Possible involvement of cone opsins in distinct photoresponses of intrinsically photosensitive dermal chromatophores in tilapia Oreochromis niloticus

Visual acuity of the summer flounder (Paralichthys dentatus) captures spatial information relevant to dynamic camouflage at close range

Dynamic camouflage is the capacity to rapidly change skin color and pattern, often for the purpose of background-matching camouflage. Summer flounder (Paralichthys dentatus) are demersal fish with an exceptional capacity for dynamic camouflage, but with eyes that face away from the substrate, it is unknown if this behavior is mediated by vision. Past studies have shown that summer flounder skin can match the pattern (i.e., spatial detail) of substrate with a high degree of precision, and for that to be achieved using sight, one testable assumption is that the resolution of vision must match the degree of detail produced in color-change performance. To test this, approaches in morphology and behavior were used to estimate visual acuity, which is the capacity of the visual system to resolve static spatial detail. Using image processing techniques, we then compared the degree of spatial detail from a relevant substrate with what may be detectable by summer flounder spatial vision. The morphological and behavioral estimates of visual acuity were calculated as 3.62 cycles per degree (CPD) ± 0.8 (s.d.) and 4.06 CPD ± 0.4 (s.d.), respectively. These estimates fall within a range of acuities known among other flatfishes and appear adequate for detecting the spatial information needed for background-matching camouflage, though only at close distances. These data provide new knowledge about summer flounder visual acuity and suggest the capacity of flounder vision to support dynamic camouflage of the skin.

Sws2 Gene Positively Regulates Melanin Production in Plectropomus leopardus Skin via Direct Regulation of the Synthesis of Retinoic Acid

Opsins are a class of transmembrane proteins encoded by opsin genes, and they play a variety of functional roles. Short wavelength-sensitive opsin 2 (sws2), one of the five classes of visual opsin genes, mainly senses blue light. Previous research has indicated that sws2 is essential for melanocyte formation in fish; however, its specific role in skin color differentiation remains to be elucidated. Here, we identified the sws2 gene in a prized reef-dwelling fish, Plectropomus leopardus. The full-length P. leopardus sws2 gene encodes a protein consisting of 351 amino acids, and exhibits substantial homology with other fish species. The expression of the sws2 gene was widespread across P. leopardus tissues, with high expression in eye and skin tissues. Through immunohistochemistry and in situ hybridization analyses, we discovered that the sws2 gene was primarily localized in the rod and cone cells of the retina, and epidermal cells of the skin. Furthermore, dsRNA interference was used for sws2 gene knockdown in living P. leopardus to elucidate its function in skin color differentiation. Black-color-related genes, melanin contents, and tyrosinase activity in the skin significantly decreased after sws2 knockdown (p < 0.05), but red-color-related genes and carotenoid and lutein contents significantly increased (p < 0.05). Retinoic acid injection produced the opposite results. Our results suggested that the sws2 gene influences P. leopardus skin color regulation by affecting vitamin synthesis and melanin-related gene expression levels. This study establishes a foundation for elucidating the molecular mechanisms by which sws2 regulates melanocyte formation in fish skin.

An Expanding Role for Nonvisual Opsins in Extraocular Light Sensing Physiology

We live on a planet that is bathed in daily and seasonal sunlight cycles. In this context, terrestrial life forms have evolved mechanisms that directly harness light energy (plants) or decode light information for adaptive advantage. In animals, the main light sensors are a family of G protein-coupled receptors called opsins. Opsin function is best described for the visual sense. However, most animals also use opsins for extraocular light sensing for seasonal behavior and camouflage. While it has long been believed that mammals do not have an extraocular light sensing capacity, recent evidence suggests otherwise. Notably, encephalopsin (OPN3) and neuropsin (OPN5) are both known to mediate extraocular light sensing in mice. Examples of this mediation include photoentrainment of circadian clocks in skin (by OPN5) and acute light-dependent regulation of metabolic pathways (by OPN3 and OPN5). This review summarizes current findings in the expanding field of extraocular photoreception and their relevance for human physiology.

Dynamic light filtering over dermal opsin as a sensory feedback system in fish color change

Dynamic color change has evolved multiple times, with a physiological basis that has been repeatedly linked to dermal photoreception via the study of excised skin preparations. Despite the widespread prevalence of dermal photoreception, both its physiology and its function in regulating color change remain poorly understood. By examining the morphology, physiology, and optics of dermal photoreception in hogfish (Lachnolaimus maximus), we describe a cellular mechanism in which chromatophore pigment activity (i.e., dispersion and aggregation) alters the transmitted light striking SWS1 receptors in the skin. When dispersed, chromatophore pigment selectively absorbs the short-wavelength light required to activate the skin's SWS1 opsin, which we localized to a morphologically specialized population of putative dermal photoreceptors. As SWS1 is nested beneath chromatophores and thus subject to light changes from pigment activity, one possible function of dermal photoreception in hogfish is to monitor chromatophores to detect information about color change performance. This framework of sensory feedback provides insight into the significance of dermal photoreception among color-changing animals.

