Morphological color changes in fish: Regulation of pigment cell density and morphology

Visual input regulates melanophore differentiation

Introduction

Developmental processes continue in organisms in which sensory systems have reached functional maturity, however, little research has focused on the influence of sensory input on cell and tissue development. Here, we explored the influence of visual system activity on the development of skin melanophores in Xenopus laevis.

Methods

Melanophore number was measured in X. laevis larvae after the manipulation of visual input through eye removal (enucleation) and/or incubation on a white or black substrate at the time when the visual system becomes functional (stage 40). To determine the developmental process impacted by visual input, migration, proliferation and differentiation of melanophores was assessed. Finally, the role of melatonin in driving melanophore differentiation was explored.

Results

Enucleating, or maintaining stage 40 larvae on a black background, results in a pronounced increase in melanophore number in the perioptic region within 24 h. Time lapse analysis revealed that in enucleated larvae new melanophores appear through gradual increase in pigmentation, suggesting unpigmented cells in the perioptic region differentiate into mature melanophores upon reduced visual input. In support, we observed increased expression of melanization genes tyr, tyrp1, and pmel in the perioptic region of enucleated or black background-reared larvae. Conversely, maintaining larvae in full light suppresses melanophore differentiation. Interestingly, an extra-pineal melatonin signal was found to be sufficient and necessary to promote the transition to differentiated melanophores.

Discussion

In this study, we found that at the time when the visual system becomes functional, X. laevis larvae possess a population of undifferentiated melanophores that can respond rapidly to changes in the external light environment by undergoing differentiation. Thus, we propose a novel mechanism of environmental influence where external sensory signals influence cell differentiation in a manner that would favor survival.

Molecular characterization, spatiotemporal expression, and background adaptation regulation of tyrosinase in loach (Misgurnus anguillicaudatus)

The Poyang Lake region is home to large-blackspot loaches (LBL), small-blackspot loaches (SBL), and non-blackspot loaches (NBL), Misgurnus anguillicaudatus. To investigate the impact of tyrosinase on spot development, the complementary DNAs (cDNA) of tyrosinase in M. anguillicaudatus (designated as Matyr) were cloned using the rapid amplification of cDNA ends (RACE)-PCR method. The full-length cDNA for Matyr was 2020 bp, and the open-reading frame comprised 1617 bp, encoding a predicted protein with 538 amino acids. Phylogenetic studies revealed that MaTyr was first grouped with Tyr of Triplophysa tibetana and Leptobotia taeniops, and then Tyr of other cyprinid fish. The quantitative reverse-transcription-PCR results show that Matyr was highly expressed in the muscle, caudal fin, and dorsal skin. The Matyr gene's messenger RNA expression pattern steadily increased from the fertilized ovum period to the somitogenesis period, and from the muscle effect stage to 6 days after fertilization, it considerably increased (p < 0.01). The Matyr hybridization signals with similar location could be found in all developmental stages of three kinds of loaches using whole-mount in situ hybridization (WISH) technology and were the strongest during the organ development period and melanin formation period. Dot hybridization signals in LBLs rapidly spread to the back of the body beginning at the period when the eyes first formed melanin, and their dimensions were larger than those of NBLs during the same time period. The body color of loaches could change reversibly with black/white background adaptation. The α-msh, mitfa, and tyr are mainly expressed in loaches adapted with a black background. Tyr gene could be involved in the development of blackspots and body color polymorphism, and contribute to organ development in the loach.

Disparities in Body Color Adaptability and Ambient Light Color Preference between Wild and Hatchery-Reared Marbled Rockfish (Sebastiscus marmoratus)

Hatchery rearing significantly influences the phenotypic development of fish, with potential adverse effects for the post-release performance of hatchery-reared individuals in natural environments, especially when targeted for stock enhancement. To assess the suitability of releasing hatchery-reared fish, a comprehensive understanding of the phenotypic effects of captive rearing, through comparisons with their wild conspecifics, is essential. In this study, we investigated the divergence in body coloration between wild and hatchery-reared marbled rockfish Sebastiscus marmoratus. We examined the selection preferences for different light colors and assessed the impact of different ambient light colors on the morphological color-changing ability of juvenile marbled rockfish. Our findings revealed significant differences in body color between wild and hatchery-reared marbled rockfish. The hue and saturation values of wild marbled rockfish were significantly higher than those of their hatchery-reared counterparts, indicative of deeper and more vibrant body coloration in the wild population. Following a ten-day rearing period under various light color environments, the color of wild marbled rockfish remained relatively unchanged. In contrast, hatchery-reared marbled rockfish tended to change their color, albeit not reaching wild-like coloration. Light color preference tests demonstrated that wild juvenile marbled rockfish exhibited a preference for a red-light environment, while hatchery-reared individuals showed a similar but weaker response. Both wild and hatchery-reared marbled rockfish displayed notable negative phototaxis in the presence of yellow and blue ambient light. These results highlight the impact of hatchery rearing conditions on the body color and morphological color-changing ability, and provide insight into light color selection preferences of marbled rockfish. To mitigate the divergence in phenotypic development and produce more wild-like fish for stocking purposes, modifications to the hatchery environment, such as the regulation of ambient light color, should be considered.

Functional relation of agouti signaling proteins (ASIPs) to pigmentation and color change in the starry flounder, Platichthys stellatus

We investigated the involvement of agouti-signaling proteins (ASIPs) in morphological pigmentation and physiological color change in flatfishes. We isolated ASIP1 and 2 mRNAs from the skin of starry flounder (Platichthys stellatus), and compared their amino acid (aa) structures to those of other animals. Then, we examined the mRNA expression levels of two ASIPs (Sf-ASIPs) in the pigmented ocular body and in the unpigmented blind body, as well as in the ordinary skin and in albino skin, in flatfishes. To investigate the role of Sf-ASIPs in physiological color change (color camouflage), we compared the expression of the two genes in two background colors (dark-green and white). Sf-ASIP1 cDNA had a 375-bp open reading frame (ORF) that encoded a protein consisting of 125 aa residues, and Sf-ASIP2 cDNA had a 402-bp ORF that encoded a protein consisting of 132 aa residues. RT-PCR revealed that the strongest Sf-ASIP1 and Sf-ASIP2 expression levels were observed in the eye and blind-skin, respectively. In Sf-ASIP1, the gene expression did not differ between the ocular-side skin and blind-side skin, nor between ordinary skin and abnormal skin of the fish. However, in Sf-ASIP2, the expression level was significantly higher in blind-side skin, compared to ocular-side skin, suggesting that the ASIP2 gene is related to the countershading body pigment pattern of the fish. In addition, the Sf-ASIP2 gene expression level was lower in the pigmented spot regions than in the unpigmented spot regions of the malpigmented pseudo-albino skins on the ocular side, implying that ASIP2 is responsible for the ocular-side pseudo-albino. Additionally, ASIP2 gene expression in the blind-side skin of ordinary fish was enhanced by a white tank, implying that a bright background color could inhibit hypermelanosis in the blind-side skin of cultured flounder by increasing the activity of the Sf-ASIP2 gene. However, we did not find any relationship of ASIPs with camouflage color changes. In conclusion, the ASIP2 gene is related to the morphological pigmentation (countershading and malpigmentation) of the skin in starry flounder, but not with physiological color changes (color camouflage) in the ocular-side skin.

