known about colour vision of reef fish. In this study, we measured the spectral sensitivity of an endemic Hawaiian coral reef fish, Thalassoma duperrey (family Labridae), and assessed the possible role of visual sensitivity in mediating intraspecific communication. Electrophysiological recordings of compound action potentials from retinal ganglion cells were used to generate spectral sensitivity curves for specific wavelengths (380–620 nm). We found at least 2 sensitivity peaks for the on response (λmax=460, 550 nm). The off response lacked a short wavelength mechanism but a medium wavelength mechanism (λmax=545 nm) and a longwave mechanism (λmax=570 nm) were found. To quantify the visual stimulus provided by a conspecific individual, spectral reflectance from the colour pattern of T. duperrey was measured with a spectroradiometer. Luminance and spectral contrast were computed between colour patches of the pattern and between the patches and natural backgrounds (i.e., water and coral). Reflectance from the blue head and contrast from the blue, green and red patches matched the sensitivity maxima of T. duperrey, although this depended on the type of background. Our results indicate that T. duperrey should be able to visually detect the colour pattern of a conspecific fish and that T. duperrey's visual system is designed to enhance target detection in the coral reef habitat with matched and offset cone mechanisms.

Juvenile rainbow trout (Oncorhynchus mykiss) were given ethambutol (900 mg kg−1 body mass per day) for 4 weeks to examine its effects on vision. Using multi-unit recording from the optic nerve, spectral sensitivity of the on-responses were significantly affected in two regions, 340–440 nm and 600–660 nm. Off-responses were statistically unaffected. Changes in sensitivity to polarised light were also observed with on-responses to vertically-polarised light decreasing relative to horizontally-polarised light. In contrast, off-responses were less affected. The treatment effects were attributed to changes in the relative contribution of the photoreceptor channels as recorded at the level of the optic nerve.

Although color-opponent neurons appear to subserve color vision, precisely how these cells encode hue is still not clear. Single-unit, extracellular recordings from the rainbow trout optic tectum were made in order to examine the possible role of action potential timing in coding chromatic stimuli. We found that color-opponent units can exhibit differences in response latency which are a function of wavelength and response sign, with the OFF response exhibiting the shorter response latency. We also found that units often responded with spike bursts characterized by early and late spikes separated by a silent period, with the relative proportion of early and late spikes varying as a function of wavelength. This type of discharge pattern appears to be a result of inhibitory, color-opponent processes. We suggest that complete inhibition of early spikes may be the mechanism underlying the observed latency differences. These findings suggest a role for action potential patterning in coding chromatic stimuli.

We used compound action potential recordings from the optic nerve of anesthetized live fish to study the spectral and polarization sensitivities of the northern anchovy and the Pacific herring. The photoreceptor structure and cone mosaic type of the (illuminated) central retina was studied by microscopy. Both species showed a single peak spectral photopic sensitivity function with λmax= 500 nm for the northern anchovy and λmax= 520 nm for the herring. However, only the northern anchovy exhibited polarization sensitivity; the response was 180° periodic with maximum sensitivity to horizontal polarization. Similar to the bay anchovy (Fineran and Nicol 1978), the central retina of the northern anchovy showed bifid cone units with cone lamellae parallel to the cones' lengths. The herring, on the other hand, had twin cones arranged in rows with the same orientation and tangentially arranged lamellae. Our results support the hypothesis that bifid cone units act as orthogonal dichroic filters rendering anchovies polarization sensitive. The lack of polarization sensitivity in the herring suggests that twin cones may not be used in polarization sensitivity or that one orientation of polarization receptors is insufficient for the animal to detect polarization direction.

Two families of fishes, the Cyprinidae and Salmonidae, exhibit ultraviolet sensitivity and polarization sensitivity (i.e., differential sensitivity to the orientation of the electric field of polarized light). Both of these families possess a square arrangement of double cones and/or their dividing partitions in the centro-temporal retina, an area where polarization sensitivity has been tested for and found. To correlate the presence of an ordered cone mosaic in the centro-temporal retina with polarization sensitivity in ultraviolet-sensitive fishes, we examined the visual system of the common white sucker (Catostomus commersoni) and compared it to those of the above-mentioned families. We found that the common white sucker possesses four cone-mediated neural mechanisms similar to those in cyprinids and salmonids, but it does not exhibit polarization sensitivity. In addition, unlike cyprinids and salmonids, the common white sucker shows a random cone mosaic in the centro-temporal retina. These results suggest that polarization sensitivity in ultraviolet-sensitive fishes requires an ordered double-cone mosaic in this area of the retina.

