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

Abstract

SUMMARYWeakly 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.

Waveform generation in the weakly electric fishGymnotus coropinae(Hoedeman): the electric organ and the electric organ discharge

“…omari, a member of one of the most wide-spread genera of South American pulse gymnotids (Castelló et al, 2000). More recently a similar organization was described in Gnathonemus petersii, an African weakly discharging electric fish having convergent evolution (von der Emde and Schwartz, 2002;Bacelo et al, 2008;Pusch et al, 2008).…”

Environmental acoustic cues guide the biosonar attention of a highly specialised echolocator

“…Gnathonemus petersii possesses two electrosensory foveae, one of which relies on scanning movements to probe the environment (Pusch et al, 2008). This species increases the rate of electric organ discharges in response to acoustic, visual, passive electrical and active electrolocation stimuli (Post and Von Der Emde, 1999).…”

Long-term Behavioral Tracking of Freely Swimming Weakly Electric Fish

“…The subtracted image is converted to a binary format by applying an intensity threshold, such that the centroid and the orientation axis can be computed from binary morphological operations. In Gymnotiforms [27][28][29] and Mormyrids [30][31][32] , the electroreceptor density is the highest near the head region; thus the head position at any moment indicates a location of the highest sensory acuity. The head and tail locations can be automatically determined by applying the image rotation and bounding-box operations.…”