Visual pigments, oil droplets, ocular media and cone photoreceptor distribution in two species of passerine bird: the blue tit (Parus caeruleus L.) and the blackbird (Turdus merula L.)

The spectral absorption characteristics of the retinal photoreceptors of the blue tit (Parus caeruleus) and blackbird (Turdus merula) were investigated using microspectrophotometry. The retinae of both species contained rods, double cones and four spectrally distinct types of single cone. Whilst the visual pigments and cone oil droplets in the other receptor types are very similar in both species, the wavelength of maximum sensitivity (λ_max) of long-wavelength-sensitive single and double cone visual pigment occurs at a shorter wavelength (557 nm) in the blackbird than in the blue tit (563 nm). Oil droplets located in the long-wavelength-sensitivesingle cones of both species cut off wavelengths below 570–573 nm, theoretically shifting cone peak spectral sensitivity some 40 nm towards the long-wavelength end of the spectrum. This raises the possibility that the precise λ_max of the long-wavelength-sensitive visual pigment is optimised for the visual function of the double cones. The distribution of cone photoreceptors across the retina, determined using conventional light and fluorescence microscopy, also varies between the two species and may reflect differences in their visual ecology. Accepted: 8 January 2000     

Scanning electron microscopy and microspectrophotometry of the photoreceptors of ictalurid catfishes

The retinal photoreceptors from larval channel catfish (Ictalurus punctatus) were studied using single cell, in situ microspectrophotometry. Rods appear at 5 days after hatch; cones are present from day one. The rods contain a visual pigment which absorbs light maximally at 540 nm. The cones contain either a green sensitive visual pigment with peak absorbance at 535 nm or a red sensitive visual pigment with peak absorbance at 608 nm. All pigments are based on vitamin A₂. Visual pigment complement does not change with age, as photoreceptors from adultI. punctatus, I. catus andI. melas contain visual pigments virtually identical to those of the larvalI. punctatus. Regardless of age, no visual pigment with peak absorbance in the short wavelength region of the spectrum was ever observed. Scanning electron microscopy of adultI. punctatus retinas showed large rods with long, cylindrical outer segments and smaller cones with short, tapered outer segments. The myoids of both rods and cones are extensable. The rods, embedded in a granular tapetal material, comprise from 50 to 60% of the photoreceptors. Only single cones are present. The data are consistent with the idea that the ictalurid catfishes spend their entire lives in an environment deficient in blue light.     

The cone photoreceptors and visual pigments of chameleons

Visual pigments, oil droplets and photoreceptor types in the retinas of four species of true chameleons have been examined by microspectrophotometry. The species occupy different photic environments: two species of Chamaeleo are from Madagascar and two species of Furcifer are from Africa and the Arabian Peninsula. In addition to double cones, four spectrally distinct classes of single cone were identified. No rod photoreceptors were observed. The visual pigments appear to be mixtures of rhodopsins and porphyropsins. Double cones contained a pale oil droplet in the principle member and both outer segments contained a long-wave-sensitive visual pigment with a spectral maximum between about 555 nm and 610 nm, depending on the rhodopsin/porphyropsin mixture. Long-wave-sensitive single cones contained a visual pigment spectrally identical to the double cones, but combined with a yellow oil droplet. The other three classes of single cone contained visual pigments with maxima at about 480–505, 440–450 and 375–385 nm, combined with yellow, clear and transparent oil droplets respectively. The latter two classes were sparsely distributed. The transmission of the lens and cornea of C. dilepis was measured and found to be transparent throughout the visible and near ultraviolet, with a cut off at about 350 nm.     