We examined the retinal cone topography in sexually mature individuals from four species of Pacific salmonid fishes by using semithin plastic sections. We identified variations in cone density and cone arrangements and noted the presence of putative ultraviolet (UV) cones. Putative UV cones were found over an area extending dorsotemporally from the center of the retina. Because most of the putative UV cones are believed to disappear in early ontogeny, their presence over a large proportion (15–20%) of the surface area of the adult retina suggests that they may be reincorporated prior to or at sexual maturity, at least in rainbow trout. Cone density varied across the retina, with highest values at the peripheral margin. Relatively high densities were observed ventrotemporally (in all specimens) and, to a lesser extent, dorsonasally (7 of 11 specimens). The higher cone density in the ventrotemporal retina may represent a retinal specialization in the part of the visual field located above and in front of the animal. Lowest cone densities were found dorsocentrally and coincided approximately with the distribution of putative UV cones, raising the possibility that these cones may not be used in visual tasks requiring the higher visual acuity normally associated with higher cone densities. We also report a novel cone arrangement that consists of rows of double cones inserted between rows composed of single–double cone pairs alternating in position. J. Comp. Neurol. 383:49–59, 1997. © 1997 Wiley-Liss, Inc.

The sensitivities of two species of sunfish (Lepomis gibbosus and Lepomis cyanellus) to the electric field (E-vector) of polarized light were assessed by compound action potential recordings from the optic nerve of live fish. Under white light and long wavelength adapting backgrounds, two cone mechanisms were found with maximum sensitivities in the long wavelength (λmax ≈ 620 nm) and middle wavelength (λmax ≈ 530 nm) regions of the spectrum. In contrast to previous findings ( Cameron and Pugh, 1991 ), no evidence of polarization sensitivity was observed for either species. We conclude from these results that post-larval sunfish do not exhibit polarization sensitivity. © 1997 Elsevier Science Ltd. All rights reserved.

Dermal specialized pigment cells (chromatophores) are thought to be one type of extraretinal photoreceptors responsible for a wide variety of sensory tasks, including adjusting body coloration. Unlike the well-studied image-forming function in retinal photoreceptors, direct evidence characterizing the mechanism of chromatophore photoresponses is less understood, particularly at the molecular and cellular levels. In the present study, cone opsin expression was detected in tilapia caudal fin where photosensitive chromatophores exist. Single-cell RT-PCR revealed co-existence of different cone opsins within melanophores and erythrophores. By stimulating cells with six wavelengths ranging from 380 to 580 nm, we found melanophores and erythrophores showed distinct photoresponses. After exposed to light, regardless of wavelength presentation, melanophores dispersed and maintained cell shape in an expansion stage by shuttling pigment granules. Conversely, erythrophores aggregated or dispersed pigment granules when exposed to short- or middle/long-wavelength light, respectively. These results suggest that diverse molecular mechanisms and light-detecting strategies may be employed by different types of tilapia chromatophores, which are instrumental in pigment pattern formation.

Summary 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.

Polarization sensitivity (PS) in vertebrate vision is controversial, perhaps because its underlying mechanism has remained obscure. An issue that might have added to the controversy is that rainbow trout (Oncorhynchus mykiss), which have served as the primary model system for polarization-based orientation, lose their ability to orient relative to celestial polarized-light patterns when parr (fry) transform into migratory smolts (juveniles), which would benefit most from polarization-based orientation. Here we addressed two key questions: (1) what is the mechanism underling PS?, and (2) how can the paradoxical loss of PS in trout smolts be reconciled? We assessed PS from optic nerve recordings in parr and smolts and found that the retinal region with enhanced PS shifted from the ventral retina in parr to the dorsal retina in smolts. This adaptation may allow fish to use the most reliable polarization field encountered at each life stage, the celestial polarization field in the shallow-swimming parr and the depth-insensitive underwater polarization field in the deep-swimming smolts. In addition, we assessed spectral sensitivity across the retina and during ontogeny and fit a cascade retinal model to PS data. We found that differential contribution of two cone detectors with orthogonal PS could drive the variation in PS and that feedback from horizontal cells to cones could explain the differential amplification of PS. This elegant arrangement, in which weak PS of cones is amplified and tuned by retinal networks, allows for PS without interfering with sampling of other visual information and illustrates how sensory systems may simultaneously process disparate aspects of physical environments.