Analysis of opsin gene family of Crimson snapper (Lutjanus erythropterus)

Opsin is a fellow of the G protein-coupled receptors (GPCRs) superfamily. It can be divided into visual and non-visual opsin according to whether it is directly involved in visual imaging. Opsin plays an important role in visual image formation and the regulation of non-image forming functions such as circadian entrainment in the growth, development and evolution of fish. Crimson snapper belongs to Perciforme mainly found in the Indo-West Pacific and the South China Sea. It is one of the most influential economic fishes in the South China Sea. In order to study the existence and expression of opsin gene in Crimson snapper, we sequenced the genome and tissue sample transcriptome of Crimson snapper. In this study, 32 opsin genes were identified from the genome of Crimson snapper. The length of these genes ranged from 1061 bp to 86203 bp and were distributed on 15 different chromosomes. The analysis of opsin gene family of Crimson snapper showed that the sws2 had two extra copies as compared with that of Zebrafish. Domain and motif analysis revealed that all the 32 opsin genes have seven-(pass)-transmembrane domain receptors (7TM receptors) each, and the opsin family contained 10 common motifs. The expression level of opsin gene, confirmed by RT-qPCR, was analyzed by using nine tissues transcriptome databases of Crimson snapper. The results showed that almost all opsin genes were highly expressed in the retina and brain, except opn7a and opn7b which were expressed in intestine and red skin, and almost no expression in other tissues. Our results provide a comprehensive basic knowledge for the opsin gene family of Crimson snapper, which has significance for the study of the function of opsin in Lutjanidaes.

Identification and Characterization of a Rhodopsin Kinase Gene in the Suckers of Octopus vulgaris: Looking around Using Arms?

Simple Summary

Octopus arms are a fascinating and evolutionarily unique sensory organ, with hundreds of motile suckers, each with thousands of sensory cells, lining eight highly flexible arms. Scientifically, there are many open questions regarding the sensory capabilities of the arms and specifically the highly innervated suckers. In our present work, we used a multidisciplinary approach to fully characterize the light-sensing molecule, Ov-GRK1, in the suckers, skin and retina of Octopus vulgaris. We sequenced the O. vulgaris GRK1 gene, defining a phylogenetic tree and performing a 3D structure model prediction. We found differences in the relative expression of mRNA in different sucker types at several locations along the arm, which might indicate a functional difference. Using labeling methods, we localized the expression to the highly sensitive sucker rim. Our findings indicate that octopus suckers, in specific areas of the arm, might have the ability for light sensing. We therefore suggest that suckers are tactile, chemical and light sensors.

Abstract

In their foraging behavior octopuses rely on arm search movements outside the visual field of the eyes. In these movements the environment is explored primarily by the suckers that line the entire length of the octopus arm. In this study, for the first time, we report the complete characterization of a light-sensing molecule, Ov-GRK1, in the suckers, skin and retina of Octopus vulgaris. We sequenced the O. vulgaris GRK1 gene, defining a phylogenetic tree and performing a 3D structure model prediction. Furthermore, we found differences in relative mRNA expression in different sucker types at several arm levels, and localized it through in situ hybridization. Our findings suggest that the suckers in octopus arms are much more multimodal than was previously shown, adding the potential for light sensing to the already known mechanical and chemical sensing abilities.

Fish skin pigmentation in aquaculture: The influence of rearing conditions and its neuroendocrine regulation

Skin pigmentation pattern is a species-specific characteristic that depends on the number and the spatial combination of several types of chromatophores. This feature can change during life, for example in the metamorphosis or reproductive cycle, or as a response to biotic and/or abiotic environmental cues (nutrition, UV incidence, surrounding luminosity, and social interactions). Fish skin pigmentation is one of the most important quality criteria dictating the market value of both aquaculture and ornamental species because it serves as an external signal to infer its welfare and the culture conditions used. For that reason, several studies have been conducted aiming to understand the mechanisms underlying fish pigmentation as well as the influence exerted by rearing conditions. In this context, the present review focuses on the current knowledge on endocrine regulation of fish pigmentation as well as on the aquaculture conditions affecting skin coloration. Available information on Iberoamerican fish species cultured is presented.

