A device or organ that detects magnetic fields, particularly the earth’s magnetic field. Some form of magnetic sense is found in a wide range of animals, including insects, fishes, amphibians, reptiles, birds, and mammals, but the nature of the magnetoreceptor organs remains elusive. Investigators have sought small particles of the magnetic material magnetite, which are postulated to transduce the magnetic field into nervous impulses. But the only confirmed examples of magnetite in organisms are found in magnetotactic bacteria, which use the earth’s magnetic field to guide them when swimming. Photoreceptors may also be involved, and there is evidence to support this in various invertebrate and vertebrate species. One theory is that changes in electron spin resonance caused by the earth’s magnetic field elicit nerve impulses. The light-sensitive proteins—cryptochromes—that occur in the retina of the eye have also been postulated to form the basis of an internal magnetic compass in certain animals. Certain fishes, particularly sharks and rays, may use their electroreceptors as magnetoreceptors. This is possible because the animals themselves act as conductors moving through the magnetic field, and so generate electric currents, albeit very weak ones, by electromagnetic induction. Seawater is the conductor that completes the circuit and so could permit detection by the animal’s own sense organs. Whatever the basis for it, such a sense is exploited by the animals in navigating through the marine environment. It is suggested that electromagnetic induction might underlie a magnetic field-sensing mechanism operating in the semicircular canals of the inner ear in some birds.