Photosensitive pigments from the retinae of certain deep-sea fishes

Active swimming and transphort by currents observed in Japanese eels (Anguilla japonica) acoustically tracked in the western North Pacific

The mechanisms of oceanic animal migration remain enigmatic. Adult Japanese eels start their long-distance oceanic migration from coastal areas to breed near the West Mariana Ridge. We tracked acoustically tagged eels released in the Kuroshio Current (KC) area near Japan (five silver-phase eels, three of which had impaired swim bladders) and a tropical/subtropical (TS) area near/in the spawning area (two yellow-phase and three silver-phase eels). We analyzed their active swimming and transport by water currents. The strong flow of the KC dominated the eels’ movements in the north, and TS area; their swimming influenced their movements. In the KC area, greater distances were covered at night than during the day, because eels swam in shallower layers with strong currents at night. Three and one eel in the TS and KC area in the upper 400 m showed counterclockwise and clockwise movements around the time of solar culmination, respectively. The meta-analysis showed that eels released at middle latitudes (20°–34° N) generally swam southward through currents, whereas those released at low latitudes (12°–13° N) generally swam northward through currents. Our study suggests the influence of the surrounding current and a potential effect of solar cues on the movements of Japanese eels.

Morphological abnormalities in seven American round ray specimens: A review of America's batomorph anomalies

Although morphological abnormalities in several rays and skate species around the American continents have frequently mentioned, their numbers are unknown. The present work record morphological abnormalities in four Urotrygonidae species. Two anophthalmic specimens were detected (Urotrygon microphthalmum and Urobatis halleri). Two individuals lacked caudal fins (Urobatis maculatus and Urotrygon chilensis). Two round rays showed incomplete fusion of the pectoral fin to the head (U. microphthalmum and U. chilensis). Vertebral compression and fusion were found in a 6-year-old female Urotrygon rogersi. In addition, 118 abnormal batomorph specimens were gathered from the available bibliography, spanning the last six decades (1959-2021). Amblyraja doellojuradoi was the species with the highest number of abnormalities (18). The most common anomaly was an incomplete fusion of the pectoral fin with the head. Since 2010, at least 30 anomalous batomorphs have been recorded every 5 years. Sixty-nine abnormal specimens occurred in the Northern Hemisphere (1.00-60.00 N). The Cortezian (Pacific) and Southeastern Brazil (Atlantic) marine ecoregions stood out with the highest number of these specimens. Mexico recorded 58 anomalous specimens, followed by Brazil (n = 36). Biological, abiotic and anthropogenic factors are probably the leading causes. However, additional studies are necessary to elucidate these speculations.

The exceptional diversity of visual adaptations in deep-sea teleost fishes

The deep-sea is the largest and one of the dimmest habitats on earth. In this extreme environment, every photon counts and may make the difference between life and death for its inhabitants. Two sources of light are present in the deep-sea; downwelling light, that becomes dimmer and spectrally narrower with increasing depth until completely disappearing at around 1000 m, and bioluminescence, the light emitted by animals themselves. Despite these relatively dark and inhospitable conditions, many teleost fish have made the deep-sea their home, relying heavily on vision to survive. Their visual systems have had to adapt, sometimes in astonishing and bizarre ways. This review examines some aspects of the visual system of deep-sea teleosts and highlights the exceptional diversity in both optical and retinal specialisations. We also reveal how widespread several of these adaptations are across the deep-sea teleost phylogeny. Finally, the significance of some recent findings as well as the surprising diversity in visual adaptations is discussed.

Variation in rod spectral sensitivity of fishes is best predicted by habitat and depth

Rod spectral sensitivity data (λ_max ), measured by microspectrophotometry, were compiled for 403 species of ray-finned fishes in order to examine four hypothesized predictors of rod spectral sensitivity (depth, habitat, diet and temperature). From this database, a subset of species that were known to be adults and available on a published phylogeny (n = 210) were included in analysis, indicating rod λ_max values averaging 503 nm and ranging from 477 to 541 nm. Linear models that corrected for phylogenetic relatedness showed that variation in rod sensitivity was best predicted by habitat and depth, with shorter wavelength λ_max values occurring in fishes found offshore or in the deep sea. Neither diet, nor the interaction of diet and habitat, had significant explanatory power. Although temperature significantly correlated with rod sensitivity, in that fishes in temperate latitudes had longer wavelength rod λ_max values than those in tropical latitudes, sampling inequity and other confounds require the role of the temperature to be studied further. Together, these findings indicate that fish rod λ_max is influenced by several ecological factors, suggesting that selection can act on even small differences in fish spectral sensitivity.