Light organ photosensitivity in deep-sea shrimp may suggest a novel role in counterillumination

Extraocular photoreception, the ability to detect and respond to light outside of the eye, has not been previously described in deep-sea invertebrates. Here, we investigate photosensitivity in the bioluminescent light organs (photophores) of deep-sea shrimp, an autogenic system in which the organism possesses the substrates and enzymes to produce light. Through the integration of transcriptomics, in situ hybridization and immunohistochemistry we find evidence for the expression of opsins and phototransduction genes known to play a role in light detection in most animals. Subsequent shipboard light exposure experiments showed ultrastructural changes in the photophore similar to those seen in crustacean eyes, providing further evidence that photophores are light sensitive. In many deep-sea species, it has long been documented that photophores emit light to aid in counterillumination - a dynamic form of camouflage that requires adjusting the organ's light intensity to "hide" their silhouettes from predators below. However, it remains a mystery how animals fine-tune their photophore luminescence to match the intensity of downwelling light. Photophore photosensitivity allows us to reconsider the organ's role in counterillumination - not only in light emission but also light detection and regulation.

Expression of extraocular opsin genes and light-dependent basal activity of blind cavefish

Background

Animals living in well-lit environments utilize optical stimuli for detecting visual information, regulating the homeostatic pacemaker, and controlling patterns of body pigmentation. In contrast, many subterranean animal species without optical stimuli have evolved regressed binocular eyes and body pigmentation. Interestingly, some fossorial and cave-dwelling animals with regressed eyes still respond to light. These light-dependent responses may be simply evolutionary residuals or they may be adaptive, where negative phototaxis provides avoidance of predator-rich surface environments. However, the relationship between these non-ocular light responses and the underlying light-sensing Opsin proteins has not been fully elucidated.

Methods

To highlight the potential functions of opsins in a blind subterranean animal, we used the Mexican cave tetra to investigate opsin gene expression in the eyes and several brain regions of both surface and cave-dwelling adults. We performed database surveys, expression analyses by quantitative reverse transcription PCR (RT-qPCR), and light-dependent locomotor activity analysis using pinealectomized fish, one of the high-opsin expressing organs of cavefish.

Results

Based on conservative criteria, we identified 33 opsin genes in the cavefish genome. Surveys of available RNAseq data found 26 of these expressed in the surface fish eye as compared to 24 expressed in cavefish extraocular tissues, 20 of which were expressed in the brain. RT-qPCR of 26 opsins in surface and cavefish eye and brain tissues showed the highest opsin-expressing tissue in cavefish was the pineal organ, which expressed exo-rhodopsin at 72.7% of the expression levels in surface fish pineal. However, a pinealectomy resulted in no change to the light-dependent locomotor activity in juvenile cavefish and surface fish. Therefore, we conclude that, after 20,000 or more years of evolution in darkness, cavefish light-dependent basal activity is regulated by a non-pineal extraocular organ.

De novo transcriptomics reveal distinct phototransduction signaling components in the retina and skin of a color-changing vertebrate, the hogfish (Lachnolaimus maximus)

Across diverse taxa, an increasing number of photoreceptive systems are being discovered in tissues outside of the eye, such as in the skin. Dermal photoreception is believed to serve a variety of functions, including rapid color change via specialized cells called chromatophores. In vitro studies of this system among color-changing fish have suggested the use of a phototransduction signaling cascade that fundamentally differs from that of the retina. Thus, the goal of this study was to identify phototransduction genes and compare their expression in the retina and skin of a color-changing fish, the hogfish Lachnolaimus maximus. De novo transcriptomics revealed the expression of genes that may underlie distinct, yet complete phototransduction cascades in L. maximus retina and skin. In contrast to the five visual opsin genes and cGMP-dependent phototransduction components expressed in the retina of L. maximus, only a single short-wavelength sensitive opsin (SWS1) and putative cAMP-dependent phototransduction components were expressed in the skin. These data suggest a separate evolutionary history of phototransduction in the retina and skin of certain vertebrates and, for the first time, indicate an expression repertoire of genes that underlie a non-retinal phototransduction pathway in the skin of a color-changing fish.

