The active electrosensory range of Gymnotus omarorum

This article reports a biophysical and behavioral assessment of the active electrolocation range of Gymnotus omarorum. Physical measurements show that the stimulus field of a point on the sensory mosaic (i.e. the potential positions in which an object may cause a significant departure of the transcutaneous field from basal in the absence of an object) consists of relatively extended volumes surrounding this point. The shape of this stimulus field is dependent on the position of the point on the receptive mosaic and the size of the object. Although the limit of stimulus fields is difficult to assess (it depends on receptor threshold), departure from the basal field decays rapidly, vanishing at about 1.5 diameters for conductive spheres. This short range was predictable from earlier theoretical constructs and experimental data. Here, we addressed the contribution of three different but synergetic mechanisms by which electrosensory signals attenuate with object distance. Using novelty responses as an indicator of object detection we confirmed that the active electrosensory detection range is very short. Behavioral data also indicate that the ability to precisely locate a small object of edible size decays even more rapidly than the ability to detect it. The role of active electroreception is discussed in the context of the fish's habitat.