Facultative mimicry: cues for colour change and colour accuracy in a coral reef fish

Mimetic species evolve colours and body patterns to closely resemble poisonous species and thus avoid predation (Batesian mimicry), or resemble beneficial or harmless species in order to approach and attack prey (aggressive mimicry). Facultative mimicry, the ability to switch between mimic and non-mimic colours at will, is uncommon in the animal kingdom, but has been shown in a cephalopod, and recently in a marine fish, the bluestriped fangblenny Plagiotremus rhinorhynchos, an aggressive mimic of the juvenile cleaner fish Labroides dimidiatus. Here we demonstrate for the first time that fangblennies adopted mimic colours in the presence of juvenile cleaner fish; however, this only occurred in smaller individuals. Field data indicated that when juvenile cleaner fish were abundant, the proportion of mimic to non-mimic fangblennies was greater, suggesting that fangblennies adopt their mimic disguise depending on the availability of cleaner fish. Finally, measurements of spectral reflectance suggest that not only do mimic fangblennies accurately resemble the colour of their cleaner fish models but also mimic other species of fish that they associate with. This study provides insights into the cues that control this remarkable facultative mimicry system and qualitatively measures its accuracy.     

A simple method for measuring colour in wild animals: validation and use on chest patch colour in geladas (Theropithecus gelada)

Adaptive hypotheses about colour variation are widespread in behavioural ecology, and several methods of objective colour assessment have been proposed and validated for use in a wide variety of taxa. However, to date, the most objective and reliable methods of assessing colour are not readily applied to wild animals. In the present study, we present a simple method for assessing colour in unrestrained, wild subjects using digital photography. The method we describe uses a digital camera, a colour standard, and colour analysis software, and can be used to measure any part of the visible colour spectrum. We demonstrate that the method: (1) is accurate and precise across different light conditions; (2) satisfies previous criteria regarding linearity and red, green, and blue equality; and (3) can be independently validated visually. In contrast with previous digital methods, this method can be used under natural light conditions and can be readily applied to subjects in their natural habitat. To illustrate this, we use the method to measure chest colour in wild geladas ( Theropithecus gelada ). Unique among primates, geladas have a red patch of skin on their chest and neck, which, for males, is thought to be a sexually selected signal. Offering some support to this hypothesis, we found differences in chest 'redness' for males across different age groups, with males in their reproductive prime exhibiting the reddest chests.  © 2008 The Linnean Society of London, Biological Journal of the Linnean Society , 2008, 94 , 231–240.     

Novel identification of the different types of cones in the retina of the chicken

1. Although single cones and double cones in the chicken retina had been documented for more than 30 years, the exact morphology of these cells had never been studied by the scanning electronmicroscopy. In this brief report, we present the evidence for the first time the existence of two types of single cones and three types of double cones (termed as types A, B, and C), each with distinct morphology.2. The proportion of type A:type B:type C double cones, as estimated from the midperipheral and central regions of our scanning specimens, was about 30%:50%:20%.3. Based on the literature data that red oil droplets reside in single cones and yellow oil droplets in chief cones of the double cones, the proportion of single to double cones was deciphered and the relative proportion was estimated to be 1:0.91 in the central region, 1:0.92 in the midperiphery, and 1:0.84 in the periphery.     

Polarization-based brightness discrimination in the foraging butterfly, Papilio xuthus

The human eye is insensitive to the angular direction of the light e-vector, but several animal species have the ability to discriminate differently polarized lights. How the polarization is detected is often unclear, however. Egg-laying Papilio butterflies have been shown to see false colours when presented with differently polarized lights. Here we asked whether this also holds in foraging butterflies. After training individuals to feed on nectar in front of an unpolarized spectral light, we carried out three dual-choice tests, where the discrimination of (i) the spectral content, (ii) the light intensity, and (iii) the e-vector orientation were investigated. In the first test, the butterflies selected the trained spectrum irrespective of its intensity, and in the second test they chose the light with the higher intensity. The result of the e-vector discrimination test was very similar to that of the second test, suggesting that foraging butterflies discriminate differently polarized lights as differing in brightness rather than as differing in colour. Papilio butterflies are clearly able to use at least two modes of polarization vision depending on the behavioural context.     