Diverse Distributions of Extraocular Opsins in Crustaceans, Cephalopods, and Fish

Non-visual and extraocular photoreceptors are common among animals, but current understanding linking molecular pathways to physiological function of these receptors is lacking. Opsin diversity in extraocular tissues suggests that many putative extraocular photoreceptors utilize the "visual" phototransduction pathway-the same phototransduction pathway as photoreceptors within the retina dedicated to light detection for image sensing. Here, we provide a brief overview of the current understanding of non-visual and extraocular photoreceptors, and contribute a synopsis of several novel putative extraocular photoreceptors that use both visual and non-visual phototransduction pathways. Crayfish, cephalopods, and flat fish express opsins in diverse tissues, suggesting the presence of extraocular photoreceptors. In most cases, we find that these animals use the same phototransduction pathway that is utilized in the retinas for image-formation. However, we also find the presence of non-visual phototransduction components in the skin of flounders. Our evidence suggests that extraocular photoreceptors may employ a number of phototransduction pathways that do not appear to correlate with purpose or location of the photoreceptor.

From crypsis to mimicry: changes in colour and the configuration of the visual system during ontogenetic habitat transitions in a coral reef fish

Animals often change their habitat throughout ontogeny; yet, the triggers for habitat transitions and how these correlate with developmental changes – e.g. physiological, morphological, and behavioural – remain largely unknown. Here, we investigated how ontogenetic changes in body colouration and of the visual system relate to habitat transitions in a coral-reef fish. Adult dusky dottybacks, Pseudochromis fuscus, are aggressive mimics that change colour to imitate various fishes in their surroundings; however, little is known about the early life stages of this fish. Using a developmental time-series in combination with the examination of wild caught specimens we uncover that dottybacks change colour twice during development: (i) nearly translucent cryptic pelagic larvae change to a grey camouflage colouration when settling on coral reefs; and (ii) juveniles change to mimic yellow or brown coloured fishes when reaching a size capable of consuming juvenile fish prey. Moreover, microspectrophotometric (MSP) and quantitative real time PCR (qRT-PCR) experiments show developmental changes of the dottyback visual system, including the use of a novel adult specific visual gene (RH2 opsin). This gene is likely to be coexpressed with other visual pigments to form broad spectral sensitivities that cover the medium-wavelength part of the visible spectrum. Surprisingly, the visual modifications precede changes in habitat and colour, possibly because dottybacks need to first acquire the appropriate visual performance before transitioning into novel life stages.

Visual phototransduction components in cephalopod chromatophores suggest dermal photoreception

Cephalopod mollusks are renowned for their colorful and dynamic body patterns, produced by an assemblage of skin components that interact with light. These may include iridophores, leucophores, chromatophores and (in some species) photophores. Here, we present molecular evidence suggesting that cephalopod chromatophores – small dermal pigmentary organs that reflect various colors of light – are photosensitive. RT-PCR revealed the presence of transcripts encoding rhodopsin and retinochrome within the retinas and skin of the squid Doryteuthis pealeii, and the cuttlefish Sepia officinalis and Sepia latimanus. In D. pealeii, Gqα and squid TRP channel transcripts were present in the retina and in all dermal samples. Rhodopsin, retinochrome and Gqα transcripts were also found in RNA extracts from dissociated chromatophores isolated from D. pealeii dermal tissues. Immunohistochemical staining labeled rhodopsin, retinochrome and Gqα proteins in several chromatophore components, including pigment cell membranes, radial muscle fibers, and sheath cells. This is the first evidence that cephalopod dermal tissues, and specifically chromatophores, may possess the requisite combination of molecules required to respond to light.

Functional characterisation of the chromatically antagonistic photosensitive mechanism of erythrophores in the tilapia Oreochromis niloticus

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.

Ontogeny of melanophore photosensitivity in rainbow trout (Oncorhynchus mykiss)

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.

The influence of chromatic background on the photosensitivity of tilapia erythrophores

Non-mammalian vertebrates and invertebrates use extraretinal photoreceptors to detect light and perform diverse non-image-forming functions. Compared to well-studied visual systems, the effect of ambient light conditions on photosensory systems of extraretinal photoreceptors is poorly understood. Chromatophores are photosensitive dermal pigment cells that play an important role in the formation of body color patterns to fit the surrounding environment. Here, we used tilapia erythrophores to investigate the relationship between environmental light and chromatophore photoresponses. All erythrophores from three spectral conditions aggregated their pigment granules in UV/short wavelengths and dispersed in middle/long wavelengths. Unlike retinal visual systems, environmental light did not change the usage of the primary opsins responsible for aggregation and dispersion. In addition, short wavelength-rich and red-shifted background conditions led to an inhibitory effect on erythrophore photoresponses. We suggest that, as extraretinal photoreceptors for non-image-forming functions, chromatophores directly adjust their photoresponse sensitivity via changes in opsin expression levels rather than opsin types when environmental light changes.