Eastward slow, the sea slugs.

Eastward slow, the sea slugs

Slow maybe, but not without direction -- thatis the way of Tritonia diomedia. No bread crumbs, North Star or mossy trees point this sea slug species toward shallow water to feed and mate. But it does have a special neuron that apparently uses the earth's magnetic field and the phases of the moon as guides, say scientists.

Using simple, elegant experiments, A.O. Dennis Willos and Kenneth J. Lohmann of the University of Washington in Seattle and its Friday Harbor Laboratories have manipulated the magnetic fields across tanks holding slugs from the Pacific Ocean. They found in the first series of experiments that most of the slugs faced east when exposed to the earth's magnetic field, but remained randomly oriented when that field was canceled.

Yet tests over the following monthsfailed to prove that the slugs usually orient toward the east. The mystery was solved when the researchers incorporated the phases of the moon into their experiments, as reported in the Jan 16 SCIENCE. A majority of the slugs turned east in the earth's magnetic field when there was a full moon. No such turning preference was seen during new-moon periods.

This is not the first finding of such amoon-magnetic field phenomenon in animals. Responses to magnetic fields by homing pigeons, fruit flies and flatworms also are apparently affected by lunar phase. But the lowly sea slug promises a significant contribution to this area of biology research.

"The real crux of the findings is . . .that they have an organ [that senses magnetic fields]," Willos told SCIENCE NEWS. "In virtually no case has the existence of such an organ been shown." He notes that there have been descriptions of small magnetic particles in bacteria (SN: 12/21 & 28/85, p.396) and of electric organs in sharks that detect changing magnetic fields.

But the accessibility, and relativelylarge size of the sea slug's neurons make this model a prime choice for studies of the neurophysiology of magnetic field detection. Willows says more recent studies have confirmed suspicions that a single neuron is the slug's magnetic organ. Among the evidence for this was the finding that the neuron's electrical activity increases when magnetic fields are altered.

The scientists have not yet studiedslugs from the Atlantic Ocean. But Willows says he suspects they would orient toward the west -- and friendly shallow waters.