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Electroreception (detection of electric fields) is a sense present in sharks, rays, electric fish and some amphibians. The ampullae of Lorenzini of elasmobranchs detect electric fields as low as 15 nV/cm equivalent to detecting the field of a 1.5 V battery 1500 km away. This extreme sensitivity allows them to locate prey buried in sand through the bioelectric field of their muscles and nerves.

The dipole electric field in salt water (conductivity 4 S/m) falls as E 1/r³ (much faster than light or sound). This limits the range of electroreception to distances of 1050 cm in most species. However, in dense, conductive seawater, even small bioelectric potentials produce currents detectable by receptors.

Apteronotid and mormyrid fish generate their own electric fields (electrical organ discharge, EOD) and detect disturbances of this field caused by nearby objects (active electrolocation). The EOD produces a dipolar field whose distortion when passing next to a conductive or dielectric object is detected by the difference between blisters on the same and different sides of the body, with spatial resolution of millimeters.

Magnetoreception uses two proposed mechanisms: (1) magnetite (FeO) crystals in specialized cells that act as mechanical compasses the torque = m·B·sin() aligns the dipole with the Earth's field and this rotation activates mechanoreceptors; (2) radical electron pairs (cryptochromes in the eyes) whose chemistry is sensitive to the magnetic field through the radical pair mechanism.

The induced EMF = v·B·L when a fish swims at speed v in the Earth's magnetic field B_T generates a voltage across the body that can be detected by its own electroreceptors. With v = 1 m/s, B_T = 50 T and L = 0.5 m, 25 nV at the resolution limit of the Lorenzini ampoules. This hypothesis would explain how sharks and turtles use the Earth's magnetic field for ocean navigation.

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