Triphenyltin induced darker body coloration by disrupting melanocortin system and pteridine metabolic pathway in a reef fish, Amphiprion ocellaris

Triphenyltin (TPT) is a typical persistent organic pollutant whose occurrence in coral reef ecosystems may threaten the survival of reef fishes. In this study, a brightly colored representative reef fish, Amphiprion ocellaris was used to explore the effects of TPT at environmental levels (1, 10, and 100 ng/L) on skin pigment synthesis. After the fish were exposed to TPT for 60 days, the skin became darker, owing to an increase in the relative area of black stripes, a decrease in orange color values while an increase in brown color values, and an increase in the number of melanocytes in the orange part of the skin tissues. To explore the mechanisms by which TPT induces darker body coloration, the enzymatic activity and gene expression levels of the members of melanocortin system that affect melanin synthesis were evaluated. Leptin levels and lepr expression were found to be increased after TPT exposure, which likely contributed to the increase found in pomc expression and α-melanocyte-stimulating hormone (α-MSH) levels. Then Tyr activity and mc1r, tyr, tyrp1, mitf, and dct were upregulated, ultimately increasing melanin levels. Importantly, RT-qPCR results were consistent with the transcriptome analysis of trends in lepr and pomc expression. Because the orange color values decreased, pterin levels and the pteridine metabolic pathway were also evaluated. The results showed that TPT induced BH4 levels and spr, xdh, and gch1 expression associated with pteridine synthesis decreased, ultimately decreasing the colored pterin content (sepiapterin). We conclude that TPT exposure interferes with the melanocortin system and pteridine metabolic pathway to increase melanin and decrease colored pterin levels, leading to darker body coloration in A. ocellaris. Given the importance of body coloration for the survival and reproduction of reef fishes, studies on the effects of pollutants (others alongside TPT) on body coloration are of high priority.

Comparative transcriptome analysis reveals growth and molecular pathway of body color regulation in turbot (Scophthalmus maximus) exposed to different light spectrum

Fish body color changes play vital roles in adapting to ecological light environment and influencing market value. However, the initial mechanisms governing the changes remain unknown. Here, we scrutinized the impact of light spectrum on turbot (Scophthalmus maximus) body coloration, exposing them to red, blue, and full light spectra from embryo to 90 days post hatch. Transcriptome and quantitative real-time PCR (qRT-PCR) analyses were employed to elucidate underlying biological processes. The results showed that red light induced dimorphism in turbot juvenile skin pigmentation: some exhibited black coloration (Red_Black_Surface, R_B_S), while others displayed lighter skin (Red_White_Bottom, R_W_B), with red light leading to reduced skin lightness (L*) and body weight, particularly in R_B_S group. Transcriptomic and qRT-PCR analyses showcased upregulated gene expressions related to melanin synthesis in R_B_S individuals, notably tyrosinase (tyr), tyrosinase-related protein 1 (tyrp1), and dopachrome tautomerase (dct), alongside solute carrier family 24 member 5 (slc24a5) and oculocutaneous albinism type II (oca2) as pivotal regulators. Nervous system emerged as a critical mediator in spectral environment-driven color regulation. N-methyl d-aspartate (NMDA) glutamate receptor, and calcium signaling pathway emerged as pivotal links intertwining spectral conditions, neural signal transduction, and color regulation. The individual differences in NMDA glutamate receptor expression and subsequent neural excitability seemed responsible for dichromatic body coloration in red light-expose juveniles. This study provides new insights into the comprehending of fish adaptation to environment and methods for fish body color regulation and could potentially help enhance the economic benefit of fish farming industry.

Light-specific wavelengths differentially affect the exploration rate, opercular beat, skin color change, opsin transcripts, and the oxi-redox system of the longsnout seahorse Hippocampus reidi

Light is a strong stimulus for the sensory and endocrine systems. The opsins constitute a large family of proteins that can respond to specific light wavelengths. Hippocampus reidi is a near-threatened seahorse that has a diverse color pattern and sexual dimorphism. Over the years, H. reidi's unique characteristics, coupled with its high demand and over-exploitation for the aquarium trade, have raised concerns about its conservation, primarily due to their significant impact on wild populations. Here, we characterized chromatophore types in juvenile and adult H. reidi in captivity, and the effects of specific light wavelengths with the same irradiance (1.20 mW/cm2) on color change, growth, and survival rate. The xanthophores and melanophores were the major components of H. reidi pigmentation with differences in density and distribution between life stages and sexes. In the eye and skin of juveniles, the yellow (585 nm) wavelength induced a substantial increase in melanin levels compared to the individuals kept under white light (WL), blue (442 nm), or red (650 nm) wavelengths. In addition, blue and yellow wavelengths led to a higher juvenile mortality rate in comparison to the other treatments. Adult seahorses showed a rhythmic color change over 24 h, the highest reflectance values were obtained in the light phase, representing a daytime skin lightening for individuals under WL, blue and yellow wavelength, with changes in the acrophase. The yellow wavelength was more effective on juvenile seahorse pigmentation, while the blue wavelength exerted a stronger effect on the regulation of adult physiological color change. Dramatic changes in the opsin mRNA levels were life stage-dependent, which may infer ontogenetic opsin functions throughout seahorses' development. Exposure to specific wavelengths differentially affected the opsins mRNA levels in the skin and eyes of juveniles. In the juveniles, skin transcripts of visual (rh1, rh2, and lws) and non-visual opsins (opn3 and opn4x) were higher in individuals under yellow light. While in the juvenile's eyes, only rh1 and rh2 had increased transcripts influenced by yellow light; the lws and opn3 mRNA levels were higher in juveniles' eyes under WL. Prolonged exposure to yellow wavelength stimulates a robust increase in the antioxidant enzymes sod1 and sod2 mRNA levels. Our findings indicate that changes in the visible light spectrum alter physiological processes at different stages of life in H. reidi and may serve as the basis for a broader discussion about the implications of artificial light for aquatic species in captivity.

A fish can change its stripes: investigating the role of body colour and pattern in the bluelined goatfish

Bluelined goatfish (Upeneichthys lineatus) rapidly change their body colour from a white horizontally banded pattern to a seemingly more conspicuous vertically banded red pattern, often when foraging. Given the apparent conspicuousness of the pattern to a range of observers, it seems unlikely that this colour change is used for camouflage and instead may be used for communication/signalling. Goatfish often drive multispecies associations, and it is possible that goatfish use this colour change as a foraging success signal to facilitate cooperation, increase food acquisition, and reduce predation risk through a 'safety in numbers' strategy. Using a novel approach, we deployed 3D model goatfish in different colour morphs-white without bands, white with black vertical bands, and white with red vertical bands-to determine whether the red colouration is an important component of the signal or if it is only the vertical banding pattern, regardless of colour, that fish respond to as an indicator of foraging success. Use of remote underwater video allowed us to obtain information without the influence of human observers on the communities and behaviours of other fish in response to these different colours exhibited by goatfish. We found that conspecifics were more abundant around the black- and red-banded model fish when compared with the white models. Conspecifics were also more likely to forage around the models than to pass or show attraction, but this was unaffected by model colour. No difference in the abundance and behaviour of associated heterospecifics around the different models was observed, perhaps due to the static nature of the models. Some species did, however, spend more time around the red- and black-banded fish, which suggests the change in colour may indicate benefits in addition to food resources. Overall, the results suggest that the body colour/pattern of U. lineatus is likely a signalling tool but further work is required to explore the benefits to both conspecifics and heterospecifics and to further determine the behavioural functions of rapid colour change in U. lineatus.

Weighted correlation network analysis of the genes in the eyes of juvenile Plectropomus leopardus provide novel insights into the molecular mechanisms of the adaptation to the background color

Plectropomus leopardus is a valuable marine fish whose skin color is strongly affected by the background color. However, the influence of the visual sense on the skin color variation of P. leopardus remains unknown. In the present study, transcriptome analysis was used to examine the visual response mechanism under different background colors. Paraffin sections of the eyes showed that the background color caused morphological changes in the pigment cells (PCs) and outer nuclear layer (ONL) and the darkening of the iris color. The transcriptome analysis results indicated that the gene expressions in the eyes of P. leopardus were significantly different for different background colors. We identified 4845, 3069, 5874, and 6309 differentially expressed genes (DEGs) in the pairwise comparisons of white vs. initial, blue vs. initial, red vs. initial, and black vs. initial groups, respectively. Some hub genes and key pathways regulating the adaptive mechanism of P. leopardus's eyes to the background color were identified, i.e., the JAK-STAT, mTOR, and Ras signaling pathways, and the ndufb7, slc6a13, and novel.3553 gene. This adaptation was achieved through the synthesis of stress proteins and energy balance supply mediated by hub genes and key pathways. In addition, the phenylalanine metabolism, tyrosine metabolism, and actin cytoskeleton-related processes or pathways and genes were responsible for iris and skin color adaptation. In summary, we inferred that stress protein synthesis, phenylalanine metabolism, and energy homeostasis were critical stress pathways for P. leopardus to adapt its skin color to the environment. These new findings indicate that the P. leopardus skin color variation may have been caused by the environmental adaption of the eyes. The results provide new insights into the molecular mechanisms underlying the skin color adaptation of P. leopardus.