Colour vision in the passeriform bird,Leiothrix lutea: correlation of visual pigment absorbance and oil droplet transmission with spectral sensitivity

The visual receptors in the retina of the passeriform bird Leiothrix lutea were examined microspectro-photometrically. The rods had a maximum absorbance close to 500 nm. Four spectrally different classes of single cone were identified with typical combinations of photopigments and oil droplets: a long-wave sensitive cone with a photopigment P568 and a droplet with a cut-off wavelength at 564 nm, a middle-wave sensitive cone with a P499 and a droplet with a cut-off at 506 nm, a short-wave sensitive cone with a P454 and a droplet with maximum absorbance below 410nm and an ultraviolet sensitive cone with a P355 and a transparent droplet. Double cones possessed a P568 in both the principal and accessory members. A pale droplet with variable absorbance (maximal at about 420 nm) was associated with the principal member whereas the ellipsoid region of the accessory member contained only low concentrations of carotenoid. The effective spectral sensitivities of the different cone classes were calculated from the characteristic combinations of oil droplets and photopigments and corrected for the absorbance of the ocular media. Comparison of these results with the behavioural spectral sensitivity function of Leiothrix lutea suggests that the increment threshold photopic spectral sensitivity of this avian species is mediated by the 4 single cone classes modified by neural opponent mechanisms.Abbreviations LWS long wave sensitive - MWS middle wave sensitive - SWS short wave sensitive (cones)     

Visual pigments and oil droplets in the penguin,Spheniscus humboldti

Summary The photoreceptors of the penguin,Spheniscus humboldti, were examined using a microspectrophotometer. The cones could be divided into three classes based on their visual pigment absorbance spectra [max 403, 450 and 543 nm (Fig. 1)], and into five classes based on their visual pigment-oil droplet combination (Fig. 4). Oil droplets were of three types (Fig. 2). The rods contained a rhodopsin with max at 504 nm. No double cones were observed. The penguin should be capable of good wavelength discrimination in the blue-green region of the spectrum but with poor discrimination at longer wavelengths. It is concluded that the spectral properties of the cone types indicate that the photopic vision ofS. humboldti is adapted to the spectral qualities of its aquatic environment.     

The photoreceptors and visual pigments of the garter snake (Thamnophis sirtalis): a microspectrophotometric, scanning electron microscopic and immunocytochemical study

Scanning electron microscopy, immunocytochemistry, and single cell microspectrophotometry were employed to characterize the photoreceptors and visual pigments in the retina of the garter snake, Thamnophis sirtalis. The photoreceptor population was found to be comprised entirely of cones, of which four distinct types were identified. About 45.5% of the photoreceptors are double cones consisting of a large principal member joined near the outer segment with a much smaller accessory member. About 40% of the photoreceptors are large single cones, and about 14.5% are small single cones forming two subtypes. The outer segments of the large single cones and both the principal and accessory members of the doubles contain the same visual pigment, one with peak absorbance near 554 nm. The small single cones contain either a visual pigment with peak absorbance near 482 nm or one with peak absorbance near 360 nm. Two classes of small single cones could be distinguished also by immunocytochemistry and scanning electron microscopy. The small single cones with the 360-nm pigment provide the garter snake with selective sensitivity to light in the near ultraviolet region of the spectrum. This ultraviolet sensitivity might be important in localization of pheromone trails. Accepted: 10 March 1997     

Photoreceptors and visual pigments in the retina of the fully anadromous green sturgeon (<Emphasis Type="Italic">Acipenser medirostrus</Emphasis>) and the potamodromous pallid sturgeon (<Emphasis Type="Italic">Scaphirhynchus albus</Emphasis>)

Green sturgeon and pallid sturgeon photoreceptors were studied with scanning electron microscopy (SEM), microspectrophotometry and, in the case of the green sturgeon, retinal whole-mounts. The retinas of both species contain both rods and cones: cones comprise between 23% (whole-mount) and 36% (SEM) of the photoreceptors. The cone population of both species is dominated by large single cones, but a rare small single cone is also present. In both species, most rods have long outer segments of large diameter. A rod with a relatively thin outer segment is present in the pallid sturgeon retina. Mean cone packing density for the entire green sturgeon retina is 4,690±891 cones/mm², with the dorsal retina 14% more dense than the ventral. There is evidence for a horizontal visual streak just above and including the optic disc. Mean rod packing density is 16,006±1,668 rods/mm² for the entire retina, and fairly uniform throughout. Both species have rods with peak absorbance near 540 nm, as well as short-wavelength-sensitive cones (green: 464.5±0.7 nm; pallid: 439.7±3.5 nm); middle-wavelength-sensitive cones (green: 538.0±1.4 nm; pallid: 537.0±1.7 nm); and long-wavelength-sensitive cones (green: 613.9±3.0 nm; pallid: 617.8±7.6 nm).     