Rhodopsin in the Dark Hot Sea: Molecular Analysis of Rhodopsin in a Snailfish, Careproctus rhodomelas, Living near the Deep-Sea Hydrothermal Vent

Visual systems in deep-sea fishes have been previously studied from a photobiological aspect; however, those of deep-sea fish inhabiting the hydrothermal vents are far less understood due to sampling difficulties. In this study, we analyzed the visual pigment of a deep-sea snailfish, Careproctus rhodomelas, discovered and collected only near the hydrothermal vents of oceans around Japan. Proteins were solubilized from the C. rhodomelas eyeball and subjected to spectroscopic analysis, which revealed the presence of a pigment characterized by an absorption maximum (λ_max) at 480 nm. Immunoblot analysis of the ocular protein showed a rhodopsin-like immunoreactivity. We also isolated a retinal cDNA encoding the entire coding sequence of putative C. rhodomelas rhodopsin (CrRh). HEK293EBNA cells were transfected with the CrRh cDNA and the proteins extracted from the cells were subjected to spectroscopic analysis. The recombinant CrRh showed the absorption maximum at 480 nm in the presence of 11-cis retinal. Comparison of the results from the eyeball extract and the recombinant CrRh strongly suggests that CrRh has an A₁-based 11-cis-retinal chromophore and works as a photoreceptor in the C. rhodomelas retina, and hence that C. rhodomelas responds to dim blue light much the same as other deep-sea fishes. Because hydrothermal vent is a huge supply of viable food, C. rhodomelas likely do not need to participate diel vertical migration and may recognize the bioluminescence produced by aquatic animals living near the hydrothermal vents.

Visual sensitivity of deepwater fishes in Lake Superior

The predator-prey interactions in the offshore food web of Lake Superior have been well documented, but the sensory systems mediating these interactions remain unknown. The deepwater sculpin, (Myoxocephalus thompsoni), siscowet (Salvelinus namaycush siscowet), and kiyi (Coregonus kiyi) inhabit low light level environments. To investigate the potential role of vision in predator-prey interactions, electroretinography was used to determine visual sensitivity for each species. Spectral sensitivity curves revealed peak sensitivity at 525 nm for each species which closely corresponds to the prevalent downwelling light spectrum at depth. To determine if sufficient light was available to mediate predator-prey interactions, visual sensitivity was correlated with the intensity of downwelling light in Lake Superior to construct visual depth profiles for each species. Sufficient daytime irradiance exists for visual interactions to approximately 325 m for siscowet and kiyi and 355 m for the deepwater sculpin during summer months. Under full moon conditions, sufficient irradiance exists to elicit ERG response to light available at approximately 30 m for the siscowet and kiyi and 45 m for the deepwater sculpin. Visual interactions are therefore possible at the depths and times when these organisms overlap in the water column indicating that vision may play a far greater role at depth in deep freshwater lakes than had been previously documented.

The reniform reflecting superposition compound eyes of Nephrops norvegicus: optics, susceptibility to light-induced damage, electrophysiology and a ray tracing model

The large reniform eyes of the reptant, tube-dwelling decapod Nephrops norvegicus are described in detail. Optically these reflecting superposition compound eyes are a little unusual in that they are laterally flattened, a feature that may enhance their sensitivity in that region, albeit at the expense of resolution. Electrophysiological and anatomical investigations suggest that the eyes are tuned to appropriate spectral and temporal sensitivities in the long and short term through movement of proximal pigments and possibly rhabdom adaptation. Although exposure to ambient surface light intensities is shown to cause damage to the retinal layer, especially in deeper living animals, there is no evidence yet that demonstrates an impact of eye damage on their survival. It is suggested that experimentation on marine decapods, with sensitive eyes, requires that particular attention is paid to their light environment.