Exploring the mechanisms and impacts of melatonin on fish colouration

The pineal hormone melatonin is a multi-functional molecule with a recognized role in pigment aggregation in chromatophores, mediating its actions through binding to subtypes of its specific receptors. Since its discovery, melatonin has been known to be responsible for pigment aggregation towards the cell centre in fishes, including their embryos, as an adaptation to reduced light and thus results in pale body colouration. Diversity exists in the sensitivity of melanophores towards melatonin at interspecies, intraspecific levels, seasons, and amongst chromatophores at different regions of the animal body. In most of the fishes, melatonin leads to their skin paling at night. It is indicated that the melatonin receptors have characteristically maintained to show the same aggregating effects in fishes and other vertebrates in the evolutionary hierarchy. However, besides this aggregatory effect, melatonin is also responsible for pigment dispersion in certain fishes. Here is the demand in our review to explore further the nature of the dispersive behaviour of melatonin through the so-called β-melatonin receptors. It is clear that the pigment translocations in lower vertebrates under the effect of melatonin are mediated through the melatonin receptors coupled with other hormonal receptors as well. Therefore, being richly supplied with a variety of receptors, chromatophores and melanocytes can be used as in vitro test models for pharmacological applications of known and novel drugs. In this review, we present diverse effects of melatonin on chromatophores of fishes in particular with appropriate implications on most of the recent findings.

Uric acid is a major chemical constituent for the whitish coloration in the medaka leucophores

In whitish parts of teleost skin, the coloration is attributed to a light scattering phenomenon within light-reflecting chromatophores, namely leucophores and iridophores, which contain high refractive index materials in their cytoplasmic organelles, leucosomes and light-reflecting platelets, respectively. Previous chemical examinations revealed that guanine is a major constituent of the materials in the platelets of the iridophores, while, in leucophores, the detailed chemical nature of the materials contained in the leucosomes has not been reported. Here, using liquid chromatography-tandem mass spectroscopy, we investigated the chemical features of materials eluted from scales, larvae, and single chromatophores of the medaka. Results of the liquid chromatography-tandem mass spectroscopy suggested that uric acid is a major constituent of the high refractive index materials in medaka leucophores and is a unique marker to investigate the presence of leucophores in the fish. The whitish appearance of the medaka leucophores may be attributed to the light-scattering phenomenon in leucosomes, which contain highly concentrated uric acid.

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.

Background matching through fast and reversible melanin-based pigmentation plasticity in tadpoles comes with morphological and antioxidant changes

Facultative colour change is widespread in the animal kingdom, and has been documented in many distantly related amphibians. However, experimental data testing the extent of facultative colour change, and associated physiological and morphological implications are comparatively scarce. Background matching in the face of spatial and temporal environmental variation is thought to be an important proximate function of colour change in aquatic amphibian larvae. This is particularly relevant for species with long larval periods such as the western spadefoot toad, Pelobates cultripes, whose tadpoles spend up to six months developing in temporary waterbodies with temporally variable vegetation. By rearing tadpoles on different coloured backgrounds, we show that P. cultripes larvae can regulate pigmentation to track fine-grained differences in background brightness, but not hue or saturation. We found that colour change is rapid, reversible, and primarily achieved through changes in the quantity of eumelanin in the skin. We show that this increased eumelanin production and/or maintenance is also correlated with changes in morphology and oxidative stress, with more pigmented tadpoles growing larger tail fins and having an improved redox status.

Effects of monochromatic lights on the melanophores arrangement in the spotted snakehead fish Channa punctata (Bloch, 1793)

Some freshwater teleost fish have pigment cells whose arrangement and shape are affected by the environment. Natural light has a wide range of light intensity. Fish are sensitive to the background and exposed light colour. Fish body colour is a significant criterion in fixing its market value, whether it is ornamental or edible. By favourable light exposure, a culturist may get a good market value of fish on most ethical grounds. In this study, we recorded the changes in melanophore response with the changes in light colour on Channa punctata. Adult fish were treated with monochromatic lights (darkness, white, blue and red light) for 5 and 28 days. After treatment, their body colour and melanophore size, number, length and the number of dendrites were studied. The results showed a significant influence of monochromatic light on melanophore arrangement in fish skin. The data showed that blue light is appropriate for the overall species colour of photic C. punctata. Continuous black or white light caused severe damage to the fish's appearance.

An Attempt to Identify the Medaka Receptor for Somatolactin Alpha Using a Reverse Genetics Approach

Somatolactin alpha (SLα) is a fish-specific hormone involved in body color regulation. The growth hormone (GH) is another hormone that is expressed in all vertebrates and promotes growth. These peptide hormones act by binding to receptors (SLα receptor (SLR) and GH receptor (GHR)); however, the relationships between these ligands and their receptors vary among species. Here, we first performed phylogenetic tree reconstruction by collecting the amino-acid sequences classified as SLR, GHR, or GHR-like from bony fish. Second, we impaired SLR or GHR functions in medaka (Oryzias sakaizumii) using CRISPR/Cas9. Lastly, we analyzed SLR and GHR mutants for phenotypes to deduce their functions. Phylogenetic tree reconstruction was performed using a total of 222 amino-acid sequences from 136 species, which revealed that many GHRa and GHRb are vaguely termed as GHR or GHR-like, while showing no orthologous/paralogous relationships. SLR and GHR mutants were successfully established for phenotyping. SLR mutants exhibited premature lethality after hatching, indicating an essential role for SLR in normal growth. GHR mutations did not affect viability, body length, or body color. These results provide no evidence that either SLR or GHR functions as a receptor for SLα; rather, phylogenetically and functionally, they seem to be receptors for GH, although their (subfunctionalized) roles warrant further investigation.

The Effect of Background Color on Skin Color Variation of Juvenile Plectropomus leopardus

Fish skin color is usually strongly affected by the background color of their environment. The study investigated the effects of five different background colors on the skin color of leopard coral groupers (Plectropomus leopardus). More than 450 juveniles were reared in Blue, Red, Black, White, and Transparent background tanks for 56 days. The paraffin section showed that the skin melanin zone of fish in the White group was smaller, whereas the Black and Red groups (especially Black) were nearly the largest. The apparent skin color of P. leopardus was red on the white background, which darkened in response to the other color backgrounds. The Black group revealed the blackest skin color, followed by the transparent group. Moreover, the White group had the highest L*, a*, and b* values. The melanin content and tyrosinase activity in the dorsal and ventral skin of the Black group were significantly higher than those in the other groups (p < 0.05), and the serum α-MSH level was higher in the Black group as well. The carotenoid and lutein contents showed completely different trends among the experimental groups, as carotenoid content was higher in the Red and White groups, while lutein content was higher in the Transparent group. The expression level of scarb1 was highest in the Blue and White groups, followed by the Transparent group, and lowest in the Black group (p < 0.05). The expression trend of scarb1 was similar to the skin color in different backgrounds, indicating that the background color regulated scarb1 expression level through visual center, then influenced the uptake and transport of carotenoids, then influenced the skin color formation of P. leopardus. Moreover, lighter colors inhibited the formation of melanocytes and had a significant effect on carotenoid and lutein contents. Pigment-related genes were involved in the regulation of fish skin color, and they were affected by background color in P. leopardus. These results indicate that a white background is more conducive to maintaining red skin color in juvenile P. leopardus.

