Rock pool fish use a combination of colour change and substrate choice to improve camouflage

Adaptation for crypsis versus conspicuous social signalling following transitions across an extreme ecotone

A key selection pressure in most habitats is predation, and a common strategy adopted by prey is crypsis through background matching. Many marine blenny fishes are in the process of a dramatic transition across one of the world's most extreme ecotones: the invasion of land across the intertidal zone. We investigated the impact of this transition on body crypsis versus the conspicuousness of visual signals across 56 blenny taxa relative to 59 biologically relevant backgrounds, as viewed by conspecifics and four representative fish and avian predators. We computed 33 colour and 23 pattern indices from standardised digital photographs of six individuals for each taxa (median sample). Six of these indices were selected for detailed analysis following phylogenetic Principal Component Analysis. While phylogenetic regressions revealed some aspects of body crypsis appeared to have changed adaptively with the progressive transition to land (specifically a reduction in body colour saturation), colonisation was primarily facilitated by a generalist form of crypsis. That is, the colours and patterns of aquatic blennies were already well matched to the range of terrestrial backgrounds where amphibious and terrestrial species were observed out of water. Predation appears to have been an important selection pressure constraining the colour and pattern of the dorsal fins used in social communication, which also matched visual backgrounds. Our data implies anti-predator strategies that translate well across habitats and different predator regimes will facilitate colonisation by either reducing predation risk or allowing species to persist long enough to respond adaptively to environmental change.

Phenotypic and dispersal plasticity are not alternative strategies for organisms to face thermal changes

To buffer the effects of local environmental changes, organisms may modify their phenotypic traits (i.e. phenotypic plasticity) or disperse towards other potential habitats (i.e. dispersal plasticity). Despite extensive work studying either 'local phenotypic plasticity' or 'dispersal plasticity' independently, little is known about their potential covariation and interplay. These strategies are classically viewed as alternatives. However, this expectation has been challenged by theoretical work suggesting that they may instead evolve together under some environmental contexts. Here, we experimentally quantified morphological, movement and dispersal plasticity in response to thermal changes in 12 strains of the ciliate Tetrahymena thermophila. We showed that phenotypic and dispersal plasticity are not alternative strategies, with half of the strains expressing simultaneously all dimensions of plasticity in response to thermal changes. Furthermore, the extent of morphological and movement plasticity weakly but significantly differed between residents and dispersers. Interestingly, we found no covariation between these different plasticity dimensions, suggesting that they may evolve independently, which pleads for studying which environmental contexts favour the evolution of each. The fact that phenotypic and dispersal plasticity are not alternative strategies and may affect the expression of one another opens interesting perspectives about their joint evolution and the potential consequences of their interplay.

Dynamic color change in the grouper Variola louti during interspecific interactions and swimming

Animals can change their body color for various ecological functions. In fish, rapid dynamic color change is primarily known in contexts of intraspecific communication and camouflage, while examples in interspecific contexts are rare. We studied dynamic color changes and their associated behaviors in the grouper Variola louti in its native coral reef environment in the Red Sea. Using underwater videos to record natural behaviors and color-calibrated still images to measure body colors, we quantified color displays as the brightness of the body and the contrast of three distinct patterns: body patches, head stripe, and side bars. V. louti exhibited a diverse range of pattern displays, which rapidly transformed according to its behavioral shifts. A high-contrast head stripe pattern was observed when V. louti engaged in agonistic interspecific interactions, but was interestingly absent when hunting alone or in cooperation with moray eels. The brightness of V. louti's body color and the contrasts of the body patches and side bars were associated with its swimming behavior. Darker body colors and high contrast body patches and side bars were expressed when the fish rested on the bottom, whereas bright and uniform body colors were displayed when swimming higher above the reef. Our results suggest that V. louti utilizes dynamic color displays for camouflage and interspecific communication in agonistic and competitive interspecific interactions. These findings highlight the importance of dynamic color changes for communication and provide valuable insights into the behavioral ecology of animals.