Molecular cloning of cDNA encoding red opsin gene in the retinas of five Antarctic notothenioid fishes

Previous evidence suggested that notothenioid fish had lost red-sensitive (LWS) visual pigment and photoreceptors, but retained ultraviolet-sensitive (SWS1), blue-sensitive (SWS2), and green-sensitive (RH2) pigments. We used RT-PCR and Southern blot to isolate the LWS opsin gene in five notothenioid species. We determined full-coding LWS opsin sequences and genomic sequences. The expected peak absorbance of the LWS opsin, based on the five-sites rule that is primarily responsible for the spectral sensitivities in vertebrates, ranged from 541 to 553 nm. In Antarctic waters, light of this wavelength penetrates to dozens of meters. Thus, we conclude that notothenioids use tetrachromatic color vision in shallower waters, at least during the Antarctic summer.     

Developmental Changes in the Visual Pigments of the Yellowfin Tuna,Thunnus Albacares

Previous studies have suggested that adult tunas have only two visual pigments in their retinas - a rod pigment with a wavelength at maximum absorbance (u max) around 485 nm and one with similar u max in both twin and single cones inferred from extraction data. Using microspectrophotometry we confirm the presence of a u max 483 nm visual pigment in the rods of adult yellowfin tuna and a u max 485 nm pigment in both members of the twin cones. However, all single cones contain a previously undetected violet visual pigment with u max 426 nm making the adult yellowfin tuna a photopic dichromat. The situation for larvae and early juveniles is different from that of the adults. The all single-cone retina of preflexion larvae shows a wide distribution in individual cone absorbances suggesting not only mixtures of the two adult cone pigments, but the presence of at least a third visual pigment with u max greater than 560 nm. With growth, the variation in cone absorbances decreases with convergence to the adult condition coincident with cone twinning. The significance of u max variability, multiple visual pigment expression and age-related differences are discussed in terms of the visual ecology of larval, juvenile and adult tunas.     

Retinal photoreceptors and visual pigments in Boa constrictor imperator

The photoreceptors of Boa constrictor, a boid snake of the subfamily Boinae, were examined with scanning electron microscopy and microspectrophotometry. The retina of B. constrictor is duplex but highly dominated by rods, cones comprising 11% of the photoreceptor population. The rather tightly packed rods have relatively long outer segments with proximal ends that are somewhat tapered. There are two morphologically distinct, single cones. The most common cone by far has a large inner segment and a relatively stout outer segment. The second cone, seen only infrequently, has a substantially smaller inner segment and a finer outer segment. The visual pigments of B. constrictor are virtually identical to those of the pythonine boid, Python regius. Three different visual pigments are present, all based on vitamin A(1.) The visual pigment of the rods has a wavelength of peak absorbance (lambda(max)) at 495 +/- 2 nm. The visual pigment of the more common, large cone has a lambda(max) at 549 +/- 1 nm. The small, rare cone contains a visual pigment with lambda(max) at 357 +/- 2 nm, providing the snake with sensitivity in the ultraviolet. We suggest that B. constrictor might employ UV sensitivity to locate conspecifics and/or to improve hunting efficiency. The data indicate that wavelength discrimination above 430 nm would not be possible without some input from the rods.     