Transcriptome Analysis Reveals the Complex Regulatory Pathway of Background Color in Juvenile Plectropomus leopardus Skin Color Variation

Fish skin color is often strongly affected by background color. We hypothesized that the regulatory mechanism of variations in skin color in P. leopardus is linked to the background color. In this study, we conducted transcriptome analysis of Plectropomus leopardus cultured under different background colors to compare gene expression levels and the important signaling pathways. The RNA-seq analysis yielded 26,675 known mRNAs, 3278 novel mRNAs, and 3179 differentially expressed genes (DEGs). The DEGs related to melanin synthesis were screened out. Some key melanin-related genes were identified, specifically tyr, slc7a11, mc1r, ednrb, dct, tat, and wnt1. These DEGs were mainly involved in melanogenesis, including tyrosine metabolism, the Wnt signaling pathway, and the cAMP signaling pathway. The expression levels of some key genes were upregulated when background color deepened, such as α-msh, wnt, and gf. The α-MSH/cAMP-dependent, Wnt/β-catenin, and PI3K/Akt signaling pathways were activated, resulting in the accumulation of intracellular mitf. mitf promoted melanin production by binding to the tyr/tyrp1/dct promoter region. In the present study, we explored the molecular mechanism underlying the darkened skin color pattern of P. leopardus, providing a theoretical basis for the molecular mechanism underlying pigmentation in P. leopardus.

At the Intersection of Natural Structural Coloration and Bioengineering

Most of us get inspired by and interact with the world around us based on visual cues such as the colors and patterns that we see. In nature, coloration takes three primary forms: pigmentary coloration, structural coloration, and bioluminescence. Typically, pigmentary and structural coloration are used by animals and plants for their survival; however, few organisms are able to capture the nearly instantaneous and visually astounding display that cephalopods (e.g., octopi, squid, and cuttlefish) exhibit. Notably, the structural coloration of these cephalopods critically relies on a unique family of proteins known as reflectins. As a result, there is growing interest in characterizing the structure and function of such optically-active proteins (e.g., reflectins) and to leverage these materials across a broad range of disciplines, including bioengineering. In this review, I begin by briefly introducing pigmentary and structural coloration in animals and plants as well as highlighting the extraordinary appearance-changing capabilities of cephalopods. Next, I outline recent advances in the characterization and utilization of reflectins for photonic technologies and and discuss general strategies and limitations for the structural and optical characterization of proteins. Finally, I explore future directions of study for optically-active proteins and their potential applications. Altogether, this review aims to bring together an interdisciplinary group of researchers who can resolve the fundamental questions regarding the structure, function, and self-assembly of optically-active protein-based materials.

Exoskeleton ageing and its relation to longevity and fecundity in female Australian leaf insects (

Senescence is a decline in reproduction and survival rate with advancing age resulting from deterioration of somatic tissues and systems throughout the body. Age-related somatic changes (somatic ageing) have been studied extensively in vertebrates but are less well known in other animals, including insects. Since adult insects have very limited ability to repair their exoskeleton, somatic ageing could involve deterioration and discolouration of the cuticle. We investigated age-related changes in wing pigmentation and abdominal cuticle necrosis in females of the Australian leaf insect Phyllium monteithi. Adult females varied markedly in the extent and pattern of pigmentation on their bodies, and we found that pigment spots on the forewings increased in size with age in most individuals. As females aged, most individuals also exhibited increasing levels of abdominal cuticle necrosis, resulting in the loss of abdominal cuticle along the margin of the abdomen. Neither the extent of pigmentation nor cuticle loss were clearly associated with reduced fecundity or longevity in the protected laboratory environment, but it remains unknown whether these age-related changes have functional implications in the wild. Our results show that the P. monteithi exoskeleton undergoes complex changes with age, with potential implications for functional traits and fitness.

Hybrid confocal fluorescence and photoacoustic microscopy for the label-free investigation of melanin accumulation in fish scales

Lower vertebrates, including fish, can rapidly alter skin lightness through changes in melanin concentration and melanosomes’ mobility according to various factors, which include background color, light intensity, ambient temperature, social context, husbandry practices and acute or chronic stressful stimuli. Within this framework, the determination of skin chromaticity parameters in fish species is estimated either in specific areas using colorimeters or at the whole animal level using image processing and analysis software. Nevertheless, the accurate quantification of melanin content or melanophore coverage in fish skin is quite challenging as a result of the laborious chemical analysis and the typical application of simple optical imaging methods, requiring also to euthanize the fish in order to obtain large skin samples for relevant investigations. Here we present the application of a novel hybrid confocal fluorescence and photoacoustic microscopy prototype for the label-free imaging and quantification of melanin in fish scales samples with high spatial resolution, sensitivity and detection specificity. The hybrid images are automatically processed through optimized algorithms, aiming at the accurate and rapid extraction of various melanin accumulation indices in large datasets (i.e., total melanin content, melanophores’ area, density and coverage) corresponding to different fish species and groups. Furthermore, convolutional neural network-based algorithms have been trained using the recorded data towards the classification of different scales’ samples with high accuracy. In this context, we demonstrate that the proposed methodology may increase substantially the precision, as well as, simplify and expedite the relevant procedures for the quantification of melanin content in marine organisms.

Evaluation of Age-Dependent Changes in the Coloration of Male Killifish Nothobranchius Guentheri Using New Photoprocessing Methods

Simple Summary

This paper proposes a new methodology for evaluating fish coloration, which allows us to identify differences in the intensity of coloration of specific areas of the body. Changes in fish coloration occur during growth and under the influence of environmental factors. Male fish belonging to the family Nothobranchius are characterized by extremely diverse coloration, depending on the age of the fish, environmental factors, and social hierarchical status. As the lifespan of this genus of fish is very short (12–14 months), studies on age-dependent changes are possible. In this study, we demonstrate correlations between the coloration of particular body zones of male Nothobranchius guentheri and age using new photofixation methods and image processing software. This methodology can be applied to other fish with unique coloration patterns, for example, family Cichlidae and order Cyprinodontiformes.

Abstract

Fish as model objects have found wide applications in biology and fundamental medicine and allow studies of behavioral and physiological responses to various environmental factors. Representatives of the genus Nothobranchius are one of the most convenient objects for such studies. Male fish belonging to the family Nothobranchiidae are characterized by extremely diverse coloration, which constantly changes, depending on the age of the fish, environmental factors, and social hierarchical status. These fish species are characterized by a short life cycle, which allows changes in coloration, an indicator of the ontogenesis stage, to be estimated. Existing methods of fish color assessments do not allow the intensity of coloration of particular body zones to be clearly differentiated. In the present study, we suggest a method of two-factor assessment of specific fish body zones using modified methods of photofixation and image processing software. We describe the protocol of the method and the results of its application to different-aged groups of male Nothobranchius guentheri. The coloration of selected areas (i.e., red spot on the gill cover (RSGC), black border on the caudal fin (BBCF), and white border on the dorsal fin (WBDF)) differed significantly according to the size and age of the fish (p < 0.05). The data obtained suggest that N. guentheri can be a model for studying aging by the intensity of body coloration in males.

Type II Opsins in the Eye, the Pineal Complex and the Skin of Xenopus laevis: Using Changes in Skin Pigmentation as a Readout of Visual and Circadian Activity

The eye, the pineal complex and the skin are important photosensitive organs. The African clawed frog, Xenopus laevis, senses light from the environment and adjusts skin color accordingly. For example, light reflected from the surface induces camouflage through background adaptation while light from above produces circadian variation in skin pigmentation. During embryogenesis, background adaptation, and circadian skin variation are segregated responses regulated by the secretion of α-melanocyte-stimulating hormone (α-MSH) and melatonin through the photosensitivity of the eye and pineal complex, respectively. Changes in the color of skin pigmentation have been used as a readout of biochemical and physiological processes since the initial purification of pineal melatonin from pigs, and more recently have been employed to better understand the neuroendocrine circuit that regulates background adaptation. The identification of 37 type II opsin genes in the genome of the allotetraploid X. laevis, combined with analysis of their expression in the eye, pineal complex and skin, is contributing to the elucidation of the role of opsins in the different photosensitive organs, but also brings new questions and challenges. In this review, we analyze new findings regarding the anatomical localization and functions of type II opsins in sensing light. The contribution of X. laevis in revealing the neuroendocrine circuits that regulate background adaptation and circadian light variation through changes in skin pigmentation is discussed. Finally, the presence of opsins in X. laevis skin melanophores is presented and compared with the secretory melanocytes of birds and mammals.