Claw coloration in the fiddler crab Leptuca uruguayensis has no correlation with male quality

Sexual selection is thought to play a major role in the evolution of color due to the correlation between a signaler's physiological state and the displayed color. As such, researchers often investigate how color correlates to the quality of the signaler. However, research on the relationship between color and individual quality is often taxonomically limited and researchers typically investigate how color phenotypes relate to one index of quality, such as a linear measure of body size. Here, we investigated the relationship among body size, claw size, claw muscle mass, lipid content, and the color of the claw in male fiddler crabs (Leptuca uruguayensis) which wield an exaggerated claw that varies in color from brown to red. We hypothesized that if the color was correlated to one or more indices of male quality, the color displayed on the claws of male L. uruguayensis could be under sexual selection. We found L. uruguayensis claw color varies substantially among the individuals we photographed. However, we did not find a correlation between claw color and indices of quality; neither brightness nor hue correlated to the indices of quality we measured. Our findings suggest that claw color in L. uruguayensis is unlikely to have evolved to signal quality, but may instead function as a species identity or as a non-indicator sexual signal.

Dynamic colour change in zebrafish (Danio rerio) across multiple contexts

Many animals are capable of rapid dynamic colour change, which is particularly well represented in fishes. The proximate mechanisms of dynamic colour change in fishes are well understood; however, less attention has been given to understanding its ecological relevance. In this study, we investigate dynamic colour change in zebrafish (Danio rerio) across multiple contexts, using a protocol to image the colouration of live fish without anaesthesia under standardized conditions. We show that zebrafish respond to different visual environments by darkening their overall colouration in a dark environment and lightening in a light environment. This is consistent with crypsis through background matching as a function of dynamic colour change. Additionally, we find that zebrafish use dynamic colour change to increase the internal contrast of their striped patterning in the presence of conspecifics. We speculate that this may function in social signalling and/or dazzle colouration. We find no effect of a predator stimulus on dynamic colour change. Finally, we discuss the potential for zebrafish to use multiple colouration strategies simultaneously as distance-dependent effects, considering the typical viewing distances of zebrafish and their predators.

Background selection for camouflage shifts in accordance with color change in an intertidal prawn

To maximize camouflage across visually heterogeneous habitats, animals have evolved a variety of strategies, including polyphenism, color change, and behavioral background matching. Despite the expected importance of behavioral processes for mediating camouflage, such as selection for matching substrates, behavior has received less attention than color traits themselves, and interactions between color change and behavior are largely unexplored. Here, we investigated behavioral background matching in green and red chameleon prawns (Hippolyte varians) over the course of a color change experiment. Prawns were housed on mismatching green and red seaweeds for 30 days and periodically given a choice test between the same seaweeds in y-choice trials over the experiment. We found that, as prawns change color and improve camouflage (to the perspective of a fish predator), there is a reinforcing shift in behavior. That is, as prawns shift from red to green color, or vice versa, their seaweed color preference follows this. We provide key empirical evidence that plasticity of appearance (color) is accompanied by a plastic shift in behavior (color preference) that reinforces camouflage in a color changing species on its natural substrate. Overall, our research highlights how short-term plasticity of behavior and longer-term color change act in tandem to maintain crypsis over time.

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.

Association of a single amino acid replacement with dorsal pigmentation in a lizard from the Qinghai-Tibetan Plateau

Reptiles can evolve adaptive colors in different environments, but relatively little is known about the genetic mechanisms. Here, we identified the MC1R gene and its association with intraspecific color variation in the lizard Phrynocephalus erythrurus. Analysis of the MC1R sequence in 143 individuals from dark South Qiangtang Plateau (SQP) and light North Qiangtang plateau (NQP) populations, revealed two amino acid sites that showed significant differences in frequency between two areas. One SNP, corresponding to Glu183Lys residue, was found to be a highly significant outlier and differentially fixed for SQP and NQP populations. This residue is located in an extracellular area in the second small extracellular loop within the secondary structure of MC1R, which represents an "attachment pocket" part of the 3D structure. Cytological expression of MC1R alleles with the Glu183Lys replacement showed a 39 % increase in intracellular agonist-induced cyclic AMP levels and a 23.18 % greater cell surface expression of MC1R protein in the SQP relative to the NQP allele. Further in silico 3D modeling and in vitro binding experiments indicated a higher MC1R-α-MSH binding for the SQP allele, and elevated melanin synthesis. We provide an overview of how a single amino acid replacement leads to fundamental changes in MC1R function, and hence shapes variation in dorsal pigmentation in lizards from different environments.