The ecology of the visual pigments of snappers (Lutjanidae) on the Great Barrier Reef

The visual pigments in the retinal photoreceptors of 12 species of snappers of the genus Lutjanus (Teleostei; Perciformes; Lutjanidae) were measured by microspectrophotometry. All the species were caught on the Great Barrier Reef (Australia) but differ in the colour of the water in which they live. Some live in the clear blue water of the outer reef, some in the greener water of the middle and inshore reefs and some in the more heavily stained mangrove and estuarine water. All the species had double cones, each member of the pair containing a different visual pigment. Using Baker's and Smith's (1982) model to predict the spectral distribution of ambient light from chlorophyll and dissolved organic matter it was found that the absorption spectra of the visual pigments in the double cones were close to those that confer the maximum sensitivity in the different water types. Single cones contained a blue or violet-sensitive visual pigment. The visual pigments in the rods showed little variation, their wavelength of maximum absorption always being in the region 489–502 nm.Abbreviations DOC dissolved organic carbon - DOM dissolved organic material - MSP microspectrophotometry deceased     

Tetrachromatic colour vision in the duck (Anas platyrhynchos L.):microspectrophotometry of visual pigments and oil droplets

Summary Microspectrophotometric examination of the visual receptors of the duck,Anas platyrhynchos, revealed four types of single cone containing visual pigments absorbing maximally at about 420 nm, 452 nm, 502 nm and 570 nm. A single population of double cones contained the P570 in both members. Rods absorbed maximally at 505 nm.Within the single cones, three types of oil droplet, acting as cut-off filters, were identified by the wavelength at which 50% transmission occurred, approximately 580, 515 and 450 nm. A further droplet, transparent throughout the visible spectrum, was also found in a small population of single cones. A fifth droplet type with a variable cutoff between 475–500 nm was located in the principal member of the double cones.The optical density of the anterior half of the eye, established by spectrophotometry, was used, in conjunction with the visual pigment and oil droplet combinations found within intact cones, to estimate the relative spectral sensitivities of the major cone types within the retina.     

Developmental Changes in the Visual Pigments of the Yellowfin Tuna, Thunnus Albacares

Previous studies have suggested that adult tunas have only two visual pigments in their retinas - a rod pigment with a wavelength at maximum absorbance ( λmax ) around 485 nm and one with similar λmax in both twin and single cones inferred from extraction data. Using microspectrophotometry we confirm the presence of a λmax 483 nm visual pigment in the rods of adult yellowfin tuna and a λmax 485 nm pigment in both members of the twin cones. However, all single cones contain a previously undetected violet visual pigment with λmax 426 nm making the adult yellowfin tuna a photopic dichromat. The situation for larvae and early juveniles is different from that of the adults. The all single-cone retina of preflexion larvae shows a wide distribution in individual cone absorbances suggesting not only mixtures of the two adult cone pigments, but the presence of at least a third visual pigment with λmax greater than 560 nm. With growth, the variation in cone absorbances decreases with convergence to the adult condition coincident with cone twinning. The significance of λmax variability, multiple visual pigment expression and age-related differences are discussed in terms of the visual ecology of larval, juvenile and adult tunas.     

Tetrachromacy, oil droplets and bird plumage colours

There is a growing body of data on avian eyes, including measurements of visual pigment and oil droplet spectral absorption, and of receptor densities and their distributions across the retina. These data are sufficient to predict psychophysical colour discrimination thresholds for light-adapted eyes, and hence provide a basis for relating eye design to visual needs. We examine the advantages of coloured oil droplets, UV vision and tetrachromacy for discriminating a diverse set of avian plumage spectra under natural illumination. Discriminability is enhanced both by tetrachromacy and coloured oil droplets. Oil droplets may also improve colour constancy. Comparison of the performance of a pigeon's eye, where the shortest wavelength receptor peak is at 410 nm, with that of the passerine Leiothrix, where the ultraviolet-sensitive peak is at 365 nm, generally shows a small advantage to the latter, but this advantage depends critically on the noise level in the sensitivity mechanism and on the set of spectra being viewed. Accepted: 3 July 1998     

Variations of colour vision in a New World primate can be explained by polymorphism of retinal photopigments