Sequence and localization of an ultraviolet (sws1) opsin in the retina of the Japanese sardine Sardinops melanostictus (Teleostei: Clupeiformes)

A full‐length complementary (c)DNA encoding ultraviolet (UV)‐sensitive opsin (sws1) was isolated from the retina of the Japanese sardine Sardinops melanostictus. The sws1 phylogenetic tree showed a sister group relationship with the Cypriniformes, following the ray‐finned fish phylogeny. By expressing reconstituted opsin in vitro, it was determined that the maximum absorbance spectrum (λ_max) of sws1 is around 382 nm, being intermediate in position between two subtypes of sws1 pigment that are UV sensitive (λ_max = 355–380 nm) and violet sensitive (λ_max = 388–455 nm), which have been reported to date. The ocular media transmitted >20% transmittance of light in the range of 360–600 nm. In situ hybridization analyses revealed that sws1 messenger (m)RNA is localized in a central single cone surrounded by four double cones in a square mosaic. The square mosaic occupies the ventro‐temporal quadrant of the retina and the in situ hybridization signals were dominant in this area suggesting that the fish may use UV vision when looking upward. Based on these results, considerable significances of potential UV sensitivity, in relation to characteristic habits of S. melanostictus, are discussed.     

Phylogeography of colour polymorphism in the coral reef fish Pseudochromis fuscus, from Papua New Guinea and the Great Barrier Reef

Body colour has played a significant role in the evolution of coral reef fishes, but the phylogenetic level at which colour variation is expressed and the evolutionary processes driving the development and persistence of different colour patterns are often poorly understood. The aim of this study was to examine the genetic relationships between multiple colour morphs of Pseudochromis fuscus (family Pseudochromidae), both within and among geographic locations. Pseudochromis fuscus is currently described as a single species, but exhibits at least six discrete colour morphs throughout its range. In this study, P. fuscus from Papua New Guinea (PNG) and the Great Barrier Reef (GBR), Australia, formed three genetically distinct clades based on mitochondrial DNA (control region) sequence data: (1) yellow and brown morphs from the GBR and southern PNG, as well as an orange morph from southern PNG; (2) a pink morph from southern PNG; and (3) all three morphs (pink, orange and grey) found in Kimbe Bay, northern PNG. The three groups showed deep levels of divergence (d=14.6–25.4%), suggesting that P. fuscus is a complex of polychromatic species, rather than a single widespread species with many different colour morphs. Population genetic analyses indicate that the three clades have experienced unique evolutionary histories, possibly from differential effects of sea level fluctuations, barriers to gene flow and historical biogeography.     

Diurnal variation in the abundance of juvenile parasitic gnathiid isopods on coral reef fish: implications for parasite-cleaner fish interactions

 The dynamics of parasitic gnathiid isopod infestation on the fish Hemigymnus melapterus were examined at Heron Island, Great Barrier Reef, by measuring the abundance and feeding state of gnathiids on fish collected between dawn and sunset and by estimating the time required for gnathiids to become engorged on host fluids. A model was developed to estimate gnathiid abundance on fish for any given time of day and host size. Fish at dawn had 2.4 times as many gnathiids compared with fish at sunset, indicating that some gnathiids infest fish overnight. Most gnathiids had engorged guts (72–86%); the proportion of empty guts and engorged guts did not differ in three time periods of collection (<0800 h, 0800 to 1100 h, and >1100 h). In the laboratory, gnathiids fed quickly with 75% of gnathiids exposed to fish for 4 h having engorged guts. The short time required for gnathiids to become engorged and the presence of gnathiids with empty guts throughout the day suggests that gnathiids also infest fish during the day. Thus gnathiids eaten by cleaner fish during the day may be replaced by other gnathiids during the day or night suggesting that interactions between gnathiids and cleaner fish are highly dynamic. Accepted: 15 April 1999     

Red fluorescence increases with depth in reef fishes,supporting a visual function,not UV protection

Why do some marine fishes exhibit striking patterns of natural red fluorescence? In this study, we contrast two non-exclusive hypotheses: (i) that UV absorption by fluorescent pigments offers significant photoprotection in shallow water, where UV irradiance is strongest; and (ii) that red fluorescence enhances visual contrast at depths below −10 m, where most light in the ‘red’ 600–700 nm range has been absorbed. Whereas the photoprotection hypothesis predicts fluorescence to be stronger near the surface and weaker in deeper water, the visual contrast hypothesis predicts the opposite. We used fluorometry to measure red fluorescence brightness in vivo in individuals belonging to eight common small reef fish species with conspicuously red fluorescent eyes. Fluorescence was significantly brighter in specimens from the −20 m sites than in those from −5 m sites in six out of eight species. No difference was found in the remaining two. Our results support the visual contrast hypothesis. We discuss the possible roles fluorescence may play in fish visual ecology and highlight the possibility that fluorescent light emission from the eyes in particular may be used to detect cryptic prey.     

