Electroreception comes in two types, passive and active. The passive sense takes advantage of the electric fields generated by living organisms or, as has been shown in sharks, the electromagnetic field of the earth (e.g., ). Unlike passive electrosen-sation and most other sensory modalities, active electrosensation relies on signals originating from the animal itself. The fish generates an electric field through discharge of an electric organ extending along most of the caudal part of its body (Figure 8.3). The Gymnotiformes are one of two groups of teleosts that independently evolved active electrosensing . Fish of the two Gymnotiform genera treated here, Eigenmannia and Apteronotus, produce a quasi-sinusoidal electric organ discharge (EOD) waveform with frequencies between 200 and 1200 Hz, the exact range being species-specific.
Objects or animals with impedance different from that of water perturb the electric field surrounding a fish. Electroreceptors in the skin monitor these distortions and thus provide information about obstacles, approaching predators, or prey (Figure 8.3; [2,89, 90]). Nearby conspecifics also engage in electric communication, for example in the context of courtship [48, 55, 87]. Thus, the active electrosense allows weakly electric fish to forage and to communicate under conditions when other senses are more or less useless as is the case in their natural habitat: They are nocturnal animals and live in turbid tropical freshwaters, which strongly limits the usefulness of vision. Similar to echolocation in bats, active electrosensation opens an ecological niche that is safe from most diurnal predators. Additionally, it opens a new channel for intraspecific communication.
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