CAIN, SD: Identified Neurons in Two Groups of Tritonia diomedea Respond Differently to an Earth-strength Magnetic Field Stimulus

Behavioral and neurophysiological studies of magnetic field sensitivity have provided evidence that diverse animals possess the ability to use the geomagnetic field to guide orientation and migration behavior. One animal in which these investigations have been carried out is the nudibranch mollusc Tritonia diomedea, which possesses a relatively simple central nervous system and neurons that can be individually identified. Previously, a pair of neurons, left and right Pd5, was found to respond with enhanced electrical activity to rotations of an earth-strength magnetic field. Here we report that the Pd5 cells in two groups of Tritonia collected at separate locations within the Puget Sound region of Washington State, USA, respond differently to the same magnetic stimulus. The Pd5 cells from Bellingham Bay had a significantly greater change in spike frequency in response to the rotated field than did animals from Dash Point (p < 0.002). The responses from Bellingham Bay animals had latencies and magnitudes similar to previous studies, none of which studied Tritonia captured from Dash Point. By contrast, the spike rate of the Pd5 cells in Dash Point animals increased only slightly or not at all. The intrinsic properties of the Pd5 cells we measured (resting potential, spike amplitude, input resistance) did not account for the observed differences. These results indicate that the overall activity and excitability of the Pd5 neurons were similar between groups. The neural elements that underlie the observed difference are therefore likely to be in the upstream components of the magnetic orientation circuitry that are presynaptic to the Pd5 cells.