Tanya Detto

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

Mating signals are often directed at numerous senses and provide information about species identity, gender, receptiveness, individual identity and mate quality. Given the diversity of colourful body patterns in invertebrates, surprisingly few studies have examined the role of these visual signals in mate recognition. Here, we demonstrate the use of claw coloration as a species recognition signal in a fiddler crab (Uca mjoebergi). Furthermore, we show that distinct carapace colour patterns in Uca capricornis enable males to discriminate between their female neighbours and unfamiliar females. This is the first empirical evidence of the social importance of colour markings in fiddler crabs and the first example of visually mediated species and neighbour recognition in invertebrates other than insects.

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.

When and Why Do Territorial Coalitions Occur? Experimental Evidence from a Fiddler Crab

Neighboring territory owners are often less aggressive toward each other than to strangers (“dear enemy” effect). There is, however, little evidence for territorial defense coalitions whereby a neighbor will temporarily leave his/her own territory, enter that of a neighbor, and cooperate in repelling a conspecific intruder. This is surprising, as theoreticians have long posited the existence of such coalitions and the circumstances under which they should evolve. Here we document territorial defense coalitions in the African fiddler crab Uca annulipes, which lives in large colonies wherein each male defends a burrow and its surrounding area against neighbors and “floaters” (burrowless males). Fights between a resident and a floater sometimes involve another male who has left his territory to fight the floater challenging his neighbor. Using simple experiments, we provide the first evidence of the rules determining when territorial coalitions form. Our results support recent models that suggest that these coalitions arise from by‐product mutualism.

The fiddler crab Uca mjoebergi uses ultraviolet cues in mate choice but not aggressive interactions

The perception of ultraviolet light and its role in communication has received a great deal of attention in recent years. Although ultraviolet light is no different to other wavelengths in many respects, its role as a signal does deserve special consideration, if only because it is frequently overlooked by human observers who are unable to see it. Studies on the role of ultraviolet cues in inter- and intrasexual interactions are still rare, and generally focused on vertebrates with relatively complex visual systems. Here we show for the first time that crustaceans, with a relatively simple, probably dichromatic, visual system, also use ultraviolet cues in mate choice. We found that the enlarged claws of male Uca mjoebergi reflect ultraviolet wavelengths of light. Mate-searching females preferred ultraviolet-reflecting males over those whose reflectance was blocked with sunscreen. Conspecific males showed no such preference when deciding which rival to attack, suggesting that ultraviolet cues are not involved in signalling competitive ability.

Hunting and predation in a fiddler crab

Fiddler crabs are known primarily to be deposit feeders. They eat detritus, bacteria, and other small particles of organic material found in the sandy or muddy substrate on which they live. They have highly specialized mouthparts used to separate edible matter from nondigestable material. Here we provide evidence of cannibalism and predation in a fiddler crab, Uca annulipes. We additionally provide the first evidence of a fiddler crab hunting shrimp and insects. This study is an exemplary reminder that, even though an animal may have evolved highly specialized feeding traits, this need not preclude it from opportunistically acting as a generalist feeder.

Social Monogamy in A Fiddler Crab, UCA capricornis

Abstract Fiddler crabs are generally considered polygamous as they tend to live in dense mixed sex colonies with numerous neighbours and individually defended territories. We show that the Australian fiddler crab, Uca capricornis, is socially monogamous based on behavioural experiments and observations of neighbouring males and females. The unusual relationship between neighbouring males and females in U. capricornis is selected for and maintained by intrasexual aggression and the ability to recognise and defend their partner.