Perception,pattern, chance and the structure of reef fish communities

The recent history of attempts to understand the ecology of fish on coral reefs is surveyed as an example of the way in which science progresses. Scientists are trapped, both by their sensory equipment and by their preconceptions, into viewing the world in certain ways. Paradigms fall only slowly. Scientists are trained to seek pattern in their data, yet in some cases the largely stochastic variation of a system around its mean condition is the more important key to understanding its ecology. The reef fish assemblage provides such a case. It is unlikely that our present understanding of the nature of reef fish communities will survive unchanged by future research. And it is also unlikely that reef fish ecologists are the only ecologists who have difficulty discovering truth! Editorial     

Characterization of 16 microsatellite loci for a common coral reef fish,the fierce shrimpgoby Ctenogobiops feroculus

We developed 16 pairs of primers for microsatellite loci of the fierce shrimpgoby, Ctenogobiops feroculus. Analysis of 35–40 gobies per locus from five islands in French Polynesia indicated that allele frequency varied from two to 30 per locus, while observed heterozygosity ranged between 0.05 and 0.98. These microsatellites should provide insight into patterns of dispersal and connectivity among populations of this common coral reef fish.     

Two SNPs in the SILV gene are associated with silver coat colour in ponies

In horses, a pigment dilution acting only on black eumelanin is the so-called silver coat colour, which is characterized by a chocolate-to-reddish body with a white mane and tail. Using information from other species, we focused our study on SILV as a possible candidate gene for the equine silver phenotype. A 1559-bp genomic fragment was sequenced in 24 horses, and five SNPs were detected. Two of the five SNPs (DQ665301:g.697A>T and DQ665301:g.1457C>T) were genotyped in 112 horses representing eight colour phenotypes. Both mutations were completely associated with the silver phenotype: all eumelanin-producing horses (blacks and bays) with atypical white mane and tail were carriers of the [g.697T; g.1457T] haplotype. We identified this haplotype as well as the silver phenotype only in Shetland ponies and Icelandic horses. Horses without eumelanin (chestnuts) were carriers of the [g.697T; g.1457T] haplotype, but they showed no phenotypic effect. The white or flaxen mane often detected in chestnuts is presumably based on another SILV mutation or on polymorphisms in other genes.     

The colour line and the colour scale in the twentieth century

Some more recent evidence supports Du Bois’ prediction that the twentieth century would prove the century of the colour line. It indicates that men have always and everywhere shown a preference for fair complexioned women as sexual partners, whereas males seeking a mate are rarely disadvantaged by a dark complexion. In the employment market in the USA, a dark complexion is a significant disadvantage for both males and females. Though there is no properly comparable evidence from other countries, there appears to be a widespread tendency for any negative valuation of darker skin colour to be incorporated into a scale of socio-economic status. In some situations a colour scale is replacing the colour line.

Du Bois’ reference to differences of colour has been largely superseded in English-speaking countries by references to differences of race. From a policy standpoint, the switch from colour to race has had both positive and negative consequences. From a sociological standpoint, it has made it more difficult to disaggregate the dimensions of social difference and to dispel the confusions engendered by ideas of racial difference.     

Spatiotemporal distribution of nocturnal coral reef fish juveniles in Moorea Island,French Polynesia

This study aimed to investigate the spatial structure of nocturnal fish communities at settlement on coral reefs in Moorea Island lagoon, French Polynesia; and the temporal consistency of habitat selection between winter (April to June 2001) and summer (November 2001). The Moorea lagoon was divided into 12 habitat zones (i.e., coral reef zones), which were distinct in terms of depth, wave exposure, and substratum composition. Nocturnal visual censuses among the 12 habitats found that the recently settled juveniles of 25 species recorded were dispatched among three communities spatially distributed according to the distance from the reef crest (reef crest, barrier reef, and fringing reef communities). This spatial communities structure of nocturnal juveniles was consistent in both winter and summer and would be explained primarily by a current gradient in Moorea lagoon (current speed was high near the reef crest and decreased towards the beach) and by the topographic characteristics of reef zones. Among the 25 species, 13 were recorded in both winter and summer. A comparison of the spatial distribution between summer and winter for 13 species that occurred during both seasons found that only 4 differed between the two seasons. For these species, habitat selection would be organized primarily by some stochastic processes such as inter- and intraspecific competition, predation, and food availability. Overall, the present study allowed us to highlight that most nocturnal coral reef fish juveniles at Moorea Island exhibited striking patterns in their distribution and current and topographic characteristics of reef zones might exert considerable influence on the distribution of fishes.     

