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First published online February 29, 2008
Journal of Experimental Biology 211, 921-934 (2008)
Published by The Company of Biologists 2008
doi: 10.1242/jeb.014175

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Active sensing in a mormyrid fish: electric images and peripheral modifications of the signal carrier give evidence of dual foveation

Roland Pusch¹, Gerhard von der Emde¹, Michael Hollmann¹, Joao Bacelo², Sabine Nöbel¹, Kirsty Grant² and Jacob Engelmann^1,*

¹ University of Bonn, Institute of Zoology, Department Neuroethology/Sensory Ecology, Endenicher Allee 11-13, 43115 Bonn, Germany
² UNIC, CNRS, 1 Avenue de la Terrasse, 91190 Gif-sur-Yvette, France

^* Author for correspondence (e-mail: jacob.engelmann{at}uni-bonn.de)

Accepted 11 December 2007

Weakly electric fish generate electric fields with an electric organ and perceive them with cutaneous electroreceptors. During active electrolocation, nearby objects are detected by the distortions they cause in the electric field. The electrical properties of objects, their form and their distance, can be analysed and distinguished. Here we focus on Gnathonemus petersii (Günther 1862), an African fish of the family Mormyridae with a characteristic chin appendix, the Schnauzenorgan. Behavioural and anatomical results suggest that the mobile Schnauzenorgan and the nasal region serve special functions in electroreception, and can therefore be considered as electric foveae. We investigated passive pre-receptor mechanisms that shape and enhance the signal carrier. These mechanisms allow the fish to focus the electric field at the tip of its Schnauzenorgan where the density of electroreceptors is highest (tip-effect). Currents are funnelled by the open mouth (funnelling-effect), which leads to a homogenous voltage distribution in the nasal region. Field vectors at the trunk, the nasal region and the Schnauzenorgan are collimated but differ in the angle at which they are directed onto the sensory surface. To investigate the role of those pre-receptor effects on electrolocation, we recorded electric images of objects at the foveal regions. Furthermore, we used a behavioural response (novelty response) to assess the sensitivity of different skin areas to electrolocation stimuli and determined the receptor densities of these regions. Our results imply that both regions – the Schnauzenorgan and the nasal region – can be termed electric fovea but they serve separate functions during active electrolocation.

Key words: pre-receptor mechanism, active electrolocation, fovea, electric image, shape recognition, electric organ discharge, Mormyridae, Gnathonemus petersii

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