A fluorescent chromatophore changes the level of fluorescence in a reef fish

Diversity and function of fluorescent molecules in marine animals

Fluorescence in marine animals has mainly been studied in Cnidaria but is found in many different phyla such as Annelida, Crustacea, Mollusca, and Chordata. While many fluorescent proteins and molecules have been identified, very little information is available about the biological functions of fluorescence. In this review, we focus on describing the occurrence of fluorescence in marine animals and the behavioural and physiological functions of fluorescent molecules based on experimental approaches. These biological functions of fluorescence range from prey and symbiont attraction, photoprotection, photoenhancement, stress mitigation, mimicry, and aposematism to inter- and intraspecific communication. We provide a comprehensive list of marine taxa that utilise fluorescence, including demonstrated effects on behavioural or physiological responses. We describe the numerous known functions of fluorescence in anthozoans and their underlying molecular mechanisms. We also highlight that other marine taxa should be studied regarding the functions of fluorescence. We suggest that an increase in research effort in this field could contribute to understanding the capacity of marine animals to respond to negative effects of climate change, such as rising sea temperatures and increasing intensities of solar irradiation.

Biofluorescent response in lumpfish Cyclopterus lumpus to a therapeutic stressor as assessed by hyperspectral imaging

The demand for lumpfish (Cyclopterus lumpus) as a biological control for salmon lice is increasing. However, lumpfish welfare is considered a limiting factor within aquaculture operations. Identifying a noninvasive parameter that measures subclinical stress in lumpfish is a key goal for improving their welfare. The lumpfish is documented to emit green and red biofluorescence within the blue shifted light of their environment. Here we show that lumpfish fluorescence responds to a therapeutic stressor within a controlled experiment. Lumpfish (n = 60) underwent a 3-h freshwater bath therapeutant to evaluate whether fluorescence spectra produced by the species respond to external stimuli. Lumpfish were quickly scanned under a hyperspectral camera (400-1000 nm spectral range) prior to and after treatment. The lumpfish were randomly divided into 3 groups with identical treatment. All groups increased fluorescence emissions, though the level of change depended on whether the averaged, red, or green spectra were analyzed; the control group (n = 20) remained constant. All lumpfish emitted green fluorescence (~ 590-670 nm) while a portion (49%) produced red fluorescence (~ 690-800 nm). As lumpfish fluorescence shifts in response to the applied stressor, this study provides insight into how fluorescence may be incorporated into the welfare management of lumpfish.

Neon-green fluorescence in the desert gecko Pachydactylus rangei caused by iridophores

Biofluorescence is widespread in the natural world, but only recently discovered in terrestrial vertebrates. Here, we report on the discovery of iridophore-based, neon-green flourescence in the gecko Pachydactylus rangei, localised to the skin around the eyes and along the flanks. The maximum emission of the fluorescence is at a wavelength of 516 nm in the green spectrum (excitation maximum 465 nm, blue) with another, smaller peak at 430 nm. The fluorescent regions of the skin show large numbers of iridophores, which are lacking in the non-fluorescent parts. Two types of iridophores are recognized, fluorescent iridophores and basal, non-fluorescent iridophores, the latter of which might function as a mirror, amplifying the omnidirectional fluorescence. The strong intensity of the fluorescence (quantum yield of 12.5%) indicates this to be a highly effective mechanism, unique among tetrapods. Although the fluorescence is associated with iridophores, the spectra of emission and excitation as well as the small Stokes shifts argue against guanine crystals as its source, but rather a rigid pair of fluorophores. Further studies are necessary to identify their morphology and chemical structures. We hypothesise that this nocturnal gecko uses the neon-green fluorescence, excited by moonlight, for intraspecific signalling in its open desert habitat.

Shining in the dark: First record of biofluorescence in the seahorse Hippocampus reidi

Marine environments are visual domains restricted regarding light characteristics. Overall, blue monochromatic spectrum prevails in offshore areas especially below 15m depth, since long wavelengths are quickly attenuated. Light intensity is even more constrained in coastal waters, particularly those of tropical estuaries and bays, because further scattering through dissolved and suspended materials. Biofluorescence, which is the ability of organisms to absorb light and reflect it in a different wavelength, has been reported for many marine fish. In this paper, biofluorescence was recorded for the first time for the longsnout seahorse Hippocampus reidi, under natural conditions at Ilha Grande bay, Brazil, and both adult, juvenile and fry individuals kept in captivity. Although displaying the same colour emissions, seahorses differed in relation to body lighting, colour patterns, and age wherein fluorescence occurs. Newborn seahorses exhibit green biofluorescence only in the eyes and stomach. Further experiments are necessary to address whether H. reidi can change the patterns of biofluorescence emission for sensorial and social purposes.

