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First published online October 5, 2006
Journal of Experimental Biology 209, 4140-4153 (2006)
Published by The Company of Biologists 2006
doi: 10.1242/jeb.02483

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The variable colours of the fiddler crab Uca vomeris and their relation to background and predation

Jan M. Hemmi^1,2, Justin Marshall³, Waltraud Pix², Misha Vorobyev^1,3 and Jochen Zeil^1,2,*

¹ ARC Centre of Excellence in Vision Science, Australian National University, Canberra ACT 2601, Australia
² Centre for Visual Sciences, Research School of Biological Sciences, Australian National University, Canberra ACT 2601, Australia
³ Vision Touch and Hearing Research Centre, School of Biomedical Sciences, University of Queensland, Brisbane, QLD 4072, Australia

^* Author for correspondence (e-mail: jochen.zeil{at}anu.edu.au)

Accepted 10 August 2006

Colour changes in fiddler crabs have long been noted, but a functional interpretation is still lacking. Here we report that neighbouring populations of Uca vomeris in Australia exhibit different degrees of carapace colours, which range from dull mottled to brilliant blue and white. We determined the spectral characteristics of the mud substratum and of the carapace colours of U. vomeris and found that the mottled colours of crabs are cryptic against this background, while display colours provide strong colour contrast for both birds and crabs, but luminance contrast only for a crab visual system. We tested whether crab populations may become cryptic under the influence of bird predation by counting birds overflying or feeding on differently coloured colonies. Colonies with cryptically coloured crabs indeed experience a much higher level of bird presence, compared to colourful colonies. We show in addition that colourful crab individuals subjected to dummy bird predation do change their body colouration over a matter of days. The crabs thus appear to modify their social signalling system depending on their assessment of predation risk.

Key words: body colour, colour change, fiddler crab, predation