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.     

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.     

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     

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.     

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.     

Evolution of colour vision in mammals

Colour vision allows animals to reliably distinguish differences in the distributions of spectral energies reaching the eye. Although not universal, a capacity for colour vision is sufficiently widespread across the animal kingdom to provide prima facie evidence of its importance as a tool for analysing and interpreting the visual environment. The basic biological mechanisms on which vertebrate colour vision ultimately rests, the cone opsin genes and the photopigments they specify, are highly conserved. Within that constraint, however, the utilization of these basic elements varies in striking ways in that they appear, disappear and emerge in altered form during the course of evolution. These changes, along with other alterations in the visual system, have led to profound variations in the nature and salience of colour vision among the vertebrates. This article concerns the evolution of colour vision among the mammals, viewing that process in the context of relevant biological mechanisms, of variations in mammalian colour vision, and of the utility of colour vision.     

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     

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.     

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.     

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.     

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.     

A dynamic model of group-size choice in the coral reef fish Dascyllus albisella

We developed a dynamic programming model of group size choicefor settlingcoral reef fish to help understand variabilityin observed group sizes. Ratherthan calculating optimal groupsize, we modeled optimal choice and calculatedthe acceptablegroup sizes that arose from this choice. In the model, settlingindividualsweigh the fitness value of settling in a group against theexpectedfitness of searching another day and encountering other groups,choosingthe option with the higher value. Model results showed thatindividualssettling on any given day in the settling season have severalacceptablegroup sizes in which they can settle. The range of acceptablegroupsizes also changes across the season. Early in the season,when there is stilladequate time to grow, large groups (withhigher survival) have the highestfitness. Late in the season,when the ability to grow fast becomes moreimportant, smallgroups, which convey fast growth rates (although riskier),havehigher fitness. Thus, according to our model, even when fishall make thesame, simple decisions, a variety of outcomes arepossible, depending on thespecific options encountered andtemporally changing ecological pressures.Even when all fishbehave optimally, initial variability in group sizes willpersist.     

Management implications of fish trap effectiveness in adjacent coral reef and gorgonian habitats

A combination of visual census and trap sampling in St. John, USVI indicated that traps performed better in gorgonian habitat than in adjacent coral reef habitat. Although most families were seen more commonly in coral habitat, they were caught more often in gorgonian areas. Traps probably fished more effectively in gorgonian habitats, especially for migrating species, because traps provided shelter in the relatively topographically uniform environment of gorgonian dominated habitats. Recently, trap fishermen on St. John have been moving effort away from traditionally fished nearshore coral reefs and into a variety of more homogeneous habitats such as gorgonian habitat. Consequently, exploitation rates of the already over-harvested reef fish resources may be increasing. Reef fish managers and marine reserve designers should consider limiting trap fishing in gorgonian habitats to slow the decline of reef fisheries.     

Opponent colour coding is a universal strategy to evaluate the photoreceptor inputs in Hymenoptera

Summary Behavioural tests were carried out with 9 hymenopteran insect species, which ranked certain sets of coloured stimuli according to their subjective similarity to a previously memorized stimulus. Kendall's coefficient is employed for the analysis of correlation between these similarity rankings and the colour distance rankings predicted by various models of neural colour computation. The models are based on the measured spectral sensitivities of photoreceptor colour types and use a variety of simple colour coding systems to derive hypothetical colour distances. The correlation between the predictions of the models and the behavioural results serves as a measure for the likelihood of existence of a colour coding system. In all species, the similarity rankings can be best explained by assuming that colour is coded on a perceptual level by two colour opponent mechanisms. Brightness differences are ignored, indicating that an intensity-coding sub-system is not used in colour discrimination by the insects investigated. The weighting factors of the colour opponent mechanisms differ between species in detail, but not in the principles involved. It is thus possible to employ a standard measure of perceptual colour distance (colour hexagon distance) to predict the capacities of colour discrimination adequately in all the tested insects.     

Shell colour polymorphism associated with substrate colour in the intertidal snail Littorina saxatilis Olivi (Prosobranchia: Littorinidae)

Littorina saxatilis Olivi (1792), the rough winkle, is highly polymorphic in shell colour. Shell colour frequencies were studied at six locations in south-western Wales, U.K., each at a geological contact between red sandstone and grey limestone or volcanic rock. At each site shell colour frequencies were determined in samples from the contact zone and on red or grey rock on either side. Highly significant associations were found between shell colour frequencies and substrate colour. Grey shells were always more common on grey rock than on red rock, and brown shells were usually more common on red than on grey rock, suggesting selection for cryptic colouration. Shell colour frequency differences were also found between replicate samples taken only 5 m apart from the same kind of rock, and between samples from the same kind of rock at the six study sites. These latter differences suggest that selection for camouflage is not the only factor involved in maintaining shell colour polymorphism in this species.