Morphological Characteristics and Comparative Transcriptome Analysis of Three Different Phenotypes of Pristella maxillaris

Pristella maxillaris is known as the X-ray fish based on its translucent body. However, the morphological characteristics and the molecular regulatory mechanisms of these translucent bodies are still unknown. In this study, the following three phenotypes, a black-and-gray body color or wild-type (WT), a silvery-white body color defined as mutant I (MU1), and a fully transparent body with a visible visceral mass named as mutant II (MU2), were investigated to analyze their chromatophores and molecular mechanisms. The variety and distribution of pigment cells in the three phenotypes of P. maxillaris significantly differed by histological assessment. Three types of chromatophores (melanophores, iridophores, and xanthophores) were observed in the WT, whereas MU1 fish were deficient in melanophores, and MU2 fish lacked melanophores and iridophores. Transcriptome sequencing of the skin and peritoneal tissues of P. maxillaris identified a total of 166,089 unigenes. After comparing intergroup gene expression levels, more than 3,000 unigenes with significantly differential expression levels were identified among three strains. Functional annotation and Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analyses of the differentially expressed genes (DEGs) identified a number of candidates melanophores and iridophores genes that influence body color. Some DEGs that were identified using transcriptome analysis were confirmed by quantitative real-time PCR. This study serves as a global survey of the morphological characteristics and molecular mechanism of different body colors observed in P. maxillaris and thus provides a valuable theoretical foundation for the molecular regulation of the transparent phenotype.

Teleost Fish-Specific Preferential Retention of Pigmentation Gene-Containing Families After Whole Genome Duplications in Vertebrates

Vertebrate pigmentation is a highly diverse trait mainly determined by neural crest cell derivatives. It has been suggested that two rounds (1R/2R) of whole-genome duplications (WGDs) at the basis of vertebrates allowed changes in gene regulation associated with neural crest evolution. Subsequently, the teleost fish lineage experienced other WGDs, including the teleost-specific Ts3R before teleost radiation and the more recent Ss4R at the basis of salmonids. As the teleost lineage harbors the highest number of pigment cell types and pigmentation diversity in vertebrates, WGDs might have contributed to the evolution and diversification of the pigmentation gene repertoire in teleosts. We have compared the impact of the basal vertebrate 1R/2R duplications with that of the teleost-specific Ts3R and salmonid-specific Ss4R WGDs on 181 gene families containing genes involved in pigmentation. We show that pigmentation genes (PGs) have been globally more frequently retained as duplicates than other genes after Ts3R and Ss4R but not after the early 1R/2R. This is also true for non-pigmentary paralogs of PGs, suggesting that the function in pigmentation is not the sole key driver of gene retention after WGDs. On the long-term, specific categories of PGs have been repeatedly preferentially retained after ancient 1R/2R and Ts3R WGDs, possibly linked to the molecular nature of their proteins (e.g., DNA binding transcriptional regulators) and their central position in protein-protein interaction networks. Taken together, our results support a major role of WGDs in the diversification of the pigmentation gene repertoire in the teleost lineage, with a possible link with the diversity of pigment cell lineages observed in these animals compared to other vertebrates.

Widespread bone-based fluorescence in chameleons

Fluorescence is widespread in marine organisms but uncommon in terrestrial tetrapods. We here show that many chameleon species have bony tubercles protruding from the skull that are visible through their scales, and fluoresce under UV light. Tubercles arising from bones of the skull displace all dermal layers other than a thin, transparent layer of epidermis, creating a ‘window’ onto the bone. In the genus Calumma, the number of these tubercles is sexually dimorphic in most species, suggesting a signalling role, and also strongly reflects species groups, indicating systematic value of these features. Co-option of the known fluorescent properties of bone has never before been shown, yet it is widespread in the chameleons of Madagascar and some African chameleon genera, particularly in those genera living in forested, humid habitats known to have a higher relative component of ambient UV light. The fluorescence emits with a maximum at around 430 nm in blue colour which contrasts well to the green and brown background reflectance of forest habitats. This discovery opens new avenues in the study of signalling among chameleons and sexual selection factors driving ornamentation.

