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Fiddler Crabs Use the Visual Horizon to Distinguish Predators from Conspecifics: A Review of the Evidence

Published online by Cambridge University Press:  11 May 2009

John Layne ,
Michael Land  and
Jochen Zeil
John Layne
Affiliation:
Duke University Marine Station, Beaufort, NC 28516, USA.
Michael Land
Affiliation:
School of Biological Sciences, University of Sussex, Brighton, BN1 9QG.
Jochen Zeil
Affiliation:
Research School of Biological Sciences, Australian National University, Canberra, ACT 2601, Australia
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Extract

Male fiddler crabs, Uca pugilator (Crustacea: Decapoda), respond to conspecifics by claw waving, and to predators by freezing or escape. In field experiments it was found that this distinction was not made on the basis of angular size and speed, nor was shape important. The remaining possibilities were either the absolute size of the stimulus, determined from angular size and distance, or the position of the stimulus relative to the horizon. To distinguish between these, a crab was placed in a glass dish, and moved black stimuli on a white background, at a distance of 22 cm. Stimuli below the crab's horizon hardly ever evoked escape. However, identical stimuli partially or wholly above the crab's horizon produced escape responses whose frequency varied with the angular size of the stimulus. Halving the distance of the stimulus showed that it was angular and not absolute size that determines escape frequency; and experiments with a tilted horizon showed that it is the position of the stimulus relative to the eye equator that is important, rather than the geographical horizon itself. It has been concluded that crabs categorize stimuli as dangerous or otherwise by their position relative to the crabs’ visual horizon.

Type
Research Article
Information
Journal of the Marine Biological Association of the United Kingdom , Volume 77 , Issue 1 , February 1997 , pp. 43 - 54
Copyright
Copyright © Marine Biological Association of the United Kingdom 1997

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