Signals from 'crabworld': Cuticular reflections in a fiddler crab colony

Fiddler crabs inhabit intertidal sand- and mudflats, where they live in den se colonies and are active on the surface during low tide. They exhibit a r ich behavioural repertoire, with frequent interactions between animals in t he context of territorial and mating activities. Male fiddler crabs have on e massively enlarged and conspicuously coloured claw, which they use in wav ing displays and in fights with other males. The crabs carry their eyes on long, vertically oriented stalks high above the body and, as a consequence, see the bodies of conspecifics in the ventral visual field, below the loca l visual horizon, and against the mudflat surface as background. We filmed events in a colony of Uca vomeris with a normal video camera and an ultravi olet-sensitive camera placed at the eye height of an average crab, approxim ately 2-3 cm above ground. We also used a spectrographic imager and linear polarized filters to analyse the cues potentially available to the animals for detecting, monitoring and possibly identifying each other. Areas of hig h contrast in mudflat scenes include specular reflections on the wet cuticl e of crabs that are horizontally polarised. Besides specular reflections, s ome parts of the cuticle generate high-contrast signals against the mudflat background, both at wavelengths between 400 and 700 nm, and in the ultravi olet region between 300 and 400 nm. Uca vomeris can be very colourful: the different parts of the large claw of the male are white, orange or red. The carapace colours of both males and females can range from a mottled yellow ish green brown, to a brilliant light blue. White and blue colours contrast starkly with the mudflat background, especially in the ultraviolet wavelen gths. Under stress, the blue and white colours can change within minutes to a duller and darker blue or to a dull white.