The weaker sex: Male lingcod (Ophiodon elongatus) with blue color polymorphism are more burdened by parasites than are other sex-color combinations

The unusual blue color polymorphism of lingcod (Ophiodon elongatus) is the subject of much speculation but little empirical research; ~20% of lingcod individuals exhibit this striking blue color morph, which is discrete from and found within the same populations as the more common brown morph. In other species, color polymorphisms are intimately linked with host–parasite interactions, which led us to ask whether blue coloration in lingcod might be associated with parasitism, either as cause or effect. To test how color and parasitism are related in this host species, we performed parasitological dissection of 89 lingcod individuals collected across more than 26 degrees of latitude from Alaska, Washington, and California, USA. We found that male lingcod carried 1.89 times more parasites if they were blue than if they were brown, whereas there was no difference in parasite burden between blue and brown female lingcod. Blue individuals of both sexes had lower hepatosomatic index (i.e., relative liver weight) values than did brown individuals, indicating that blueness is associated with poor body condition. The immune systems of male vertebrates are typically less effective than those of females, due to the immunocompromising properties of male sex hormones; this might explain why blueness is associated with elevated parasite burdens in males but not in females. What remains to be determined is whether parasites induce physiological damage that produces blueness or if both blue coloration and parasite burden are driven by some unmeasured variable, such as starvation. Although our study cannot discriminate between these possibilities, our data suggest that the immune system could be involved in the blue color polymorphism–an exciting jumping-off point for future research to definitively identify the cause of lingcod blueness and a hint that immunocompetence and parasitism may play a role in lingcod population dynamics.

Social context affects camouflage in a cryptic fish species

Crypsis, or the ability to avoid detection and/or recognition, is an important and widespread anti-predator strategy across the animal kingdom. Many animals are able to camouflage themselves by adapting their body colour to the local environment. In particular, rapid changes in body colour are often critical to the survival of cryptic prey which rely on evading detection by predators. This is especially pertinent for animals subject to spatio-temporal variability in their environment, as they must adapt to acute changes in their visual surroundings. However, which features of the local environment are most relevant is not well understood. In particular, little is known about how social context interacts with other environmental stimuli to influence crypsis. Here, we use a common cryptic prey animal, the goby (Pseudogobius species 2) to examine how the presence and body colour of conspecifics influence the rate and extent to which gobies change colour. We find that solitary gobies change colour to match their background faster and to a greater extent than gobies in pairs. Further, we find that this relationship holds irrespective of the colour of nearby conspecifics. This study demonstrates the importance of social context in mediating colour change in cryptic animals.

Effects of background color and feeding status on the expression of genes associated with body color regulation in the goldfish Carassius auratus

Alpha-melanocyte-stimulating hormone (α-MSH), a peptide derived from proopiomelanocortin (POMC), and melanin-concentrating hormone (MCH), act as neuromodulators and regulate food intake in vertebrates. In teleosts, these peptides are also involved competitively in body color regulation; α-MSH induces a dark body color, while MCH induces a pale body color. Similarly, members of the growth hormone (GH) family, somatolactin (SL) and prolactin (PRL), which are involved in the regulation of energy metabolism, are also associated with body color regulation in teleosts. Since these hormones are involved in both body color regulation and energy metabolism, it is possible that feeding status can affect body color. Here, we examined the effects of fasting on the response of goldfish body coloration to changes in background color. Goldfish were acclimated for one week in tanks with a white or black background under conditions of periodic feeding or fasting. The results showed that body color and expression levels of pmch1 and pomc were affected by background color, irrespective of feeding status. Expression levels of sla were higher in fish maintained in tanks with a black background than in tanks with a white background, and higher in the fasted fish compared to the fed fish. However, the pattern of slb expression was almost the opposite of that observed in sla expression. The expression levels of gh and prl in the pituitary, and pmch2a and pmch2b in the brain, were not affected by background color. These results suggest that MCH, α-MSH, SLα, and SLβ might be involved in body color regulation and that they are affected by background color in goldfish. The results also suggest that feeding status may affect body color regulation via SLα and SLβ, although these effects might be limited compared to the effect of background color.

Effects of environmental antidepressants on colour change and locomotor behaviour in juvenile shore crabs, Carcinus maenas

Juvenile crabs of Carcinus maenas thrive in coastal waters reputed to be the receptacle of continental pollution. Amongst the many pollutants encountered, antidepressants, such as fluoxetine (FLX) and venlafaxine (VEN), often detected at the ng•L^-1 range, are particularly worrying because of their action on the levels of monoamines, such as serotonin, noradrenaline and dopamine. In crustaceans, those monoamines are involved in colour change through their action on neuropeptide hormones. In addition, they are known to have a role in different behaviours, such as locomotion. Both colour change and locomotion are strategies used by juvenile crabs to hide and escape from predators. To investigate if the presence of antidepressants may alter behaviours of ecological importance, juvenile crabs were exposed to environmentally realistic concentrations of either 5 ng•L^-1 of FLX alone or in combination with VEN at 5 ng•L^-1. The ability to change colour depending on the environment and the locomotor activity of juvenile crabs were monitored weekly over 25 days. Animals exposed to antidepressants displayed a different pattern of colour change than the controls, especially those exposed to the combination of FLX and VEN at 5 ng•L^-1 each, and were less efficient to adapt to their environment, i.e., they were not as pale and not as dark as controls or crabs exposed to FLX at 5 ng•L^-1. Moreover, juvenile crabs exposed to the combination of antidepressants exhibited an enhanced locomotor activity throughout the exposure period with a higher velocity and distance moved as well as more time spend moving. The alteration of cryptic behaviours, such as colour change and locomotion by antidepressants persistently present in marine environment at low concentrations may have an impact on the survival of juvenile of C. maenas on the long term.

The thermal dependence and molecular basis of physiological color change in Takydromus septentrionalis (Lacertidae)

One of the main functions of physiological color change is thermoregulation. This change occurs much more rapidly than morphological color change, but the underlying mechanism remains poorly understood. Here, we studied the thermal dependence and molecular basis of physiological color change in lizards using Takydromus septentrionalis (Lacertidae) as the model system. Body color was thermally sensitive, becoming increasingly light as body temperatures deviated from the level (∼30°C) preferred by this species. We identified 3389 differentially expressed genes (DEGs) between lizards at 24°C and 30°C, and 1,097 DEGs between lizards at 36°C and 30°C. Temperature affected the cAMP signal pathway, motor proteins, cytoskeleton, and the expression of genes related to melanocyte-stimulating hormone (MSH) and melanocyte-concentrating hormone (MCH). Our data suggest that the role of physiological color change in thermoregulation is achieved in T. septentrionalis by altering the arrangement of pigments and thus the amount of solar radiation absorbed and reflected. G protein-coupling system inhibits adenylate cyclase activity to transform ATP into cAMP and thereby causes rapid pigment aggregation. MCH deactivates the G proteins and thereby initiates pigment dispersion. This mechanism differs from that reported for teleost fish where MCH activates the G proteins and thereby causes pigment aggregation.

This article has an associated First Person interview with the first author of the paper.

Summary: G protein-coupling system inhibits adenylate cyclase activity to transform ATP into cAMP and thereby causes rapid pigment aggregation. MCH deactivates the G proteins and thereby initiates pigment dispersion.

Overview of the betta fish genome regarding species radiation, parental care, behavioral aggression, and pigmentation model relevant to humans

BACKGROUND

The Siamese fighting fish (Betta splendens, also known as the betta) is well known in aquarium markets, and also presents an exciting new research model for studying parental care, aggressive behavior, and cryptically diverse pigmentation. However, concentrated efforts are required, both in the context of conservation biology and in its genetics, to address the problems of ongoing outbreeding depression, loss of biodiversity, and lack of scientific biological information.