Anatomical Analysis of the Retinal Specializations to a Crypto-Benthic, Micro-Predatory Lifestyle in the Mediterranean Triplefin Blenny Tripterygion delaisi

The environment and lifestyle of a species are known to exert selective pressure on the visual system, often demonstrating a tight link between visual morphology and ecology. Many studies have predicted the visual requirements of a species by examining the anatomical features of the eye. However, among the vast number of studies on visual specializations in aquatic animals, only a few have focused on small benthic fishes that occupy a heterogeneous and spatially complex visual environment. This study investigates the general retinal anatomy including the topography of both the photoreceptor and ganglion cell populations and estimates the spatial resolving power (SRP) of the eye of the Mediterranean triplefin Tripterygion delaisi. Retinal wholemounts were prepared to systematically and quantitatively analyze photoreceptor and retinal ganglion cell (RGC) densities using design-based stereology. To further examine the retinal structure, we also used magnetic resonance imaging (MRI) and histological examination of retinal cross sections. Observations of the triplefin's eyes revealed them to be highly mobile, allowing them to view the surroundings without body movements. A rostral aphakic gap and the elliptical shape of the eye extend its visual field rostrally and allow for a rostro-caudal accommodatory axis, enabling this species to focus on prey at close range. Single and twin cones dominate the retina and are consistently arranged in one of two regular patterns, which may enhance motion detection and color vision. The retina features a prominent, dorso-temporal, convexiclivate fovea with an average density of 104,400 double and 30,800 single cones per mm2, and 81,000 RGCs per mm2. Based on photoreceptor spacing, SRP was calculated to be between 6.7 and 9.0 cycles per degree. Location and resolving power of the fovea would benefit the detection and identification of small prey in the lower frontal region of the visual field.

Fish with red fluorescent eyes forage more efficiently under dim, blue-green light conditions

Background

Natural red fluorescence is particularly conspicuous in the eyes of some small, benthic, predatory fishes. Fluorescence also increases in relative efficiency with increasing depth, which has generated speculation about its possible function as a “light organ” to detect cryptic organisms under bluish light. Here we investigate whether foraging success is improved under ambient conditions that make red fluorescence stand out more, using the triplefin Tripterygion delaisi as a model system. We repeatedly presented 10 copepods to individual fish (n = 40) kept under a narrow blue-green spectrum and compared their performance with that under a broad spectrum with the same overall brightness. The experiment was repeated for two levels of brightness, a shaded one representing 0.4% of the light present at the surface and a heavily shaded one with about 0.01% of the surface brightness.

Results

Fish were 7% more successful at catching copepods under the narrow, fluorescence-friendly spectrum than under the broad spectrum. However, this effect was significant under the heavily shaded light treatment only.

Conclusions

This outcome corroborates previous predictions that fluorescence may be an adaptation to blue-green, heavily shaded environments, which coincides with the opportunistic biology of this species that lives in the transition zone between exposed and heavily shaded microhabitats.

Electronic supplementary material

The online version of this article (doi:10.1186/s12898-017-0127-y) contains supplementary material, which is available to authorized users.

Red fluorescence of the triplefin Tripterygion delaisi is increasingly visible against background light with increasing depth

The light environment in water bodies changes with depth due to the absorption of short and long wavelengths. Below 10 m depth, red wavelengths are almost completely absent rendering any red-reflecting animal dark and achromatic. However, fluorescence may produce red coloration even when red light is not available for reflection. A large number of marine taxa including over 270 fish species are known to produce red fluorescence, yet it is unclear under which natural light environment fluorescence contributes perceptively to their colours. To address this question we: (i) characterized the visual system of Tripterygion delaisi, which possesses fluorescent irides, (ii) separated the colour of the irides into its reflectance and fluorescence components and (iii) combined these data with field measurements of the ambient light environment to calculate depth-dependent perceptual chromatic and achromatic contrasts using visual modelling. We found that triplefins have cones with at least three different spectral sensitivities, including differences between the two members of the double cones, giving them the potential for trichromatic colour vision. We also show that fluorescence contributes increasingly to the radiance of the irides with increasing depth. Our results support the potential functionality of red fluorescence, including communicative roles such as species and sex identity, and non-communicative roles such as camouflage.

Optic-nerve-transmitted eyeshine, a new type of light emission from fish eyes

BACKGROUND

Most animal eyes feature an opaque pigmented eyecup to assure that light can enter from one direction only. We challenge this dogma by describing a previously unknown form of eyeshine resulting from light that enters the eye through the top of the head and optic nerve, eventually emanating through the pupil as a narrow beam: the Optic-Nerve-Transmitted (ONT) eyeshine. We characterize ONT eyeshine in the triplefin blenny Tripterygion delaisi (Tripterygiidae) in comparison to three other teleost species, using behavioural and anatomical observations, spectrophotometry, histology, and magnetic resonance imaging. The study's aim is to identify the factors that determine ONT eyeshine occurrence and intensity, and whether these are specifically adapted for that purpose.

