E26 Electric and Magnetic Fields

Topic

This form of electroreception—the detection of an electric field as a distinct sense—is found in sharks, rays, electric fish, and certain amphibians; the ampullae of Lorenzini in these elasmobranchs detect such electric fields with extreme sensitivity, comparable to sensing the field of a common household battery from many kilometers away. Consequently, this extreme sensitivity enables the elasmobranch to locate prey buried in the sand by detecting the bioelectric field generated solely by the prey's own muscles and nerves. If the buried prey generates a stronger bioelectric field, the elasmobranch can locate it from an even greater distance within the sand.

Within the surrounding saltwater, this dipolar electric field decays with distance much more rapidly than light or sound—phenomena described elsewhere in this series—thereby limiting the effective range of electroreception to a relatively short distance for most of these species. However, the dense, highly conductive seawater allows even a weak bioelectric potential to generate a current that remains detectable by the ampullae of Lorenzini. Because this dipolar electric field decays with distance even more rapidly than the rate mentioned earlier, the effective range of electroreception is consequently even shorter. In contrast, a fish such as an apteronotid or a mormyrid generates its own electric field and subsequently detects the disturbances that a nearby object produces in that field—a form of active electrolocation quite distinct from the passive electroreception with which this model began. This self-generated dipolar electric field becomes distorted when passing near a conductive or dielectric object; this distortion is detected via the difference between the signals from the ampullae of Lorenzini on one side of the fish's body and those on the opposite side—a process comparable to the bilateral comparison used in another model in this set to localize a sound source. Consequently, this comparison between the two sides of the fish's body enables it to achieve extraordinarily fine spatial resolution regarding the nearby object. If the object near the fish is more conductive or more dielectric, the distortion of the fish's self-generated electric field becomes more intense and easier to detect across the two sides of its body.

Magnetoreception, on the other hand, employs two mechanisms that are quite distinct from one another: a mechanical mechanism, in which a magnetite crystal within a specialized cell acts as a mechanical compass—aligning with the Earth's magnetic field and thereby activating a mechanoreceptor (a component also featured in another model in this set) through this rotation; and a purely chemical mechanism, in which a radical electron pair within a magnetic-field-sensitive molecule—located in the organism's eye—is affected by the Earth's magnetic field through its own internal chemistry. The fact that the organism employs this second chemical mechanism—rather than the first, mechanical one—means that its magnetoreception relies on a chemical reaction sensitive to the magnetic field, rather than on a mechanically rotating magnetite crystal within its body.

Consequently, this model—incorporating the extreme electrosensitivity and magnetoreception mentioned at the outset—predicts the appearance of an induced voltage across the body of a fish swimming within the Earth's magnetic field; this induced voltage is detectable by the fish's own electroreceptors, precisely because its typical magnitude falls right at the resolution limit of the very ampullae of Lorenzini introduced earlier in the model. This hypothesis would explain how a shark or sea turtle manages to utilize the Earth's magnetic field for oceanic navigation over vast distances. Furthermore, if the fish swims faster through the Earth's magnetic field, the resulting induced voltage across its body increases, making it easier for those same electroreceptors to detect.

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