A Flamboyant Nudist with Gross Nerves

Why call this animal a nudist? And, if you’re naked, why be so colourful? The animal pictured is a nudibranch, one of over 3,000 species.
Nudibranchs are related to snails and are often called sea slugs because they crawl on a foot, like snails and land slugs. They have a snail-like shell as larvae when they live in the plankton, but when they settle to the bottom or benthos to live, they lose their shell and the cavity inside it, called the mantle cavity. Hence, they are ‘nudus’ meaning naked and their gills or ‘branchi’ are exposed. This particular species, the golden or orange-frosted nudibranch, Dirona pellucia, has been found from Oregon north and west to Russia and the Sea of Japan.
Actually, instead of true gills, finger-like projections called cerata (keratos = horn) are a feature of many species. The cerata of this golden nudibranch are leaf-like with white lines that get broader to the tips. Without the protection of the shell, these flamboyant slugs challenge predators, like crabs, to remember them, either by their colour or their taste. Some sponge-eating nudibranchs sequester toxic or distasteful chemicals from the sponge in their cerata to discourage predators. Other nudibranchs feed on sea anemones and their relatives, which have nematocysts that, when discharged, can be toxic enough to kill people.
In 1929, the Danish physiologist August Krogh wrote that "for such a large number of problems there will be some animal of choice, or a few such animals, on which it can be most conveniently studied." This has become known as Krogh’s Principle, and sea slugs illustrate the point perfectly. The nerves in the brains of the sea slug Aplysia are the largest nerve cells in the animal kingdom, 50 times larger than most mammalian neurons; they truly are gross! These gross neurons have made sea slugs model organisms for scientists studying how nerves work. Neurophysiologists can easily see these neurons and can recognize different ones by their shape and position. Among their many interesting discoveries, they have shown that these nerves are sensitive to the Earth’s magnetic field, and this enables the sea slug to find its way around on the bottom of the ocean. Perhaps our nerves are also sensitive to the Earth’s and other magnetic fields too!

Denis Lynn is a Professor Emeritus in Integrative Biology, University of Guelph, and an Adjunct Professor in Zoology, University of British Columbia. He taught various courses in biology during his 32-year career and still researches his favourite wee beasties, the ciliated protozoa.

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