RESULTS

ONT eyeshine intensity benefits from locally reduced head pigmentation, a thin skull, the gap between eyes and forebrain, the potential light-guiding properties of the optic nerve, and, most importantly, a short distance between the head surface and the optic nerves.

CONCLUSIONS

The generality of these factors and the lack of specifically adapted features implies that ONT eyeshine is widespread among small fish species. Nevertheless, its intensity varies considerably, depending on the specific combination and varying expression of common anatomical features. We discuss whether ONT eyeshine might affect visual performance, and speculate about possible functions such as predator detection, camouflage, and intraspecific communication.

Fluorescence characterisation and visual ecology of pseudocheilinid wrasses

BACKGROUND

Wrasses represent the second largest family of marine fishes and display a high diversity of complex colours linked to ecological functions. Recently, red autofluorescent body colouration has been reported in some of these fishes. However, little is known about the distribution of such fluorescent body patterns in wrasses or the animals' ability to perceive such colours.

RESULTS

Against this background, we (1) investigated long-wavelength emission autofluorescence in thirteen species of pseudocheilinid wrasses and (2) characterised the spectral absorbance of visual pigments in one of the examined species, the fairy wrasse Cirrhilabrus solorensis. Spectrophotometric analysis revealed that fluorescent body colouration is widespread and diverse within this clade, with considerable variation in both fluorescent pattern and maximum emission wavelength between species. Characterisation of visual pigments in retinal photoreceptors showed a single class of rod and three spectrally distinct cone photoreceptors, suggesting possible trichromacy.

CONCLUSION

Combining the emission characteristics of fluorescence body colouration and the spectral sensitivity data of retinal cells suggests that the visual system of C. solorensis is sensitive to pseudocheilinid fluorescence.

The consistent difference in red fluorescence in fishes across a 15 m depth gradient is triggered by ambient brightness, not by ambient spectrum

Background

Organisms adapt to fluctuations or gradients in their environment by means of genetic change or phenotypic plasticity. Consistent adaptation across small spatial scales measured in meters, however, has rarely been reported. We recently found significant variation in fluorescence brightness in six benthic marine fish species across a 15 m depth gradient. Here, we investigate whether this can be explained by phenotypic plasticity alone, using the triplefin Tripterygion delaisi as a model species. In two separate experiments, we measure change in red fluorescent brightness to spectral composition and ambient brightness, two central parameters of the visual environment that change rapidly with depth.

Results

Changing the ambient spectra simulating light at −5 or −20 m depth generated no detectable changes in mean fluorescence brightness after 4–6 weeks. In contrast, a reduction in ambient brightness generated a significant and reversible increase in mean fluorescence, most of this within the first week. Although individuals can quickly up- and down-regulate their fluorescence around this mean value using melanosome aggregation and dispersal, we demonstrate that this range around the mean remained unaffected by either treatment.

Conclusion

We show that the positive association between fluorescence and depth observed in the field can be fully explained by ambient light brightness, with no detectable additional effect of spectral composition. We propose that this change is achieved by adjusting the ratio of melanophores and fluorescent iridophores in the iris.

Electronic supplementary material

The online version of this article (doi:10.1186/s13104-016-1911-z) contains supplementary material, which is available to authorized users.

Easy approach to assembling a biomimetic color film with tunable structural colors

The self-assembly of silica microspheres into a close-packed array is a simple method of fabricating three-dimensional photonic crystal structural color films. However, the color is very dull because of the interferences of scattering and background light. In this study, we added a small quantity of surface-modified carbon black (CB) to the system of colloidal silica in n-propanol. The use of n-propanol as a dispersant is beneficial to the rapid development of photonic crystal films during the process of dip-coating. The doping of CB into silica microspheres can absorb background and scattering light, resulting in vivid structural colors.

Fairy wrasses perceive and respond to their deep red fluorescent coloration

Fluorescence enables the display of wavelengths that are absent in the natural environment, offering the potential to generate conspicuous colour contrasts. The marine fairy wrasse Cirrhilabrus solorensis displays prominent fluorescence in the deep red range (650–700 nm). This is remarkable because marine fishes are generally assumed to have poor sensitivity in this part of the visual spectrum. Here, we investigated whether C. solorensis males can perceive the fluorescence featured in this species by testing whether the presence or absence of red fluorescence affects male–male interactions under exclusive blue illumination. Given that males respond aggressively towards mirror-image stimuli, we quantified agonistic behaviour against mirrors covered with filters that did or did not absorb long (i.e. red) wavelengths. Males showed significantly fewer agonistic responses when their fluorescent signal was masked, independent of brightness differences. Our results unequivocally show that C. solorensis can see its deep red fluorescent coloration and that this pattern affects male–male interactions. This is the first study to demonstrate that deep red fluorescent body coloration can be perceived and has behavioural significance in a reef fish.