Vertebrate rhodopsin adaptation to dim light via rapid meta-II intermediate formation

Rhodopsin is a photoreceptive protein present in vertebrate rod photoreceptor cells, which are responsible for scotopic vision. Recent molecular studies have shown that several aquatic vertebrate species have independently acquired rhodopsin containing Asp83Asn, Glu122Gln, and Ala292Ser substitutions, causing a blue shift in the rhodopsin absorption spectra for adaptation to the blue-green photic environment in deep water. Here, we provide new evidence for the evolutionary and functional relevance of the Asp83Asn substitution. Spectroscopic and kinetic analyses of rhodopsins in six cichlid fishes from the East African Great Lakes using charge-coupled device spectrophotometer revealed that the Asp83Asn substitution accelerated the formation of meta-II, a rhodopsin intermediate crucial for activation of the G-protein transducin. Because rapid formation of meta-II likely results in effective transduction of photic signals, it is reasonable to assume that deep-water cichlid species have acquired rhodopsin containing Asn83 to adapt to dim lighting. Remarkably, rhodopsin containing Asn83 has been identified in terrestrial vertebrates such as bats, and these rhodopsin variants also exhibit accelerated meta-II formation. Our results indicated that the Asp83Asn substitution observed in a variety of animal species was acquired independently in many different lineages during vertebrate evolution for adaptation to dimly lit environments.

Spectral sensitivities of the seahorsesHippocampus subelongatusandHippocampus barbouriand the pipefishStigmatopora argus

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

The molecular basis for spectral tuning of rod visual pigments in deep-sea fish

Most species of deep-sea fish possess of a rod-only retina with a pigment that is generally shortwave shifted in λmax towards the blue region of the spectrum. In addition, the λmax values of different species tend to cluster at particular points in the spectrum. In this study, the rod opsin gene sequences from 28 deep-sea fish species drawn from seven different Orders are compared. The λmax values of the rod pigments vary from approximately 520 nm to <470 nm, with the majority lying between 490 nm and 477 nm. The 520 nm pigment in two species of dragon fish is associated with a Phe261Tyr substitution, whereas the shortwave shifts of the pigments in the other 26 species are accountable by substitutions at a further eight sites (83, 122, 124, 132, 208, 292, 299 and 300). Clustering of λmax values does not, however, involve a common subset of these substitutions in the different species. A phylogenetic analysis predicts that the pigment in the ancestral species would have had a λmax of approximately 480 nm. A total of 27 changes is required to generate the pattern of substitutions seen in the different species, with many sites undergoing multiple changes.

The photosensitive retinal pigments of fishes from relatively turbid coastal waters

Digitonin extracts have been prepared from the retinae of a dozen species of marine and euryhaline teleost fishes from turbid water habitats. Spectrophotometric analysis of the extracts shows that the photosensitive retinal pigments of these species have maximum absorption above 500 mµ. In nine species there are retinene₁ pigments with λ_max between 504 and 512 mµ. In the marine but euryhaline mullet, Mugil cephalus, there is a porphyropsin with λ_max 520 mµ. A mixture of rhodopsin and porphyropsin in an extract of a marine puffer, Sphoeroides annulatus, was disclosed by partial bleaching with colored light. In addition, one other species has a 508 mµ pigment, of which the nature of the chromophore was not determined. The habitats in which these fishes live are relatively turbid, with the water greenish or yellowish in color. The spectral transmission of such waters is probably maximal between 520 and 570 mµ. It is suggested that the fishes have become adapted to these conditions by small but significant shifts in spectral absorption of their retinal pigments. These pigments are decidedly more effective than rhodopsin in absorption of wavelengths above 500 mµ. This offers a possible interpretation of the confusing array of retinal pigments described from marine and euryhaline fishes.

