Detection and Use of the Earth's Magnetic Field by Aquatic Vertebrates

Although the hypothesis that animals use a magnetic sense to navigate over long distances in the sea is intuitively appealing, evidence that aquatic vertebrates respond to the magnetic field in nature has been difficult to obtain until recent years. Aquatic vertebrates have, however, been prominent in laboratory-based demonstration and analysis of the magnetic sense and its mechanism. The key conclusions of these studies have been that the magnetic sense exhibits fundamental properties found in other specialized sensory systems and that the magnetic senses of aquatic vertebrates and birds exhibit substantial similarities. In particular, the magnetic sense appears to be selective for the magnetic field stimulus; that is, it responds only to the magnetic field stimulus and does not extract magnetic field information from interactions of the magnetic field with the detector components in other specialized sensory systems. The magnetic sense of aquatic vertebrates is also likely to be highly sensitive to small changes in magnetic fields, with its detector cells operating at close to the limit set by background thermal energy. Finally, it seems likely that the magnetic senses of birds and aquatic vertebrates exhibit substantial similarities in their structure and function. Laboratory experiments have demonstrated behavioral and neural responses to magnetic direction and intensity in species from four classes of aquatic vertebrates. Magnetic impairment experiments also strongly imply that magnetic field detection in both sea turtles and elasmobranchs is based on single-domain particles of magnetite. At the receptor level, an array of new imaging and microscopic techniques has identified magnetoreceptor cells that contain 1-µm-long chains of single-domain magnetite crystals within the olfactory lamellae of rainbow trout. These chains of magnetite crystals will respond only to magnetic fields and appear to have been selected for high sensitivity to small changes in magnetic field stimuli. Recent experiments have demonstrated that the magnetic sense of birds is also based on magnetite located in the nasal region and that the same nerve carries magnetic field information to the brain in both fishes and birds. It therefore seems likely that magnetite is the basis of magnetic field detection in a wide range of vertebrate groups. We conclude that, in the aquatic vertebrates, the magnetic sense can now be demonstrated and analyzed in the laboratory using experimental approaches developed for the study of other sensory modalities. Careful selection of experimental subjects will be required, however, to overcome the challenge of applying insights gained in the laboratory to experimental analysis of the use of the magnetic field in the aquatic environment