Changes in group behaviour in response to a preferred environment reflect positive affect

When observed in their preferred environments, animals display behavioural changes, such as an increase in resting or a reduction in agonism, suggestive of positive affect and improved welfare. However, most studies focus on the behaviour of individuals or, at most, pairs of animals; even though in group-living animals beneficial environmental changes may impact on how the group behaves as a whole. In this study, we investigated whether experiencing a preferred visual environment affected the shoaling behaviour of zebrafish (Danio rerio) groups. We first confirmed a group preference for an image of gravel placed underneath the base of a tank compared to a plain white image. Second, we observed replicated groups either with or without the preferred (gravel) image present to determine if a visually enriched and preferred environment could elicit changes in shoaling behaviour. We found a significant interaction between the observation time and test condition, with differences in shoaling behaviour reflective of increased relaxation emerging gradually over time in the gravel condition. The findings of this study reveal that experiencing a preferred environment can alter group behaviour, making such holistic changes valuable as potential indicators of positive welfare.

Scorpionfish rapidly change colour in response to their background

BACKGROUND

To facilitate background matching in heterogenous environments, some animals rapidly change body colouration. Marine predatory fishes might use this ability to hide from predators and prey. Here, we focus on scorpionfishes (Scorpaenidae), well-camouflaged, bottom-dwelling sit-and-wait predators. We tested whether Scorpaena maderensis and Scorpaena porcus adjust body luminance and hue in response to three artificial backgrounds and thereby achieve background matching. Both scorpionfish species are also red fluorescent, which could contribute to background matching at depth. Therefore, we tested whether red fluorescence is also regulated in response to different backgrounds. The darkest and the lightest backgrounds were grey, while the third background was orange of intermediate luminance. Scorpionfish were placed on all three backgrounds in a randomised repeated measures design. We documented changes in scorpionfish luminance and hue with image analysis and calculated contrast to the backgrounds. Changes were quantified from the visual perspective of two potential prey fishes, the triplefin Tripterygion delaisi and the goby Pomatoschistus flavescens. Additionally, we measured changes in the area of scorpionfish red fluorescence. Because scorpionfish changed quicker than initially expected, we measured luminance change at a higher temporal resolution in a second experiment.

RESULTS

Both scorpionfish species rapidly adjusted luminance and hue in response to a change of background. From prey visual perspective, scorpionfishes' body achromatic and chromatic contrasts against the background were high, indicating imperfect background matching. Chromatic contrasts differed considerably between the two observer species, highlighting the importance of choosing natural observers with care when studying camouflage. Scorpionfish displayed larger areas of red fluorescence with increasing luminance of the background. With the second experiment, we showed that about 50% of the total luminance change observed after one minute is achieved very rapidly, in five to ten seconds.

CONCLUSION

Both scorpionfish species change body luminance and hue in response to different backgrounds within seconds. While the achieved background matching was suboptimal for the artificial backgrounds, we propose that the observed changes were intended to reduce detectability, and are an essential strategy to camouflage in the natural environment.

The physiological cost of colour change: evidence, implications and mitigations

Animals benefit from phenotypic plasticity in changing environments, but this can come at a cost. Colour change, used for camouflage, communication, thermoregulation and UV protection, represents one of the most common plastic traits in nature and is categorised as morphological or physiological depending on the mechanism and speed of the change. Colour change has been assumed to carry physiological costs, but current knowledge has not advanced beyond this basic assumption. The costs of changing colour will shape the evolution of colour change in animals, yet no coherent research has been conducted in this area, leaving a gap in our understanding. Therefore, in this Review, we examine the direct and indirect evidence of the physiological cost of colour change from the cellular to the population level, in animals that utilise chromatophores in colour change. Our Review concludes that the physiological costs result from either one or a combination of the processes of (i) production, (ii) translocation and (iii) maintenance of pigments within the colour-containing cells (chromatophores). In addition, both types of colour change (morphological and physiological) pose costs as they require energy for hormone production and neural signalling. Moreover, our Review upholds the hypothesis that, if repetitively used, rapid colour change (i.e. seconds-minutes) is more costly than slow colour change (days-weeks) given that rapidly colour-changing animals show mitigations, such as avoiding colour change when possible. We discuss the potential implications of this cost on colour change, behaviour and evolution of colour-changing animals, generating testable hypotheses and emphasising the need for future work to address this gap.

