Neuroethology of Magnetic Orientation in Sea Slugs
Behavioral experiments
have demonstrated that diverse animals can sense the Earth’s magnetic
field and use it as a cue for guiding movements over both short and long
distances. Little is known, however,
about the neural circuitry that underlies magnetic orientation behavior.
Most research on
magnetic orientation has focused on vertebrate animals such as migratory birds
and sea turtles. Although such
animals have proven to be excellent subjects for behavioral experiments, they
are not ideal for neurobiological research, in part because the complexity of
the vertebrate nervous system makes cellular-level investigations of neural
circuitry challenging.
The marine mollusc Tritonia diomedea represents a favorable model system for
studying how the nervous system detects magnetic cues, processes them, and
generates appropriate motor responses.
Behavioral experiments have demonstrated that Tritonia can orient
magnetically. In addition, this
animal has large, individually identifiable brain cells and a relatively simple
nervous system amenable to cellular-level electrophysiological analyses.
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The marine mollusc Tritonia diomedea is a nudibranch or sea slug, a kind of snail that has lost its
shell in the course of evolution. The animals are found in the Pacific
northwest ( |
Intracellular
electrophysiological recordings have demonstrated that three bilaterally
symmetric pairs of identifiable neurons respond with altered electrical
activity to changes in Earth-strength magnetic field. Two of these pairs, known as the Pd5 and
Pd6 neurons, are excited by changes in the direction of the ambient field. The third pair, known as the Pd7 cells,
is inhibited by the same magnetic stimuli that excite the others. All of these cells presumably function
in the neural circuitry underlying magnetic orientation behavior.
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Above: The
brain of Tritonia
with arrows indicating the locations of the six large,
magnetically responsive neurons. Right:
Electrophysiological traces from the four neurons that are excited by changes in the
ambient magnetic field. At the time marked by the red arrow,
the ambient field was rotated 60 degrees clockwise. |
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The six magnetically
responsive neurons in Tritonia
represent the first individually identifiable cells known to respond to Earth-strength
magnetic fields in any animal.
Recent evidence suggests that at least some of these cells are involved
in the motor output of the magnetic orientation circuitry. The Pd5 and Pd6 neurons probably control
or modulate the activity of cilia on which the slug crawls and thus influence
the direction toward which the animal moves. Given the relative simplicity of the Tritonia nervous
system, it may eventually be possible to characterize, at the level of
individual neurons, the entire neural circuitry that gives rise to magnetic
orientation behavior in this neuroethological model
animal.
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References:
Cain, S.D., Wang, J.H., and K. J. Lohmann. 2005. Immunochemical and electrophysiological analyses of magnetically responsive neurons in the mollusc Tritonia diomedea. Journal of Comparative Physiology A. In press.
Cain, S. D., Boles, L. C., Wang, J.
H., and K. J. Lohmann. 2005.
Magnetic orientation and navigation in marine turtles, lobsters, and molluscs: concepts and conundrums. Integrative and
Comparative Biology 45: 539-546.
Lohmann, K. J., and A. O. D. Willows. 1987.
Lunar-modulated geomagnetic orientation by a marine mollusk. Science.
235: 331-334.
Lohmann, K. J., A. O. D. Willows, and R. B.
Pinter. 1991. An identifiable molluscan neuron
responds to changes in earth-strength magnetic fields. Journal of
Experimental Biology. 161:1-24.
Popescu,
Wang, J. H., Cain, S. D., and K. J. Lohmann.
2003. Identification of magnetically responsive neurons in the mollusc Tritonia diomedea. Journal of Experimental Biology. 206:
381-388. [Download pdf]
Wang, J. H., Cain, S. D. and K. J. Lohmann.
2004. Identifiable neurons inhibited by Earth-strength magnetic stimuli in the mollusc Tritonia diomedea. Journal of Experimental Biology 207:
1043-1049. [Download pdf]
Willows, A. O. D. 1999. Shoreward orientation involving geomagnetic cues in the nudibranch mollusk Tritonia diomedea. Marine and Freshwater Behaviour and Physiology 32: 181-192.