Electroreception and the compass sense of sharks

Elasmobranchs have an electric sense that is sensitive enough to detect electric fields as weak as those induced through their bodies as they swim through the earth's magnetic field. Because the intensity and direction of these fields are related to the speed and direction of the movements that cause them, elasmobranchs could use their electric sense in navigation. There is evidence that they do. According to a current theory, elasmobranchs can determine their direction of motion in an Earth-fixed frame using a computation involving electroreceptor voltages, swimming speed and the local geomagnetic field vector. However, this theory is inconsistent with physical and biological constraints, notably that elasmobranch electroreceptors can not measure d.c. voltages, and that a voltage due to water flow in the ocean is not uniquely interpretable in terms of the speed and direction of flow at the point where the electrical measurement is made. This paper presents a new theory that explains how an elasmobranch could use its electric sense to determine a compass bearing as it swims. According to this theory, the direction cue is the directional asymmetry of the change in induced electroreceptor voltage during turns. A neural network could use this cue to determine swimming direction by comparing vestibular and electrosensory signals.