Analysis of pigment cell composition, pigment content, tyrosinase content and activity of three kinds of loaches Misgurnus anguillicaudatus from Poyang Lake

Pigment cell composition, pigment content, tyrosinase content and activity analysis were investigated on three kinds of loaches Misgurnus anguillicaudatus: big blackspot loaches (BBL), small blackspot loaches (SBL) and non-blackspot loaches (NBL), from Poyang Lake. Results showed that there were three types of skin pigment cells, namely melanophores, xanthophores and iridophores. Melanophores in dorsum were more than those in abdomen. Melanophore cytosomes in BBL were larger than those in SBL and NBL, and melanosomes were the largest in stage four. The melanophores in dorsal skin of SBL or NBL were small cell bodies, spindle-like and in chain distribution. There was an extremely significant difference in melanin content in BBL between the dorsum and abdomen (P < 0.01). There were no significant differences in melanin abdominal content, lutein and carotenoid contents among three kinds of loaches (P > 0.05). In dorsal skin, tyrosinase content was the highest in BBL, and it was significantly lower in NBL than in BBL and SBL (P < 0.01). This study reveals the differences in pigment and tyrosinase content in three kinds of loaches and provides a theoretical basis for further study of the mechanism of black spot formation.

Evidence for coloration plasticity in the yellow-bellied toad, Bombina variegata

Phenotypic adaptation in terms of background color matching to the local habitat is an important mechanism for survival in prey species. Thus, intraspecific variation in cryptic coloration is expected among localities with dissimilar habitat features (e.g., soil, vegetation). Yellow-bellied toads (Bombina variegata) display a dark dorsal coloration that varies between populations, assumed to convey crypsis. In this study, we explored I) geographic variation in dorsal coloration and II) coloration plasticity in B. variegata from three localities differing in substrate coloration. Using avian visual modeling, we found that the brightness contrasts of the cryptic dorsa were significantly lower on the local substrates than substrates of other localities. In experiments, individuals from one population were able to quickly change the dorsal coloration to match a lighter substrate. We conclude that the environment mediates an adaptation in cryptic dorsal coloration. We suggest further studies to test the mechanisms by which the color change occurs and explore the adaptive potential of coloration plasticity on substrates of varying brightness in B. variegata and other species.

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.

Distinct type II opsins in the eye decode light properties for background adaptation and behavioural background preference

Crypsis increases survival by reducing predator detection. Xenopus laevis tadpoles decode light properties from the substrate to induce two responses: a cryptic coloration response where dorsal skin pigmentation is adjusted to the colour of the substrate (background adaptation) and a behavioural crypsis where organisms move to align with a specific colour surface (background preference). Both processes require organisms to detect reflected light from the substrate. We explored the relationship between background adaptation and preference and the light properties able to trigger both responses. We also analysed which retinal photosensor (type II opsin) is involved. Our results showed that these two processes are segregated mechanistically, as there is no correlation between the preference for a specific background with the level of skin pigmentation, and different dorsal retina-localized type II opsins appear to underlie the two crypsis modes. Indeed, inhibition of melanopsin affects background adaptation but not background preference. Instead, we propose pinopsin is the photosensor involved in background preference. pinopsin mRNA is co-expressed with mRNA for the sws1 cone photopigment in dorsally located photoreceptors. Importantly, the developmental onset of pinopsin expression aligns with the emergence of the preference for a white background, but after the background adaptation phenotype appears. Furthermore, white background preference of tadpoles is associated with increased pinopsin expression, a feature that is lost in premetamorphic froglets along with a preference for a white background. Thus, our data show a mechanistic dissociation between background adaptation and background preference, and we suggest melanopsin and pinopsin, respectively, initiate the two responses.

All Shades of Shrimp: Preferences of Colour Morphs of a Freshwater Shrimp Neocaridina davidi (Decapoda, Atyidae) for Substrata of Different Colouration

Simple Summary

Examination of preferences of an aquarium “Red Cherry” shrimp for differently coloured backgrounds revealed common traits, irrespective of shrimp body colouration. The shrimp selected dark backgrounds and coarse patterns over light and fine patterned substrata. Thus, the use of materials with dark and uniform colouration can contribute to designing proper monitoring tools to detect biological invasions after releasing this pet into the wild, as well as to provide shrimp with comfortable conditions in captivity.

Abstract

An ornamental freshwater shrimp, Neocaridina davidi, is popular as an aquarium hobby and, therefore, a potentially invasive species. There is a growing need for proper management of this species to determine not only their optimum breeding conditions, but also their ability to colonise novel environments. We tested habitat preferences of colour morphs (brown, red, white) of N. davidi for substratum colour (black, white, grey shades, red) and fine or coarse chess-board patterns to recognise their suitable captivity conditions and predict their distribution after potential release into nature. We conducted laboratory choice experiments (n = 8) with three individuals of the same morph exposed for two hours to a range of backgrounds. Shrimp preferred dark backgrounds over light ones irrespective of their own colouration and its match with the background colour. Moreover, the brown and red morphs, in contrast to the white morph, preferred the coarse background pattern over the finer pattern. This suggests that the presence of dark, uniform substrata (e.g., rocks, macrophytes) will favour N. davidi. Nevertheless, the polymorphism of the species has little effect on its total niche breadth, and thus its invasive potential.

