LOHMANN, K.J.: Magnetic Orientation and Navigation in Marine Animals

The Earth�s magnetic field provides a pervasive source of directional information used by diverse marine animals. Behavioral experiments with sea turtles, spiny lobsters, and the mollusc Tritonia have revealed that all have a magnetic compass sense, despite vast differences in the environment each inhabits and the spatial scale over which each moves. For some of these animals, the Earth�s field also serves as a source of positional information. Hatchling loggerhead sea turtles from Florida respond to the magnetic fields found in three widely separated regions of the Atlantic Ocean by swimming in directions that would, in each case, facilitate movement along the migratory route. Thus, for young loggerheads, regional fields function as navigational markers and elicit changes in swimming direction at crucial geographic boundaries. Similarly, spiny lobsters can apparently use geomagnetic cues to determine their position relative to a goal under at least some conditions. Relatively little is known about the neural mechanisms that underlie magnetic orientation and navigation. A promising model system is the sea slug Tritonia, a mollusc possessing both a magnetic compass and a relatively simple nervous system. Six neurons in the brain of Tritonia have been identified that respond electrically to changes in ambient magnetic fields. All or most of these cells appear to be ciliary motor neurons that generate or modulate the final behavioral output of the orientation circuitry. These findings represent an encouraging first step toward an understanding of the cells and circuitry that underlie magnetic orientation behavior in a neuroethological model animal.