The structure-function relationships of a natural nanoscale photonic device in cuttlefish chromatophores

Cuttlefish, Sepia officinalis, possess neurally controlled, pigmented chromatophore organs that allow rapid changes in skin patterning and coloration in response to visual cues. This process of adaptive coloration is enabled by the 500% change in chromatophore surface area during actuation. We report two adaptations that help to explain how colour intensity is maintained in a fully expanded chromatophore when the pigment granules are distributed maximally: (i) pigment layers as thin as three granules that maintain optical effectiveness and (ii) the presence of high-refractive-index proteins-reflectin and crystallin-in granules. The latter discovery, combined with our finding that isolated chromatophore pigment granules fluoresce between 650 and 720 nm, refutes the prevailing hypothesis that cephalopod chromatophores are exclusively pigmentary organs composed solely of ommochromes. Perturbations to granular architecture alter optical properties, illustrating a role for nanostructure in the agile, optical responses of chromatophores. Our results suggest that cephalopod chromatophore pigment granules are more complex than homogeneous clusters of chromogenic pigments. They are luminescent protein nanostructures that facilitate the rapid and sophisticated changes exhibited in dermal pigmentation.

The covert world of fish biofluorescence: a phylogenetically widespread and phenotypically variable phenomenon

The discovery of fluorescent proteins has revolutionized experimental biology. Whereas the majority of fluorescent proteins have been identified from cnidarians, recently several fluorescent proteins have been isolated across the animal tree of life. Here we show that biofluorescence is not only phylogenetically widespread, but is also phenotypically variable across both cartilaginous and bony fishes, highlighting its evolutionary history and the possibility for discovery of numerous novel fluorescent proteins. Fish biofluorescence is especially common and morphologically variable in cryptically patterned coral-reef lineages. We identified 16 orders, 50 families, 105 genera, and more than 180 species of biofluorescent fishes. We have also reconstructed our current understanding of the phylogenetic distribution of biofluorescence for ray-finned fishes. The presence of yellow long-pass intraocular filters in many biofluorescent fish lineages and the substantive color vision capabilities of coral-reef fishes suggest that they are capable of detecting fluoresced light. We present species-specific emission patterns among closely related species, indicating that biofluorescence potentially functions in intraspecific communication and evidence that fluorescence can be used for camouflage. This research provides insight into the distribution, evolution, and phenotypic variability of biofluorescence in marine lineages and examines the role this variation may play.

Regulation of red fluorescent light emission in a cryptic marine fish

Introduction

Animal colouration is a trade-off between being seen by intended, intra- or inter-specific receivers while not being seen by the unintended. Many fishes solve this problem by adaptive colouration. Here, we investigate whether this also holds for fluorescent pigments. In those aquatic environments in which the ambient light is dominated by bluish light, red fluorescence can generate high-contrast signals. The marine, cryptic fish Tripterygion delaisi inhabits such environments and has a bright red-fluorescent iris that can be rapidly up- and down-regulated. Here, we described the physiological and cellular mechanism of this phenomenon using a neurostimulation treatment with KCl and histology.

Results

KCl-treatment revealed that eye fluorescence regulation is achieved through dispersal and aggregation of black-pigmented melanosomes within melanophores. Histology showed that globular, fluorescent iridophores on the anterior side of the iris are grouped and each group is encased by finger-like extensions of a single posterior melanophore. Together they form a so-called chromatophore unit. By dispersal and aggregation of melanosomes into and out of the peripheral membranous extensions of the melanophore, the fluorescent iridophores are covered or revealed on the anterior (outside) of the iris.

Conclusion

T. delaisi possesses a well-developed mechanism to control the fluorescent emission from its eyes, which may be advantageous given its cryptic lifestyle. This is the first time chromatophore units are found to control fluorescent emission in marine teleost fishes. We expect other fluorescent fish species to use similar mechanisms in the iris or elsewhere in the body. In contrast to a previously described mechanism based on dendritic fluorescent chromatophores, chromatophore units control fluorescent emission through the cooperation between two chromatophore types: an emitting and an occluding type. The discovery of a second mechanism for fluorescence modulation strengthens our view that fluorescence is a relevant and adaptive component of fish colouration.