Visually mediated species and neighbour recognition in fiddler crabs (Uca mjoebergi and Uca capricornis)

Topography of vision and behaviour

Given the great range of visual systems, tasks and habitats, there is surprisingly little experimental evidence of how visual limitations affect behavioural strategies under natural conditions. Analysing this relationship will require an experimental system that allows for the synchronous measurement of visual cues and visually guided behaviour. The first step in quantifying visual cues from an animal's perspective is to understand the filter properties of its visual system. We examined the first stage of visual processing – sampling by the ommatidial array – in the compound eye of the fiddler crab Uca vomeris. Using an in vivopseudopupil method we determined sizes and viewing directions of ommatidia and created a complete eye map of optical and sampling resolution across the visual field. Our results reveal five distinct eye regions (ventral, dorsal,frontal, lateral and medial) which exhibit clear differences in the organisation of the local sampling array, in particular with respect to the balance of resolution and contrast sensitivity. We argue that, under global eye space constraints, these regional optimisations reflect the information content and behavioural relevance of the corresponding parts of the visual field. In demonstrating the tight link between visual sampling, visual cues and behavioural strategies, our analysis highlights how the study of natural behaviour and natural stimuli is essential to our understanding and interpretation of the evolution and ecology of animal behaviour and the design of sensory systems.

Peripatric speciation drives diversification and distributional pattern of reef hermit crabs (Decapoda: Diogenidae: Calcinus)

The diversity on coral reefs has long captivated observers. We examine the mechanisms of speciation, role of ecology in speciation, and patterns of species distribution in a typical reef-associated clade-the diverse and colorful Calcinus hermit crabs-to address the origin of tropical marine diversity. We sequenced COI, 16S, and H3 gene regions for approximately 90% of 56 putative species, including nine undescribed, "cryptic" taxa, and mapped their distributions. Speciation in Calcinus is largely peripatric at remote locations. Allopatric species pairs are younger than sympatric ones, and molecular clock analyses suggest that >2 million years are needed for secondary sympatry. Substantial niche conservatism is evident within clades, as well as a few major ecological shifts between sister species. Color patterns follow species boundaries and evolve rapidly, suggesting a role in species recognition. Most species prefer and several are restricted to oceanic areas, suggesting great dispersal abilities and giving rise to an ocean-centric diversity pattern. Calcinus diversity patterns are atypical in that the diversity peaks in the west-central oceanic Pacific rather than in the Indo-Malayan "diversity center." Calcinus speciation patterns do not match well-worn models put forth to explain the origin of Indo-West Pacific diversity, but underscore the complexity of marine diversification.

Individually unique body color patterns in octopus (Wunderpus photogenicus) allow for photoidentification

Studies on the longevity and migration patterns of wild animals rely heavily on the ability to track individual adults. Non-extractive sampling methods are particularly important when monitoring animals that are commercially important to ecotourism, and/or are rare. The use of unique body patterns to recognize and track individual vertebrates is well-established, but not common in ecological studies of invertebrates. Here we provide a method for identifying individual Wunderpus photogenicus using unique body color patterns. This charismatic tropical octopus is commercially important to the underwater photography, dive tourism, and home aquarium trades, but is yet to be monitored in the wild. Among the adults examined closely, the configurations of fixed white markings on the dorsal mantle were found to be unique. In two animals kept in aquaria, these fixed markings were found not to change over time. We believe another individual was photographed twice in the wild, two months apart. When presented with multiple images of W. photogenicus, volunteer observers reliably matched photographs of the same individuals. Given the popularity of W. photogenicus among underwater photographers, and the ease with which volunteers can correctly identify individuals, photo-identification appears to be a practical means to monitor individuals in the wild.

Variability of a dynamic visual signal: the fiddler crab claw-waving display

Fiddler crabs use elaborate, species-specific claw-waving displays to communicate with rivals and mates. However, detailed comparative studies of fiddler crab signal structure and structural variations are lacking. This paper provides an analysis of the claw-waving displays of seven Australian species of fiddler crab, Uca mjoebergi, U. perplexa, U. polita, U. seismella, U. signata, U. elegans and U. vomeris. We used digital video to record and analyse the fine-scale spatiotemporal properties of these movement-based visual signals. We found that the structure and timing of the displays is species-specific, exhibiting inter-specific differences that follow phylogenetic relationships. The displays showed intra-specific variation according to individual identity, geographic location and fine-scale behavioural context. The observed differences and variations are discussed in the light of the evolutionary forces that may shape their design.

Crayfish recognize the faces of fight opponents

The capacity to associate stimuli underlies many cognitive abilities, including recognition, in humans and other animals. Vertebrates process different categories of information separately and then reassemble the distilled information for unique identification, storage and recall. Invertebrates have fewer neural networks and fewer neural processing options so study of their behavior may reveal underlying mechanisms still not fully understood for any animal. Some invertebrates form complex social colonies and are capable of visual memory–bees and wasps, for example. This ability would not be predicted in species that interact in random pairs without strong social cohesion; for example, crayfish. They have chemical memory but the extent to which they remember visual features is unknown. Here we demonstrate that the crayfish Cherax destructor is capable of visual recognition of individuals. The simplicity of their interactions allowed us to examine the behavior and some characteristics of the visual features involved. We showed that facial features are learned during face-to-face fights, that highly variable cues are used, that the type of variability is important, and that the learning is context-dependent. We also tested whether it is possible to engineer false identifications and for animals to distinguish between twin opponents.