The eyes of deep-sea fish. I: Lens pigmentation, tapeta and visual pigments

Deep-sea fish, defined as those living below 200 m, inhabit a most unusual photic environment, being exposed to two sources of visible radiation; very dim downwelling sunlight and bioluminescence, both of which are, in most cases, maximal at wavelengths around 450-500 nm. This paper summarises the reflective properties of the ocular tapeta often found in these animals, the pigmentation of their lenses and the absorption characteristics of their visual pigments. Deep-sea tapeta usually appear blue to the human observer, reflecting mainly shortwave radiation. However, reflection in other parts of the spectrum is not uncommon and uneven tapetal distribution across the retina is widespread. Perhaps surprisingly, given the fact that they live in a photon limited environment, the lenses of some deep-sea teleosts are bright yellow, absorbing much of the shortwave part of the spectrum. Such lenses contain a variety of biochemically distinct pigments which most likely serve to enhance the visibility of bioluminescent signals. Of the 195 different visual pigments characterised by either detergent extract or microspectrophotometry in the retinae of deep-sea fishes, ca. 87% have peak absorbances within the range 468-494 nm. Modelling shows that this is most likely an adaptation for the detection of bioluminescence. Around 13% of deep-sea fish have retinae containing more than one visual pigment. Of these, we highlight three genera of stomiid dragonfishes, which uniquely produce far red bioluminescence from suborbital photophores. Using a combination of longwave-shifted visual pigments and in one species (Malacosteus niger) a chlorophyll-related photosensitizer, these fish have evolved extreme red sensitivity enabling them to see their own bioluminescence and giving them a private spectral waveband invisible to other inhabitants of the deep-ocean.

The contributions of the orientated photosensitive and other molecules to the absorption of whole retina

Observations have been made on the intact retinae of various animals chosen so as to give examples of golden, red and purple retinae. It is confirmed, by a method giving the absorption curve of unbleached retina, that the retinae of deep-sea fish have golden photosensitive pigments in optical densities so high that they absorb over 90% of blue-green light striking them. Despite the fact that light has passed through all the retinal layers the spectral total density curves correspond closely to those obtained on highly purified retinal extracts. After making a small correction for losses of light in the layers of retina other than the receptor layer the D min. / D max. ratios were about 0·25 for chrysopsin retinae and 0·25 for porphyropsin retinae. The losses of light in retinae from which the rod and cone layer has been brushed off, although varying little with wavelength, rise slowly in the near ultra-violet and red from a minimum in the yellow. The edge-fold preparation, useful for studying the dichroism of the retinal rods is described. Using such preparations it is shown that the spectral curves of dichroic difference in density are very close to those of total retinal density and therefore there can be little material absorbing light between 400 and 720 m μ orientated along the axes of unbleached rods. There is, however, such material absorbing near ultra-violet light present in unbleached retinae whose principal photosensitive pigment is derived from vitamin A 2 . For both vitamin A 1 and vitamin A 2 retinae the sense of the dichroism in the near ultra-violet is reversed in the first hour following the bleaching of the photosensitive pigments. This shows that the molecules of visual white (vitamin A ) in the bleached isolated retina are orientated with their axes of resonance parallel to the axes of the rods, a conclusion confirmed by studies of the polarization of the fluorescence of visual white in the bleached retina.

Adaptations of visual pigments to the photic environment of the deep sea

This is a summary of studies that bear on the problems of the adaptation of visual pigments to the photic environment of the deep sea. The results suggest that the spectral absorption of these retinal pigments is shifted toward the blue in order to match the dim, blue-green downwelling light and/or the bioluminescence of organisms that are critical to the life of the species. Through such a spectral match, greater visual sensitivity is achieved for life in the special photic condition of their habitat. This adaptation has been found for chimaerid fishes, for elasmobranchs, for teleosts, for mammals, and for certain crustaceans and cephalopods. The most convincing evidence for such an adaptive match has been found in teleosts that have red-emitting photophores. In these fishes a photopigment with absorbance shifted toward the red has been found by extraction and microspectrophotometry. A few exceptions to this idea of an adaptive match have appeared in the literature, the cone pigments, especially, being examples of such offset pigments. The malacosteid fishes have been shown to have a red-shifted retinal pigment with 11-cis-3-dehydroretinal as the chromophore and some invertebrates have also adopted this molecule to adjust the spectral absorption to the photic environment or to the bioluminescence. These studies are beginning to reveal that visual biochemistry is basically the same in vertebrates and invertebrates and that the visual pigment protein arose early in phylogeny and has been retained, with appropraite modifications, to the present.