Color vision is most beneficial when the visual system is color constant and can correct the excitations of photoreceptors for differences in environmental irradiance. A phenomenon related to color constancy is color induction, where the color of an object shifts away from the color of its surroundings. These two phenomena depend on chromatic spatial integration, which was suggested to originate at the feedback synapse from horizontal cells (HC) to cones. However, the exact retinal site was never determined. Using the electroretinogram and compound action potential recordings, we estimated the spectral sensitivity of the photoresponse of cones, the output of cones, and the optic nerve in rainbow trout. Recordings were performed before and following pharmacological inhibition of HC-cone feedback, and were repeated under two colored backgrounds to estimate the efficiency of color induction. No color induction could be detected in the photoresponse of cones. However, the efficiency of color induction in the cone output and optic nerve was substantial, with the efficiency in the optic nerve being significantly higher than in the cone output. We found that the efficiency of color induction in the cone output and optic nerve decreased significantly with the inhibition of HC-cone feedback. Therefore, our findings suggest not only that color induction originates as a result of HC-cone feedback, but also that this effect of HC-cone feedback is further amplified at downstream retinal elements, possibly through feedback mechanisms at the inner plexiform layer. This study provides evidence for an important role of HC-cone feedback in mediating color induction, and therefore, likely also in mediating color constancy.

Summary Signal reception and production form the basis of animal visual communication, and are largely constrained by environmental light. However, the role of environmental light in producing variation in either signal reception or production has not been fully investigated. To chart the effect of environmental light on visual sensitivity and body colouration throughout ontogeny, we measured spectral sensitivity, lens transmission, and body pattern reflectance from juvenile and adult Nile tilapia held under two environmental light treatments. Spectral sensitivity in juveniles reared under a broad-spectrum light treatment and a red-shifted light treatment differed mostly at short wavelengths, where the irradiance of the two light treatments differed the most. In contrast, adults held under the same two light treatments did not differ in spectral sensitivity. Lens transmission in both juveniles and adults did not differ significantly between environmental light treatments, indicating that differences in spectral sensitivity of juveniles originated in the retina. Juveniles and adults held under the two environmental light treatments differed in spectral reflectance, and adults transferred to a third, white light treatment differed in spectral reflectance from their counterparts held under the two original treatments. These results demonstrate that environmental light plays a crucial role in shaping signal reception in juveniles and signal production throughout ontogeny, reinforcing the notion that environmental light has the capacity to influence animal communication, and suggesting that the characteristics of environmental light should be considered in models of ecological speciation.

Threespine stickleback vary greatly in male nuptial display, from full red expression between the throat and pelvis to total melanism without any traces of red. The dominant expression for any population is correlated with the underwater spectral irradiance and many populations with melanistic nuptial colour occur in bog habitats where the aquatic spectrum is shifted to long wavelengths. Field observations indicate that in these bog habitats, the typical red nuptial signal is 'spectrally masked' and this might reduce the effectiveness of display during male-male and male-female interactions. We used video imaging techniques to test whether the spectral composition of the visual background influences female preference for nuptial colouration in male threespine sticklebacks. Throat colour preference tests were carried out in which females chose between video images of red and black-throated males superimposed on either a blue or a red background. Despite belonging to a red-throated population, females were found to respond preferentially to the black-throated male image viewed against a red background, indicating that preference was not determined by throat patch hue. This suggests that preference for an epigamic optical signal is likely a function of its efficacy in generating visual contrast rather than its intrinsic spectral characteristics.