The squirrel monkey (Saimiri sciureus) exhibits a polymorphism of colour vision: some animals are dichromatic, some trichromatic, and within each of these classes there are subtypes that resemble the protan and deutan variants of human colour vision. For each of ten individual monkeys we have obtained (i) behavioural measurements of colour vision and (ii) microspectrophotometric measurements of retinal photopigments. The behavioural tests, carried out in Santa Barbara, included wavelength discrimination, Rayleigh matches, and increment sensitivity at 540 and 640 nm. The microspectrophotometric measurements were made in London, using samples of fresh retinal tissue and a modified Liebman microspectrophotometer: the absorbance spectra for single retinal cells were obtained by passing a monochromatic measuring beam through the outer segments of individual rods and cones. The two types of data, behavioural and microspectrophotometric, were obtained independently and were handed to a third party before being interchanged between experimenters. From all ten animals, a rod pigment was recorded with lambda max (wavelength of peak absorbance) close to 500 nm. In several animals, receptors were found that contained a short-wave pigment (mean lambda max = 433.5 nm): these violet-sensitive receptors were rare, as in man and other primate species. In the middle- to long-wave part of the spectrum, there appear to be at least three possible Saimiri photopigments (with lambda max values at about 537,550 and 565 nm) and individual animals draw either one or two pigments from this set, giving dichromatic or trichromatic colour vision. Thus, those animals that behaviourally resembled human protanopes exhibited only one pigment in the red-green range, with lambda max = 537 nm; other behaviourally dichromatic animals had single pigments lying at longer wavelengths and these were the animals that behaviourally had higher sensitivity to long wavelengths. Four of the monkeys were behaviourally judged to be trichromatic. None of the latter animals exhibited the two widely separated pigments (close to 535 and 567 nm) that are found in the middle- and long-wave cones of macaque monkeys.(ABSTRACT TRUNCATED AT 400 WORDS)     

Visual pigments and optical habitats of surfperch (Embiotocidae) in the California kelp forest

We studied the optical microhabitat use and visual pigment variation among a group of closely related teleosts (surfperch: Embiotocidae) living along the nearshore central California coast. We employed a diver-operated spectroradiometer to record the optical microhabitat use of eight surfperch species in Monterey Bay. and microspectrophotometry to measure visual pigment absorbance for nine surfperch species. Species were dichromatic with mixtures of A1- and A2-based visual pigments exhibiting extensive maximum absorbance (lambda(max)) variation across species: 455-482 nm for SWS cones and 527-546 nm for LWS cones. Interspecific variation in sidewelling irradiance measurements (mean lambdaFmaxs) significantly accounted for 63% of the variation in surfperch LWS visual pigments and 83% of the interspecific variation in SWS visual pigments using a phylogenetically-corrected regression technique. Optimality models for maximizing relative photon capture of background radiance demonstrate that the LWS cone lambda(max) values are tuned for maximizing photon capture of the species-specific horizontal visual field, while the SWS cone lambda(max), are well offset from the dominant background radiance. This study is one of the first to demonstrate species-specific differences in habitat usage at microhabitat scales accounting for differences in photoreceptor peak absorbance among closely related, sympatric species.     

Photoreceptors and visual pigments in three species of newts

Photoreceptor composition and retinal visual pigments in three newt (Caudata, Salamandridae, Pleurodelinae) species (Pleurodeles waltl, Lissotriton (Triturus) vulgaris, and Cynops orientalis) were studied by light microscopy and single-cell microspectrophotometry. Retinas of all three species contain “red” (rhodopsin/porphyropsin) rods, large and small single cones, and double cones. Large single cones and both components of double cones contain red-sensitive (presumably LWS) visual pigment whose absorption spectrum peaks between 593 and 611 nm. Small single cones are either blue- (SWS2, maximum absorption between 470 and 489 nm) or UV-sensitive (SWS1, maximum absorption between 340 and 359 nm). Chromophore composition of visual pigments (A₁ vs. A₂) was assessed both from template fitting of absorption spectra and by the method of selective bleaching. All pigments contained a mixture of A₁ (11-cis retinal) and A₂ (11-cis-3,4-dehydroretinal) chromophore in the proportion depending on the species and cell type. In all cases, A₂ was dominant. However, in C. orientalis rods the fraction of A₁ could reach 45%, while in P. waltl and L. vulgaris cones it did not exceed 5%. Remarkably, the absorption of the newt blue-sensitive visual pigment was shifted by up to 45 nm toward the longer wavelength, as compared with all other amphibian SWS2-pigments. We found no “green” rods typical of retinas of Anura and some Caudata (ambystomas) in the three newt species studied.     