Intertidal gobies acclimate rate of luminance change for background matching with shifts in seasonal temperature

Rate of colour change and background matching capacity are important functional traits for avoiding predation and hiding from prey. Acute changes in environmental temperature are known to impact the rate at which animals change colour, and therefore may affect their survival. Many ectotherms have the ability to acclimate performance traits such as locomotion, metabolic rate and growth rate with changes in seasonal temperature. However, it remains unclear how other functional traits that are directly linked to behaviour and survival respond to long-term changes in temperature (within an individual's lifetime). We assessed whether the rate of colour change is altered by long-term changes in temperature (seasonal variation) and if rate of colour change can acclimate to seasonal thermal conditions. We used an intertidal rock-pool goby Bathygobius cocosensis, to test this and exposed individuals to representative seasonal mean temperatures (16 or 31°C, herein referred to cold- and warm-exposed fish respectively) for 9 weeks and then tested their rate of luminance change when placed on white and black backgrounds at acute test temperatures 16 and 31°C. We modelled rate of luminance change using the visual sensitives of a coral trout Plectropmus leopardus to determine how well gobies matched their backgrounds in terms of luminance contrast to a potential predator. After exposure to long-term seasonal conditions, the warm-exposed fish had faster rates of luminance change and matched their background more closely when tested at 31 than at 16°C. Similarly, the cold-exposed fish had faster rates of luminance change and matched their backgrounds more closely at 16°C than at 31°C. This demonstrates that rate of luminance change can be adjusted to compensate for long-term changes in seasonal temperature. This is the first study to show that animals can acclimate rate of colour change for background matching to seasonal thermal conditions. We also show that rapid changes in acute temperature reduce background matching capabilities. Stochastic changes in climate are likely to affect the frequency of predator-prey interactions which may have substantial knock-on effects throughout ecosystems.

Effects of substrate color on intraspecific body color variation in the toad-headed lizard, Phrynocephalus versicolor

Diversity in animal coloration is generally associated with adaptation to their living habitats, ranging from territorial display and sexual selection to predation or predation avoidance, and thermoregulation. However, the mechanism underlying color variation in toad-headed Phrynocephalus lizards remains poorly understood. In this study, we investigated the population color variation of Phrynocephalus versicolor. We found that lizards distributed in dark substrate have darker dorsal coloration (melanic lizards) than populations living in light substrates. This characteristic may improve their camouflage effectiveness. A reciprocal substrate translocation experiment was conducted to clarify the potential role of morphological adaptation and physiological plasticity of this variation. Spectrometry technology and digital photography were used to quantify the color variation of the above-mentioned melanic and nonmelanic P. versicolor populations and their native substrate. Additionally, substrate color preference in both populations was investigated with choice experiments. Our results indicate that the melanic and nonmelanic populations with remarkable habitat color difference were significantly different on measured reflectance, luminance, and RGB values. Twenty-four hours, 30 days, and 60 days of substrate translocation treatment had little effects on dorsal color change. We also found that melanic lizards choose to live in dark substrate, while nonmelanic lizards have no preference for substrate color. In conclusion, our results support that the dorsal coloration of P. versicolor, associated with substrate color, is likely a morphological adaptation rather than phenotypic plasticity.

Colour change and behavioural choice facilitate chameleon prawn camouflage against different seaweed backgrounds

Camouflage is driven by matching the visual environment, yet natural habitats are rarely uniform and comprise many backgrounds. Therefore, species often exhibit adaptive traits to maintain crypsis, including colour change and behavioural choice of substrates. However, previous work largely considered these solutions in isolation, whereas many species may use a combination of behaviour and appearance to facilitate concealment. Here we show that green and red chameleon prawns (Hippolyte varians) closely resemble their associated seaweed substrates to the vision of predatory fish, and that they can change colour to effectively match new backgrounds. Prawns also select colour-matching substrates when offered a choice. However, colour change occurs over weeks, consistent with seasonal changes in algal cover, whereas behavioural choice of matching substrates occurs in the short-term, facilitating matches within heterogeneous environments. We demonstrate how colour change and behaviour combine to facilitate camouflage against different substrates in environments varying spatially and temporally.