Simultaneous and successive colour discrimination in the honeybee (Apis mellifera)

The colour discrimination of individual free-flying honeybees (Apis mellifera) was tested with simultaneous and successive viewing conditions for a variety of broadband reflectance stimuli. For simultaneous viewing bees used form vision to discriminate patterned target stimuli from homogeneous coloured distractor stimuli, and for successive discrimination bees were required to discriminate between homogeneously coloured stimuli. Bees were significantly better at a simultaneous discrimination task, and we suggest this is explained by the inefficiency with which the bees brain can code and retrieve colour information from memory when viewing stimuli successively. Using simultaneous viewing conditions bees discriminated between the test stimuli at a level equivalent to 1 just-noticeable-difference for human colour vision. Discrimination of colours by bees with simultaneous viewing conditions exceeded previous estimates of what is possible considering models of photoreceptor noise measured in bees, which suggests spatial and/or temporal summation of colour signals for fine discrimination tasks. The results show that when behavioural experiments are used to collect data about the mechanisms facilitating colour discrimination in animals, it is important to consider the effects of the stimulus viewing conditions on results.     

Defective colour vision and taste sensitivity to PTC in the Maharashtrian population

Three caste groups of the Maharashtrian population, namely Brahmin (N=58), Maratha (N=989) and Scheduled caste (N=1073), were studied for defective colour vision and for the ability to taste phenylthiocarbamide (PTC). Comparisons of defective colour vision and PTC taste sensitivity were made with other Maharashtrian populations studied previously by various authors. The incidence of defective colour vision is lowest among the Brahmins (3.44%) and highest in the Scheduled caste people (4.28%). Taste sensitivity to PTC is highest among the Brahmins (72.5%) with a value which is close to the Vednagar Brahmins (73.3%) who show the highest frequency of the ‘T’ gene recorded so far in Maharashtra.     

Within and between day variability in coral reef fish assemblages: Implications for fish community surveys

Coral reef ecosystems are under increasing anthropogenic pressures making it ever more important to monitor changes in fish communities to implement appropriate management. In contrast to long-term spatial and temporal variation which has been extensively documented, little work has been carried out to identify variability in fish assemblages on short time scales, with few studies testing patterns of fish assemblages between and within days. Here we investigated the diurnal changes in species richness, relative abundance and assemblage composition in a shallow coral reef fish community in Egypt. To do so, a section of coral reef was filmed during the morning (0600 h), midday (1000 and 1400 h) and afternoon (1800 h) over eleven days. Dusk (0600 h) and dawn samples (1800 h) showed higher species richness compared to late morning (1000 h) and mid-day samples (1400 h) and borderline significantly higher numbers of total individuals, likely associated with feeding activity and predator avoidance. Assemblage composition varied across days and time-of-day, showing greater variability during dusk and dawn associated with a transition between day-time and night-time assemblages. Our results have implications for designing coral reef fish surveys, emphasising that short-term changes in fish communities should be considered when designing experiments to monitor fish assemblages over time. Where possible, we suggest increasing replication within sites and time scales or randomising data within a specific time window at all sites, looking to exclude dusk and dawn.     

Effects of habitat,wave exposure,and marine protected area status on coral reef fish assemblages in the Hawaiian archipelago

The relationships between fish assemblages, their associated habitat, and degree of protection from fishing were evaluated over a broad spatial scale throughout the main Hawaiian islands. Most fish assemblage characteristics showed positive responses to protection whether it was physical (e.g. habitat complexity), biological (e.g. coral cover growth forms), or human-induced (e.g. marine reserves). Fish biomass was lowest in areas of direct wave exposure and highest in areas partially sheltered from swells. Higher values for fish species richness, number of individuals, biomass, and diversity were observed in locations with higher substrate complexity. Areas completely protected from fishing had distinct fish assemblages with higher standing stock and diversity than areas where fishing was permitted or areas that were partially protected from fishing. Locations influenced by customary stewardship harbored fish biomass that was equal to or greater than that of no-take protected areas. Marine protected areas in the main Hawaiian islands with high habitat complexity, moderate wave disturbance, a high percentage of branching and/or lobate coral coupled with legal protection from fishing pressure had higher values for most fish assemblage characteristics.