Small (90 g) rainbow trout, which had lost their ultraviolet cones during normal development, were once again ultraviolet-photosensitive and small accessory corner cones were found in their retinas. These results imply that the ultraviolet-sensitive cones, although lost at one point during development, can reappear at another time during the life history of the same individual. Retinoic acid is involved in these morphogenetic processes.

We investigated retina preparations of young rainbow trout ( Oncorhynchus mykiss ) with body wt 5–40 g. Rods, single and double cones were measured in side-on orientation by microspectrophotometry, identifying five spectrally distinct visual pigments (or photoreceptors containing mixtures of visual pigments). The mean wavelength of peak absorbance (λ max ) of the α-bands were 365 and 434 nm in single cones, 531 and 576 nm in double cones, and 521 nm in the rods. The half-band width (HBW) of the main absorption bands were broader than expected of retinal-(vitamin A 1 -) based visual pigments, and thus, they were indicative of a mixed chromophore pool derived from both the vitamin A 1 and A 2 forms. One consequence of the utilization of mixed chromophores is the broadening of the α-band absorption in each pigment type. And yet, we obtained exceptionally narrow HBW for the UV-type pigment, when compared with HBW values expected on the basis of the linear trend seen in visual pigments absorbing in the visible spectrum. We conclude that the UV pigment in rainbow trout has an unusually narrow HBW. Nevertheless, this species is not exceptional in this regard, for the UV-absorbing visual pigments in other vertebrate species also have narrow HBW.

Color vision in rainbow trout was studied by characterizing the spectral sensitivity of single units in three areas of visual processing: optic nerve; optic tectum; and torus semicircularis. Sensitivity to medium wavelength stimuli was a common feature of all single units examined. Additionally, long wavelength sensitivity was found in all units that were not medium wavelength-only or monophasic. Ultraviolet and short-wavelength sensitivity was common in small, juvenile trout, with ultraviolet sensitive units found in the optic nerve and torus, and short wavelength sensitivity found in the optic nerve and tectum. The shorter wavelength inputs were excitatory and, if found in the same unit, synergistic. The most common type of unit in the trout tectum and optic nerve is trichromatic, with ON inputs from the long and short cone mechanisms and an OFF input from the medium mechanism. In contrast, goldfish color vision is dominated by L and M opponent units without S input. The segregation of ultraviolet sensitivity in the torus but not in the tectum relates to functional differences of these two areas. While the tectum serves the function of wavelength discrimination, ultraviolet inputs to the torus may contribute to prey detection and orientation.

Spectral irradiance was measured from 300 to 850 nm under clear and overcast skies in Saanich Inlet and Trevor Channel (Vancouver Island) using an underwater spectroradiometer. The values obtained were corrected for ocular media transmission and pigment absorption of the four visual pigments in small rainbow trout, Oncorhynchus mykiss (UV, short, middle, and long wavelength sensitive), to estimate the light available to stimulate each of these photoreceptors. Light levels were compared with particle counts, Chl a, and dissolved organic matter (DOM) concentrations at various sites and with similar parameters previously measured for freshwater systems. There was sufficient light to stimulate all the photoreceptors down to 15 m, which was the limiting depth in Saanich Inlet for UV cone simulation and for stimulation of the β absorption band of middle wavelength cones. UV light levels diminished with increasing DOM and Chl a concentrations; however, no statistically discernable relationship was found between UV levels and Chl a concentrations with distance from the spawning rivers. Under completely overcast conditions, directionality of the sun's rays is maintained under some types of cloud cover. This indicates that navigation using polarized light may occur under variable atmospheric conditions provided enough polarized light is present in the water column.

Integrated spike activity of axons from the optic nerve was measured in an investigation of the e-vector sensitive mechanism underlying the ability of rainbow trout (Oncorhynchus mykiss) for orientation in downwelling, linearly-polarized light. In anaesthetized, immobilized fish, one eye was exposed to incremental light flashes which were superimposed over closely controlled background conditions. Under scotopic and various photopic conditions, intensity/response curves were generated from the on-response of the optic nerve. Relative sensitivity curves were then obtained as a function of e-vector direction for the 5 kinds of receptor cells in this trout's retina: rods, ultraviolet cones (UV), short wavelength cones (S), medium wavelength cones (M), and long wavelength cones (L).