On the visual pigments of deep-sea fish

The retinal visual pigments of 52 species of deep-sea fish were measured by partial bleaching of detergent extracts. The retinae of 45 species contained only a single rhodopsin with maximum absorbance (λ_max) at a wavelength between 474 and 490 nm, matching both the region of highest intensity downwelling sunlight and the maximum emission of most deep-sea bioluminescence. Seven species were shown to have more than one visual pigment within their retinae and these had λ_max values that generally fell outside the usual range. One of these, Bonapartia pedaliota , was particularly interesting as, unlike most such multipigment species, it had one rhodopsin and one porphyropsin pigment, apparently based on different opsins. The relative proportions of the visual pigments in the seven multipigment species are presented.     

The photoreceptors and visual pigments of two species of Acipenseriformes, the shovelnose sturgeon (Scaphirhynchus platorynchus ) and the paddlefish (Polyodon spathula )

Scanning electron microscopy, microspectrophotometry, and spectrophotometry of digitonin extracts were employed to characterize the photoreceptors and visual pigments of two freshwater Acipenseriformes. The retinas of the shovelnose sturgeon, Scaphirhynchus platorynchus (Acipenseridae), and the paddlefish, Polyodon spathula (Polyodontidae) are dominated by large rods with long, broad outer segments. A second rod, rare and much narrower than the dominant rod, is present in Scaphirhynchus but not seen in Polyodon. The absorbance maximum of the visual pigment in the rods of Polyodon is near 540 nm; that of Scaphirhynchus near 534 nm. The retinas of both species contain substantial numbers of large, single cones, about 33% of the photoreceptors in Scaphirhynchus; 37% in Polyodon. Scaphirhynchus cone pigments have absorbance maxima near 610 nm, 521 nm and 470 nm, respectively. Polyodon cone pigments absorb maximally near 607 nm and 535 nm, respectively. All visual pigments are based on vitamin A₂. The data are compared to those from other Acipenseriformes and are discussed in terms of lifestyle and behavior. Accepted: 7 October 1998     

Spectral differentiation of blue opsins between phylogenetically close but ecologically distant goldfish and zebrafish

Zebrafish and goldfish are both diurnal freshwater fish species belonging to the same family, Cyprinidae, but their visual ecological surroundings considerably differ. Zebrafish are surface swimmers in conditions of broad and shortwave-dominated background spectra and goldfish are generalized swimmers whose light environment extends to a depth of elevated short wavelength absorbance with turbidity. The peak absorption spectrum (lambdamax) of the zebrafish blue (SWS2) visual pigment is consistently shifted to short wavelength (416 nm) compared with that of the goldfish SWS2 (443 nm). Among the amino acid differences between the two pigments, only one (alanine in zebrafish and serine in goldfish at residue 94) was previously known to cause a difference in absorption spectrum (14-nm lambdamax shift in newt SWS2). In this study, we reconstructed the ancestral SWS2 pigment of the two species by applying likelihood-based Bayesian statistics and performing site-directed mutagenesis. The reconstituted ancestral photopigment had a lambdamax of 430 nm, indicating that zebrafish and goldfish achieved short wavelength (-14 nm) and long wavelength (+13 nm) spectral shifts, respectively, from the ancestor. Unexpectedly, the S94A mutation resulted in only a -3-nm spectral shift when introduced into the goldfish SWS2 pigment. Nearly half of the long wavelength shift toward the goldfish pigment was achieved instead by T116L (6 nm). The S295C mutation toward zebrafish SWS2 contributed to creating a ridge of absorbance around 400 nm and broadening its spectral sensitivity in the short wavelength direction. These results indicate that the evolutionary engineering approach is very effective in deciphering the process of functional divergence of visual pigments.