Identification of magnetically responsive neurons in the marine mollusc Tritonia diomedea.

Behavioral experiments have demonstrated that the marine mollusc Tritonia diomedea can use the Earth's magnetic field as an orientation cue. Little is known, however, about the neural mechanisms that underlie magnetic orientation behavior in this or any other animal. In previous studies, two neurons in the brain of Tritonia, known as LPd5 and RPd5, were shown to respond with enhanced electrical activity to changes in earth-strength magnetic fields. We report evidence that two additional neurons, known as LPd6 and RPd6, also respond with increases in electrical activity when the magnetic field around the animal is altered. Anatomical analyses revealed that prominent neurites from the Pd6 cells are located within two ipsilateral nerves, pedal nerves 1 and 2. These nerves extend to the periphery of the animal and innervate tissues of the anterior ipsilateral foot and body wall. Electrophysiological recordings demonstrated that action potentials generated by the Pd6 cells propagate from the central ganglia toward the periphery. These results imply that the Pd6 cells play an efferent role in the magnetic orientation circuitry. Given that these cells contain cilio-excitatory peptides and that Tritonia crawls using ciliary locomotion, the Pd6 neurons may control or modulate cilia used in crawling, turning, or both.