OBJECTIVE

The evolutionary dynamics of the betta must be better understood at the genomic scale in order to resolve the phylogenetic status of unrecognized species, develop molecular markers to study variation in traits, and identify interesting sets of genes encoding various bioresource functions.

METHODS

The recent revolution in multi-omics approaches such as genomics, transcriptomics, epigenomics, and proteomics has uncovered genetic diversity and gained insights into many aspects of betta bioresources.

RESULTS

Here, we present current research and future plans in an ongoing megaproject to characterize the betta genome as de novo assemblies, genes and repeat annotations, generating data to study diverse biological phenomena. We highlight key questions that require answers and propose new directions and recommendations to develop bioresource management to protect and enhance the betta genus.

CONCLUSION

Successful accomplishment of these plans will allow the creation of a reference annotated genome and provide valuable information at the molecular level that can be utilized to sustain biodiversity and eco-management of the betta to improve breeding programs for future biomedical research.

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.

Use of alternative protein sources for fishmeal replacement in the diet of largemouth bass (Micropterus salmoides). Part II: effects of supplementation with methionine or taurine on growth, feed utilization, and health

Fishmeal has long been a staple protein feedstuff for fish, but its global shortage and high price have prompted its replacement with alternative sustainable sources. In this experiment involving largemouth bass (a carnivorous fish), a new mixture of feedstuffs (45% poultry byproduct meal, 30% soybean meal, 15% blood meal, and 10% krill shrimp meal) was added to low (14.5%) fishmeal diets along with 0.0%, 0.5% taurine, 0.5% methionine, or 0.5% taurine plus 0.5% methionine (dry matter basis). The positive control diet [65.3% fishmeal (46% crude protein on dry matter basis)] and all low-fishmeal diets contained 40% true protein and 10% lipids. There were 3 tanks per treatment group (20 fish/tank). Fish with the mean initial body weight of 16.6 g were fed to satiety twice daily. Compared with the unsupplemented low-fishmeal group, supplementing either 0.5% methionine or 0.5% methionine plus 0.5% taurine to the low-fishmeal diet improved (P < 0.05) the growth, feed utilization, retention of dietary protein and lipids, and health of largemouth bass, reduced (P < 0.05) the occurrence of black skin syndrome from ~ 40 to ~ 10%. Histological sections of tissues from the fish with black skin syndrome showed retina degeneration, liver damage, and enteritis in the intestine. Compared with methionine supplementation, supplementing 0.5% taurine alone to the low-fishmeal diet did not affect the growth or feed efficiency of fish and had less beneficial effects (P < 0.05) on ameliorating the black skin syndrome. These results indicated that: (a) the basal low-fishmeal diet was inadequate in methionine or taurine; and (b) dietary supplementation with methionine was an effective method to improve the growth performance, feed efficiency, and health of largemouth bass. Further studies are warranted to understand the pathogenesis of the black skin syndrome in largemouth bass.

Morphological colour adaptation during development in fish: involvement of growth hormone receptor 1

Morphological background adaptation is both an endocrine and a nervous response, involving changes in the amount of chromatophores and pigment concentration. However, whether this adaptation takes place at early developmental stages is largely unknown. Somatolactin (Sl) is a pituitary hormone present in fish, which has been associated to skin pigmentation. Moreover, growth hormone receptor type 1 (Ghr1) has been suggested to be the Sl receptor and was associated with background adaptation in adults. In this context, the aim of this work was to evaluate the ontogeny of morphological adaptation to background and the participation of ghr1 in this process. We found in larval stages of the cichlid Cichlasoma dimerus that the number of head melanophores and pituitary cells immunoreactive to Sl was increased in individuals reared with black backgrounds compared with that in fish grown in white tanks. In larval stages of the medaka Oryzias latipes, a similar response was observed, which was altered by ghr1 biallelic mutations using CRISPR/Cas9. Interestingly, melanophore and leucophore numbers were highly associated. Furthermore, we found that somatic growth was reduced in ghr1 biallelic mutant O. latipes, establishing the dual function of this growth hormone receptor. Taken together, these results show that morphological background adaptation is present at early stages during development and that is dependent upon ghr1 during this period.

Identification and Characterization of Highly Fluorescent Pigment Cells in Embryos of the Arabian Killifish (Aphanius Dispar)

The Arabian killifish, Aphanius dispar, is a small tropical teleost fish living in wide range of habitats in sea water and fresh water in the Middle East. Here, we report extraordinary fluorescent pigment cells in the Arabian killifish embryo. These cells appear brown in transmitted light, yellowish white in reflected light, and as strong fluorescence in GFP and RFP filters. TEM and confocal microscopy analyses show the fluorescence emanates from leucosome-like pigment organelles. The cells express the gene encoding GTP cyclohydrolase (gch), a marker for leucophores and xanthophore. Gene knockdown and knockout of gch using morpholino or CRISPR-Cas9 induced loss of fluorescence in these embryos, indicating a crucial role of the enzyme and the associated pterine biosynthesis pathway in the generation of the fluorescence. We concluded that these cells are a highly fluorescent subtype of leucophores and have named them as fluoroleucophores.

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Arabian killifish embryos possess extremely fluorescent pigment cells

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The fluorescent pigment cells show characteristics in common with leucophores

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Gene knockout of GTP cyclohydrolase removes fluorescence from the pigment cells

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The novel pigment cells are termed fluoroleucophores

Skin transcriptional profiles in Oophaga poison frogs

Aposematic organisms advertise their defensive toxins to predators using a variety of warning signals, including bright coloration. While most Neotropical poison frogs (Dendrobatidae) rely on crypsis to avoid predators, Oophaga poison frogs from South America advertise their chemical defenses, a complex mix of diet-derived alkaloids, by using conspicuous hues. The present study aimed to characterize the skin transcriptomic profiles of South American Oophaga poison frogs. Our analyses showed very similar transcriptomic profiles for these closely related species in terms of functional annotation and relative abundance of gene ontology terms expressed. Analyses of expression profiles of Oophaga and available skin transcriptomes of cryptic anurans allowed us to propose initial hypotheses for the active sequestration of alkaloid-based chemical defenses and to highlight some genes that may be potentially involved in resistance mechanisms to avoid self-intoxication and skin coloration. In doing so, we provide an important molecular resource for the study of warning signals that will facilitate the assembly and annotation of future poison frog genomes.

Effects of tank color brightness on the body color, somatic growth, and endocrine systems of rainbow trout Oncorhynchus mykiss

We investigated the effects of tank brightness on body color, growth, and endocrine systems of rainbow trout (Oncorhynchus mykiss). Five different tank colors that produce varying levels of brightness were used, including black, dark gray [DG], light gray [LG], white, and blue. The fish were reared in these tanks for 59 days under natural photoperiod and water temperature. The body color was affected by tank brightness, such that body color brightness was correlated with tank brightness (white-housed ≥ LG-housed ≥ DG-housed ≥ blue-housed ≥ black-housed). No difference in somatic growth was observed among the fish reared in the five tanks. The mRNA levels of melanin-concentrating hormone (mch1) was higher in white-housed fish than those in the other tanks, and the mRNA levels of proopiomelanocortins (pomc-a and pomc-b) were higher in fish housed in a black tank than those in other tanks. mRNA level of somatolactin, a member of growth hormone family, was higher in black-housed fish than those in white-housed fish. The mRNA levels of mch1 and mch2 in blue-housed fish were similar to those in black-housed fish, while the mRNA levels of pomc-a and pomc-b in blue-housed fish were similar to those in white-housed fish. The current results suggest that tank color is not related to fish growth, therefore any color of conventional rearing tank can be used to grow fish. Moreover, the association between somatolactin with body color changes is suggested in addition to the role of classical MCH and melanophore stimulating hormone derived from POMC.