Interspecific variation in the visual pigments of deep-sea fishes

Visual pigments in the rods of 38 species of deep-sea fish were examined by microspectrophotometry. 33 species were found to have a single rhodopsin with a wavelength of maximum absorbance (lambda max) in the range 470-495 nm. Such visual pigments have absorbance maxima close to the wavelengths of maximum spectral transmission of oceanic water. 5 species, however, did not conform to this pattern and visual pigments were found with lambda max values ranging from 451 nm to 539 nm. In 4 of these species two visual pigments were found located in two types of rod. Some 2-pigment species which have unusual red sensitivity, also have red-emitting photophores. These species have both rhodopsin and porphyropsin pigments in their retinae, which was confirmed by HPLC, and the two pigments are apparently located in separate rods in the same retinal area. In deep-sea fishes the occurrence of 'unusual' visual pigments seems to be correlated with aspects of the species' depth ranges. In addition to ecological influences we present evidence, in the form of lambda max spectral clustering, that indicates the degree of molecular constraint imposed on the evolution of visual pigments in the deep-sea.

The visual pigments of four deep-sea crustacean species

The visual pigments of four mesopelagic crustacean species were studied at sea by means of microspectrophotometry. The absorbance maxima obtained for the visual pigments and their metarhodopsins, respectively, were: 493 nm and 481 nm (Systellaspis debilis), 485 nm and 480 nm (Acanthephyra curtirostris), 491 nm and 482 nm (A. smithi), and 495 nm and 487 nm (Sergestes tenuiremis). The spectral characteristics of the rhodopsins and metarhodopsins permit high photosensitivity and facilitate photoregeneration in a nearly monochromatic environment. Photic regeneration of rhodopsins from the deep-sea environment was demonstrated, and data were obtained which are consistent with the occurrence of dark regeneration. Specific optical density of the observed visual pigments was calculated for two species.

3-Dehydroretinal in the eye of a bioluminescent squid, Watasenia scintillans

3-Dehydroretinal was found in the retina of a bioluminescent squid, Watasenia scintillans. The possible vitamin A2-visual pigment was localized in the ventral part of the eye and its proportion was about 15% of the total visual pigment.

The visual pigment sensitivity hypothesis: further evidence from fishes of varying habitats

Visual pigments were extracted from the retinas of 8 species of marine teleosts and 4 species of elasmobranchs and a comparison was made of the pigment properties from these fishes, some inhabiting surface waters, others from the mesopelagic zone, and a few migrating vertically between these two environments. An association was found between the spectral position of the absorbance curve and the habitat depth or habitat behavior, with the blue-shifted chrysopsins being the pigments of the twilight zone fishes and the rhodopsins with fishes living near the surface. The retina of the swell shark (Cephaloscyllium ventriosum) yielded extracts with two photopigments; one, a rhodopsin at 498 nm; the second, a chrysopsin at 478 nm. This fish has been reported to practice seasonal vertical migrations between the surface and the mesopelagic waters. In addition to the spectral absorbance, several properties of these visual pigments were examined, including the meta-III product of photic bleaching, regeneration with added 11-cis and 9-cis retinals, and the chromophoric photosensitivity. The chrysopsin properties were found to be fundamentally similar to those of typical vertebrate rhodopsins. Correlating the spectral data with the habitat and habitat behavior of our fishes gives us confidence in the idea that the scotopic pigments have evolved as adaptations to those aspects of their color environment that are critical to the survival of the species.

Visual pigments and environmental light

The visual pigments in the rods do not have a special absorption that gives them maximal sensitivity. The visual pigments of "deep sea" fish are an exception for these do match the environmental light to give maximum sensitivity. At the low light intensities at which the rods operate, it is the number of photons that go to make up each element of the image that limits the ability of the eye to discriminate detail and contrast. Chemically induced isomerisation of the visual pigment molecule may cause spurious visual signals that limit the ability of the eye to detect contrasts in very dim light. In bright light the spurious visual signals become insignificant in number compared to the true photon-induced visual signals. Compared to the rods, cone visual pigments do match the spectral properties of the environment except that there appear to be no visual pigments with an absorption maximum beyond the 625 nm porphyropsins in cones. U.V. absorbing pigments are know in invertebrates, birds and fish that live in very shallow water. Animals have photoreceptors in parts of the body other than the eyes. In vertebrates these sites include the pineal, chromatophores, brain, skin and harderian gland. There is evidence based on immunocytochemistry and action spectra that at least some of the skin and pineal receptors contain visual pigments, but like those of the rods, these do not match the spectral quality of the environmental light.