Colouration and colour changes of the fiddler crab, Uca capricornis: a descriptive study

Colour changes in animals may be triggered by a variety of social and environmental factors and may occur over a matter of seconds or months. Crustaceans, like fiddler crabs (genus Uca), are particularly adept at changing their colour and have been the focus of numerous studies. However, few of these studies have attempted to quantitatively describe the individual variation in colour and pattern or their adaptive significance. This paper quantitatively describes the colour patterns of the fiddler crab Uca capricornis and their ability to change on a socially significant timescale. The most dramatic changes in colour pattern are associated with moulting. These ontogenetic changes result in a general reduction of the colour pattern with increasing size, although females are more colourful and variable than similarly-sized males. Uca capricornis are also capable of rapid colour changes in response to stress, but show no endogenous rhythms associated with the semilunar and tidal cycles commonly reported in other fiddler crabs. The extreme colour polymorphism and the relative stability of the colour patterns in Uca capricornis are consistent with their use in visually mediated mate recognition.

The fiddler crab Uca mjoebergi uses colour vision in mate choice

Although the role of colour in mate choice is well known, few tests of colour vision have been based on mating behaviour. Females of the fiddler crab Uca mjoebergi have recently been shown to use claw coloration to recognize conspecific males. In this study I demonstrate that the females use colour vision for this task; preferentially approaching yellow claws over grey claws regardless of their intensity while failing to discriminate between yellow claws differing in intensity. This is one of only a handful of studies confirming the involvement of colour vision in mate choice and the first conclusive evidence in fiddler crabs.

Crustacean social behavioral changes in response to isolation

Periods of isolation during which animals have no social contact are common in the design of behavioral experiments. They are used, for example, to test memory and recognition responses, or to ensure a baseline condition before experimental manipulations commence. We investigated the effect of isolation periods on the aggressive behavior of matched pairs of the crayfish Cherax destructor in two contexts. The first experiment tested the effects of a period of isolation between two encounters. The second experiment tested the effects of isolation before an encounter by pairing one crayfish from a communal living environment with another crayfish from an isolated one. Fight outcome and aggression levels were analyzed, resulting in three conclusions about the social biology of C. destructor. First, encounters between familiar opponents are influenced by the outcome of the familiarization fight for about 2 weeks. Second, the level of aggression and the outcome of an encounter are affected over different time frames. Third, individuals that are isolated before an encounter can be disadvantaged. These data suggest that isolation, or events that occur during periods of isolation, affect multiple elements of social behavior in C. destructor. This suggestion has implications for the interpretation of previous results and future studies in crustaceans and other taxa.

Individual recognition: it is good to be different

Individual recognition (IR) behavior has been widely studied, uncovering spectacular recognition abilities across a range of taxa and modalities. Most studies of IR focus on the recognizer (receiver). These studies typically explore whether a species is capable of IR, the cues that are used for recognition and the specializations that receivers use to facilitate recognition. However, relatively little research has explored the other half of the communication equation: the individual being recognized (signaler). Provided there is a benefit to being accurately identified, signalers are expected to actively broadcast their identity with distinctive cues. Considering the prevalence of IR, there are probably widespread benefits associated with distinctiveness. As a result, selection for traits that reveal individual identity might represent an important and underappreciated selective force contributing to the evolution and maintenance of genetic polymorphisms.

Spectral sensitivity of four species of fiddler crabs (Uca pugnax,Uca pugilator,Uca vomerisandUca tangeri) measured byin situmicrospectrophotometry

Fiddler crabs have compound eyes that are structurally fairly well understood. However, there has been much debate regarding their spectral sensitivity and capacity to enable colour discrimination. We examined the visual pigments of two North-American species (Uca pugnax and U. pugilator), one species from the Indo-West Pacific (U. vomeris)and the only Eastern-Atlantic species (U. tangeri) of fiddler crabs using in situ microspectrophotometry of frozen sections of dark-adapted eyes. Only one spectral class of visual receptor was found in the larger (R1–7) retinular cells of each species, with maximum absorption peaking between 508 nm and 530 nm (depending upon species). The R8 retinular cell, that might contain a short-wavelength sensitive photopigment and provide a basis for colour vision, was too small to analyze by these methods. Rhabdoms were lined with screening pigment which strongly influenced each species'spectral sensitivity, sharpening the peak and shifting the maximum towards longer wavelengths, on occasion to as far as the 600 nm region. We hypothesize that sensitivity to longer wavelengths enhances contrast between background(blue sky or tall vegetation) and the male major claw during the waving display.