Bisphenol S exposure alters behavioral parameters in adult zebrafish and offspring

The environmental emission of bisphenol S (BPS), which is globally utilized in the manufacturing of polycarbonates, epoxy resin and thermal paper, has affected the aquatic ecosystem. Thus, effects of BPS exposure on the fitness of aquatic animals have been noted. Here, adult male and female zebrafish were used as aquatic model organisms and separately exposed to environmentally relevant doses of BPS (0, 1, 10 and 100 μg/L) for 14 days. The results showed that BPS changed the body pigment of zebrafish and slowed the maturation of oocytes in the ovary, resulting in a significant decrease in the shoaling behavior of adult zebrafish and the attraction of BPS-treated females during the mating process. Furthermore, in the subgeneration of adult zebrafish exposed to BPS for 7 days, survival behaviors, such as locomotor, phototaxis and feeding behaviors, deviated from normal behaviors. After exposing the adult zebrafish to BPS for an additional 7 days, the above described survival behaviors and light adaptation were disrupted in offspring. Our data, based on intergenerational behavioral studies, demonstrate that BPS affects the behaviors of aquatic animals and the ability of offspring to feed and avoid predators, possibly jeopardizing the survival of aquatic animals.

Melanin-concentrating hormone like and somatolactin. A teleost-specific hypothalamic-hypophyseal axis system linking physiological and morphological pigmentation

Plastic adaptation to match the skin colour to the surrounding is key to survival. Two biological responses in skin colour are associated with background adaptation. A fast "physiological response" that aggregates/disperses the pigment organelles of skin chromatophores, and a slow "morphological response" that alters the type and/or density of pigment cells in the skin. Both responses are linked by unknown mechanisms. In this review, we discuss the role in skin colour regulation of two molecules that form part of a hypothalamic-hypophyseal pathway unique to teleosts, melanin-concentrating hormone "like" (MCHL) (previously known as MCH), and somatolactin. MCHL neurons project to the neurohypophysis and to the pars intermedia pituitary, where they interact with somatolactin-expressing cells. With a white background MCHL is released into the circulation to induce rapid melanosome aggregation and skin lightening. Somatolactin is also a fish-specific peptide whose expression and secretion are altered in organisms adapted chronically to white/black backgrounds, and that regulates morphological pigmentation. We discuss the evidence for a model whereby in teleosts, MCHL and somatolactin provide the previously unknown link between physiological and morphological pigmentation.

Neural innervation as a potential trigger of morphological color change and sexual dimorphism in cichlid fish

Many species change their coloration during ontogeny or even as adults. Color change hereby often serves as sexual or status signal. The cellular and subcellular changes that drive color change and how they are orchestrated have been barely understood, but a deeper knowledge of the underlying processes is important to our understanding of how such plastic changes develop and evolve. Here we studied the color change of the Malawi golden cichlid (Melanchromis auratus). Females and subordinate males of this species are yellow and white with two prominent black stripes (yellow morph; female and non-breeding male coloration), while dominant males change their color and completely invert this pattern with the yellow and white regions becoming black, and the black stripes becoming white to iridescent blue (dark morph; male breeding coloration). A comparison of the two morphs reveals that substantial changes across multiple levels of biological organization underlie this polyphenism. These include changes in pigment cell (chromatophore) number, intracellular dispersal of pigments, and tilting of reflective platelets (iridosomes) within iridophores. At the transcriptional level, we find differences in pigmentation gene expression between these two color morphs but, surprisingly, 80% of the genes overexpressed in the dark morph relate to neuronal processes including synapse formation. Nerve fiber staining confirms that scales of the dark morph are indeed innervated by 1.3 to 2 times more axonal fibers. Our results might suggest an instructive role of nervous innervation orchestrating the complex cellular and ultrastructural changes that drive the morphological color change of this cichlid species.

Hyperpigmented melanistic skin lesions of smallmouth bass Micropterus dolomieu from the Chesapeake Bay watershed

Hyperpigmented melanistic skin lesions (HPMLs) of smallmouth bass Micropterus dolomieu are observed in the Potomac and Susquehanna rivers, Chesapeake Bay watershed, USA. Routine, nonlethal population surveys were conducted at 8 sites on the mainstem Susquehanna River and 9 on the Juniata River, a tributary of the Susquehanna River, between 2012 and 2018, and the prevalence of HPMLs was documented. A total of 4078 smallmouth bass were collected from the mainstem Susquehanna River and 6478 from the Juniata River. Lesions were primarily seen in bass greater than 200 mm, and prevalence in the Susquehanna River (8%) was higher (p < 0.001) than in the Juniata River (2%). As part of ongoing fish health monitoring projects, smallmouth bass were collected at additional sites, primarily tributaries of the Susquehanna (n = 758) and Potomac (n = 545) rivers between 2013 and 2018. Prevalence in the Susquehanna River (13%) was higher (p < 0.001) than the Potomac (3%). Microscopically, HPMLs were characterized by an increased number of melanocytes in the epidermis or within the dermis and epidermis. RNAseq analyses of normal and melanistic skin identified 3 unique sequences in HPMLs. Two were unidentified and the third was a viral helicase (E1). Transcript abundance in 16 normal skin samples and 16 HPMLs showed upregulation of genes associated with melanogenesis and cell proliferation in HPMLs. The E1 transcript was detected in 12 of the 16 melanistic areas but in no samples from normal skin. Further research will be necessary to identify the putative new virus and determine its role in melanocyte proliferation.

Male mate choice for a female ornament in a monogamous cichlid fish, Mikrogeophagus ramirezi

Male ram cichlid, Mikrogeophagus ramirezi, spent significantly more time associating with animated female images compared to static images of the same female, indicating that males of the species reliably respond to computer-animated images of conspecific females. Female M. ramirezi temporarily display a pink-coloured belly, and this study showed that males spent significantly more time associating with animated female images displaying a pink-coloured belly compared to animated female images with pink belly colour removed. This is the first study to report male mate choice in M. ramirezi, a neotropical monogamous dwarf cichlid.

Experimental manipulation of perceived predation risk and cortisol generates contrasting trait trajectories in plastic crucian carp

Most animals constitute potential prey and must respond appropriately to predator-mediated stress in order to survive. Numerous prey also adaptively tailor their response to the prevailing level of risk and stress imposed by their natural enemies, i.e. they adopt an inducible defence strategy. Predator exposure may activate the stress axis, and drive the expression of anti-predator traits that facilitate survival in a high-risk environment (the predation-stress hypothesis). Here, we quantified two key morphological anti-predator traits, body morphology and coloration, in crucian carp reared in the presence or absence of a predator (pike) in addition to experimental manipulation of physiological stress via implants containing either cortisol or a cortisol inhibitor. We found that predator-exposed fish expressed a deeper-bodied phenotype and darker body coloration as compared with non-exposed individuals. Skin analyses revealed that an increase in the amount of melanophores caused the dramatic colour change in predator-exposed fish. Increased melanization is costly, and the darker body coloration may act as an inducible defence against predation, via a conspicuous signal of the morphological defence or by crypsis towards dark environments and a nocturnal lifestyle. By contrast, the phenotype of individuals carrying cortisol implants did not mirror the phenotype of predator-exposed fish but instead exhibited opposite trajectories of trait change: a shallow-bodied morphology with a lighter body coloration as compared with sham-treated fish. The cortisol inhibitor did not influence the phenotype of fish i.e. neither body depth nor body coloration differed between this group and predator-exposed fish with a sham implant. However, our results illuminate a potential link between stress physiology and morphological defence expression.

Does soil color affect fish evolution? Differences in color change rate between lineages of the sailfin tetra

ABSTRACT Several organisms match their skin color to the prevalent background color, granting crypsis against predators. The rate at which body color changes occur varies among organisms as a result of physiological constraints and adaptation to variation in contrasts between objects and the environmental background. Faster darkening of body color is favored in environments that show higher amounts of contrast between common objects and the prevailing background. Soil types in Amazon forest streams (igarapés) create distinct environments with respect to the amount of contrast, a result of the amount of sand and clay, which offers different contrasts against dead leaves. Here, we investigated differences in the rates of color change among populations of the sailfin tetra (Crenuchus spilurus) that represent lineages that live in regions of different soil types. Populations inserted into blackwaters (sandy soil) showed higher rates of color darkening in response to exposure to a dark environment composed by dead leaves. We propose that natural selection stemming from predation can favor faster color change rate in environments where there is higher variability of contrasts between leaf litter and soil, which is common in most blackwater streams.

