Fluorescence characterisation and visual ecology of pseudocheilinid wrasses

Repeated and widespread evolution of biofluorescence in marine fishes

Biofluorescence, the absorption of high-energy light and its reemission at lower energy wavelengths, is widespread across vertebrate and invertebrate lineages, especially fishes. New observations over the past decade have significantly increased our understanding of the diversity and multifunctionality of fluorescence in fish lineages. In this study, we present a comprehensive account of all known biofluorescent teleosts and estimate the timing and frequency of the evolution of biofluorescence across this diverse group. We show that biofluorescence evolved numerous times in marine teleosts and is estimated to date back ~112 mya in Anguilliformes (true eels). Of the 459 known biofluorescent teleosts reported in this study, the majority are associated with coral reefs. We find that reef-associated species evolve biofluorescence at 10x the rate of non-reef species. Our results suggest that the chromatic and biotic conditions of coral reefs could have provided an ideal environment to facilitate the evolution and diversification of biofluorescence in teleost fishes.

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.

New observations of fluorescent organisms in the Banda Sea and in the Red Sea

Fluorescence is a widespread phenomenon found in animals, bacteria, fungi, and plants. In marine environments fluorescence has been proposed to play a role in physiological and behavioral responses. Many fluorescent proteins and other molecules have been described in jellyfish, corals, and fish. Here we describe fluorescence in marine species, which we observed and photographed during night dives in the Banda Sea, Indonesia, and in the Red Sea, Egypt. Among various phyla we found fluorescence in sponges, molluscs, tunicates, and fish. Our study extends the knowledge on how many different organisms fluoresce in marine environments. We describe the occurrence of fluorescence in 27 species, in which fluorescence has not been described yet in peer-reviewed literature. It especially extends the knowledge beyond Scleractinia, the so far best described taxon regarding diversity in fluorescent proteins.

First record of biofluorescence in lumpfish (Cyclopterus lumpus), a commercially farmed cleaner fish

This study is the first known observation of biofluorescence in the lumpfish (Cyclopterus lumpus). Individual lumpfish were illuminated with blue excitation lighting for photography with both hyperspectral and filtered multispectral cameras. All photographed juvenile lumpfish (n = 11) exhibited green biofluorescence. Light emissions were characterised with two peaks observed at 545 and 613 nm, with the greatest intensity along the tubercles of the high crest and the three longitudinal ridges. Further research on the dynamics of biofluorescence through the lifecycle of this species is required.

Cirrhilabrus finifenmaa (Teleostei, Labridae), a new species of fairy wrasse from the Maldives, with comments on the taxonomic identity of C. rubrisquamis and C. wakanda

Cirrhilabrus rubrisquamis is redescribed on the basis of the juvenile holotype and compared to known species of Cirrhilabrus. Examination of material from the Maldives identified as C. rubrisquamis reveal differences from the holotype collected from the Chagos Archipelago. Consequently, the Maldivian specimens are herein described as Cirrhilabrus finifenmaasp. nov., on the basis of the holotype and twelve paratypes. The new species differs from all congeners in having: males with anterior third to half of body bright magenta, peach to orange-pink posteriorly; lateral line with 22–26 pored scales (16–18 in the dorso-anterior series, 6–8 in the posterior peduncular series); tenth to eleventh dorsal-fin spine longest (14.0–15.5% SL); scales on the opercle, chest, isthmus, and anterior third of the body with a dark purple-red central region (purple in alcohol), the markings joining appearing crosshatched; dorsal, caudal, anal, and pelvic-fin rays purple in alcohol. Meristic details and coloration patterns of C. rubrisquamis are very similar to C. wakanda from Tanzania, Africa, although synonymy of both species cannot be determined without additional material from Chagos. This potential synonymy is briefly discussed; however, until such material becomes available, the taxonomic statuses of C. wakanda and C. rubrisquamis are here provisionally regarded as valid.

Visual system diversity in coral reef fishes

Coral reefs are one of the most species rich and colourful habitats on earth and for many coral reef teleosts, vision is central to their survival and reproduction. The diversity of reef fish visual systems arises from variations in ocular and retinal anatomy, neural processing and, perhaps most easily revealed by, the peak spectral absorbance of visual pigments. This review examines the interplay between retinal morphology and light environment across a number of reef fish species, but mainly focusses on visual adaptations at the molecular level (i.e. visual pigment structure). Generally, visual pigments tend to match the overall light environment or micro-habitat, with fish inhabiting greener, inshore waters possessing longer wavelength-shifted visual pigments than open water blue-shifted species. In marine fishes, particularly those that live on the reef, most species have between two (likely dichromatic) to four (possible tetrachromatic) cone spectral sensitivities and a single rod for crepuscular vision; however, most are trichromatic with three spectral sensitivities. In addition to variation in spectral sensitivity number, spectral placement of the absorbance maximum (λ_max) also has a surprising degree of variability. Variation in ocular and retinal anatomy is also observed at several levels in reef fishes but is best represented by differences in arrangement, density and distribution of neural cell types across the retina (i.e. retinal topography). Here, we focus on the seven reef fish families most comprehensively studied to date to examine and compare how behaviour, environment, activity period, ontogeny and phylogeny might interact to generate the exceptional diversity in visual system design that we observe.

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.

Do the fluorescent red eyes of the marine fish Tripterygion delaisi stand out? In situ and in vivo measurements at two depths

Since the discovery of red fluorescence in fish, much effort has been invested to elucidate its potential functions, one of them being signaling. This implies that the combination of red fluorescence and reflection should generate a visible contrast against the background. Here, we present in vivo iris radiance measurements of Tripterygion delaisi under natural light conditions at 5 and 20 m depth. We also measured substrate radiance of shaded and exposed foraging sites at those depths. To assess the visual contrast of the red iris against these substrates, we used the receptor noise model for chromatic contrasts and Michelson contrast for achromatic calculations. At 20 m depth, T. delaisi iris radiance generated strong achromatic contrasts against substrate radiance, regardless of exposure, and despite substrate fluorescence. Given that downwelling light above 600 nm is negligible at this depth, we can attribute this effect to iris fluorescence. Contrasts were weaker in 5 m. Yet, the pooled radiance caused by red reflection and fluorescence still exceeded substrate radiance for all substrates under shaded conditions and all but Jania rubens and Padina pavonia under exposed conditions. Due to the negative effects of anesthesia on iris fluorescence, these estimates are conservative. We conclude that the requirements to create visual brightness contrasts are fulfilled for a wide range of conditions in the natural environment of T. delaisi.

Fluorescence as a means of colour signal enhancement

Fluorescence is a physico-chemical energy exchange where shorter-wavelength photons are absorbed by a molecule and are re-emitted as longer-wavelength photons. It has been suggested a means of communication in several taxa including flowers, pitcher plants, corals, algae, worms, squid, spiders, stomatopods, fish, reptiles, parrots and humans. The surface or object that the pigment molecule is part of appears to glow due to its setting rather than an actual production of light, and this may enhance both signals and, in some cases, camouflage. This review examines some known uses of fluorescence, mainly in the context of visual communication in animals, the challenge being to distinguish when fluorescence is a functional feature of biological coloration or when it is a by-product of a pigment or other molecule. In general, we conclude that most observations of fluorescence lack enough evidence to suggest they are used in visually driven behaviours.This article is part of the themed issue 'Animal coloration: production, perception, function and application'.

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.