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Species:

Habitat: coral beds in the , hoping Earth’s magnetic field doesn’t switch polarities any time soon

With no warning you’re swept away by a strong current. Seconds earlier you were crawling over the sea floor minding your own business, but now you’re spinning head over tail in the turbulent flow. When you finally come to rest you’re in unfamiliar territory, and there’s no one else in sight.

So you are feeling a bit like Dorothy from The Wizard of Oz, and maybe mildly disappointed that you didn’t when you landed.

You need to find your way back home, to food and potential mates. But you’ve lost all sense of direction, and the seabed looks pretty much the same every way you look.

All in all, it would really help if you had a compass. If you are the shocking pink sea slug known as the , you’re in luck. Your entire body is a compass.

Pink slug

There are lots of animals that get called “sea slugs”, from several disparate groups. The rosy tritonia belongs to the biggest, the .

It spends its days in search of food and mates. Mostly mates, it must be said: . After a brief courtship and a few minutes circling each other, they copulate for over an hour.

When they do want a snack, they normally go for stationary animals called that are related to corals. Each pen has a root embedded in the seabed and a long feathery body extending up into the water. A tritonia will sidle up to the sea pen and bite off one of its “leaves”, at which point the sea pen quickly retracts into the mud.

Animal magnetism

The rosy tritonia is one of the growing list of animals that are known to sense Earth’s magnetic field to help them navigate. It was in 1987.

The discovery raised two questions: how does it do it, and why? of the University of Washington in Friday Harbor has spent the past two decades trying to get answers.

Many animals use localised sensors to detect magnetic fields – some birds seem to use special chemicals in their eyes, for instance – but Willows says the rosy tritonia has sensors throughout its body.

Early experiments identified in the rosy tritonia’s that , or in some cases , when the direction of the magnetic field was changed. But those neurons aren’t the sensors: cutting the nerves running into the brain .

Willows and colleagues have now tried recording from peripheral nerves in animals that have had their brains removed. Nerves from all over the body responded when he rotated the magnetic field. The response was stronger on the right side: 43 axons responded on the right but only 25 on the left.

Because so many nerves responded, Willows thinks the rosy tritonia must have sensors distributed throughout its body. But he doesn’t know what sort of sensors: it might be a chemical like the one birds use, or small bits of magnetic metal embedded in cells.

That way!

Sensing the Earth’s magnetic field could help rosy tritonias find their way home if they get swept away by a current.

If one gets lost it tends to head for shore. “This is not a bad strategy,” Willows says. “You cannot go too far, because you will run up against an impassable shoreline, and you are more likely to bumble onto your food and mates there than if you wander in any other direction.”

The nearest shoreline will be aligned at a particular angle to the local magnetic field, so by orienting itself with reference to the magnetic field the tritonia can head towards it. When Willows moved tritonias to distant parts of the seabed, where the field was different, they often .

Given the trouble Dorothy had with , it’s a pity she didn’t have a built-in compass of her own.

Journal reference:

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