The effects of chromatic lights on body color and gene expressions of melanin-concentrating hormone and proopiomelanocortin in goldfish (Carassius auratus)

In the present study, the effects of photic environments, such as background color (white and black) and chromatic lights (blue, green, and red), on body color and gene expressions of melanin-concentrating hormone (mch) in the brain and proopiomelanocortin (pomc) in the pituitary, as well as the roles of the eyes and brain as mediators of ambient light to these genes, were examined in goldfish (Carassius auratus). Body color of goldfish exposed to fluorescent light (FL) under white background (WBG) was paler than those under black background (BBG). Gene expression levels for mch and pomc were reciprocally different depending on background color; under WBG, mRNA levels of mch and pomc were high and low, respectively, while under BBG, these levels were reversed. mch and pomc mRNA expressions of the fish exposed to chromatic light from LED were primarily similar to those exposed to FL, while blue light stimulated the expressions of mch and pomc. Ophthalmectomized goldfish exposed to FL or blue light showed minimum expression levels of mch gene, suggesting that eyes are the major mediator of ambient light for mch gene expression. Contrastingly, mRNA expressions of pomc in ophthalmectomized goldfish exposed to FL were different from those of intact goldfish. These results suggest that eyes play a functional role in mediating ambient light to regulate pomc gene expression. Since ophthalmectomy caused an increase in pomc mRNA contents in the fish exposed to blue light, we suggest that the brain is an additional mediator to regulate pomc gene expression.

Adrenocorticotropic Hormone and Cyclic Adenosine Monophosphate are Involved in the Control of Shark Bioluminescence

Among Etmopteridae and Dalatiidae, luminous species use hormonal control to regulate bioluminescence. Melatonin (MT) triggers light emission and, conversely, alpha melanocyte-stimulating hormone (α-MSH) actively reduces ongoing luminescence. Prolactin (PRL) acts differentially, triggering light emission in Etmopteridae and inhibiting it in Dalatiidae. Interestingly, these hormones are also known as regulators of skin pigment movements in vertebrates. One other hormone, the adrenocorticotropic hormone (ACTH), also members of the skin pigmentation regulators, is here pharmacologically tested on the light emission. Results show that ACTH inhibits luminescence in both families. Moreover, as MT and α-MSH/ACTH receptors are members of the G-protein coupled receptor (GPCR) family, we investigated the effect of hormonal treatments on the cAMP level of photophores through specific cAMP assays. Our results highlight the involvement of ACTH and cAMP in the control of light emission in sharks and suggest a functional similarity between skin pigment migration and luminescence control, this latter being mediated by pigment movements in the light organ-associated iris-like structure cells.

Observation on rapid physiological color change in Giant tree frog Rhacophorus smaragdinus (Blyth, 1852) from Namdapha Tiger Reserve, Arunachal Pradesh, India

Many poikilotherms have the ability to change body color for homeostatic regulation, conspecific communication or predator deterrence. Physiological color change is a rapid, reversible mode of color change regulated by neuromuscular or neuroendocrine system and has been observed in several anuran species. Here we report the occurrence of physiological color change in the tree frog Rhacophorussmaragdinus (Blyth, 1852) (Amphibia, Anura, Rhacophoridae) for the first time from Namdapha Tiger Reserve, Arunachal Pradesh, India. Probable proximate causes of the behavior are discussed along with an overview of physiological color change in species of the family Rhacophoridae and nature of color change observed.

Effects of Ortho-Phosphate on Growth Performance, Welfare and Product Quality of Juvenile African Catfish (Clarias gariepinus)

Ortho-phosphate inside recirculation aquaculture systems is limited as a consequence of precipitation and regular water exchange rates. To improve plant growth in coupled aquaponics, phosphate fertilizer addition to hydroponics can increase PO43−-P concentrations inside the process water. We investigated the effects of four PO43−-P concentrations (<10 (P0), 40, 80, 120 mg L−1) in rearing water on growth performance, feed efficiency, and welfare traits of juvenile African catfish (Clarias gariepinus Burchell, 1822). By trend, optimum specific growth rate of 2.66% d−1 and feed conversion ratio of 0.71 were observed at 40 and 80 mg L−1 PO43−-P. Higher PO43−-P significantly affected skin coloration, swimming activity and external injuries, with the palest and inactive fish combined with most external injuries in the P120 group. Mineral and protein contents in the fish remained unaffected, while fat content inside the fillets enriched with increasing PO43−-P. Inorganic P in blood plasma increased significantly, while phosphate concentrations inside the fillet remained unchanged. We suggest that PO43−-P concentrations of 40 to 80 mg L−1 do not reduce the performance of African catfish aquaculture, while increased values of 120 mg L−1 affect fish welfare. This allows limited addition of PO43−-P fertilizer in coupled aquaponics with African catfish to support plant growth.

Melanin-Based Skin Coloration Predicts Antioxidant Capacity in the Brown Trout (Salmo trutta)

In many vertebrate species, individuals exhibit large variation in the degree of melanin-based coloration on their body. Dark and pale individuals differ in diverse physiological and behavioral traits, suggesting that melanic coloration may reveal individual quality. However, research into the relationships between physiological and skin traits, in terms of melanin-based skin coloration, in wild fish is scant. Our correlative study aimed at investigating the relationships between physiology and melanin-based coloration of the skin of free-living brown trout (Salmo trutta Linnaeus, 1758). We scrutinized the relationships between body condition (body mass and Fulton's K condition factor), oxidative status (plasma total antioxidant capacity and amount of pro-oxidant molecules), and the degree of melanin-based skin coloration assessed by digital photography and image analysis. We found heavier fish to be, on average, darker colored than paler conspecifics. Moreover, a significant covariation between plasma total antioxidant capacity and melanic coloration was noted. Our findings suggest that the melanin-based coloration of brown trout serves as a signal to communicate a better antioxidant defense to conspecifics.

Melanin-concentrating hormone is a major substance mediating light wavelength-dependent skin color change in larval zebrafish

Melanosome dispersion is important for protecting the internal organs of fish against ultraviolet light, especially in transparent larvae with underdeveloped skin. Melanosome dispersion leads to dark skin color in dim light. Melanosome aggregation, on the other hand, leads to pale skin color in bright light. Both of these mechanisms are therefore useful for camouflage. In this study, we investigated a hormone thought to be responsible for the light wavelength-dependent response of melanophores in zebrafish larvae. We irradiated larvae using light-emitting diode (LED) lights with peak wavelengths (λ_max) of 355, 400, 476, 530, and 590 nm or fluorescent light (FL) 1-4 days post fertilization (dpf). Melanosomes in skin melanophores were more dispersed under short wavelength light (λ_max ≤ 400 nm) than under FL. Conversely, melanosomes were more aggregated under mid-long wavelength light (λ_max ≥ 476 nm) than under FL. In addition, long-term (1-12 dpf) irradiation of 400 nm light increased melanophores in the skin, whereas that of 530 nm light decreased them. In teleosts, melanin-concentrating hormone (MCH) aggregates melanosomes within chromatophores, whereas melanocyte-stimulating hormone, derived from proopiomelanocortin (POMC), disperses melanosomes. The expression of a gene for MCH was down-regulated by short wavelength light but up-regulated by mid-long wavelength light, whereas a gene for POMC was up-regulated under short wavelength light. Melanosomes in larvae (4 dpf) exposed to a black background aggregated when immersing the larvae in MCH solution. Yohimbine, an α₂-adrenergic receptor antagonist, attenuated adrenaline-dependent aggregation in larvae exposed to a black background but did not induce melanosome dispersion in larvae exposed to a white background. These results suggest that MCH plays a key role in the light wavelength-dependent response of melanophores, flexibly mediating the transmission of light wavelength information between photoreceptors and melanophores.