A Blue View: Nudibranchs

You might not expect a sea slug to be particularly pretty, but nudibranchs—toxic sea slugs—could take the crown in an undersea beauty pageant.

Published April 26, 2016

nudibranch

The brilliantly colored creatures owe it all to their food. Epitomizing the expression “You are what you eat,” nudibranchs derive their vibrant hues from their prey, which ranges from algae, sponges and anemones to barnacles, corals and even other nudibranchs. 

As you probably guessed, sea slugs aren’t the quickest predators, and their tiny eyes provide limited sight. But nudibranchs are hardly helpless. On their heads, two highly sensitive tentacles, called rhinophores, allow them to smell, taste and feel their way around. These appendages receive chemical signals helping them to identify potential prey. 

nudibranch

Once they’ve found their next meal, nudibranchs scrape up their food with a file-like organ on the mouth, called a radula. It’s covered with chitinous teeth that help tear apart their prey. Pretty fierce for a delicate-looking sea slug, right?

nudibranch

Nudibranchs aren’t afraid to be adventurous with their food either, chowing down on the poisonous anemones that most aquatic creatures avoid. The sea slugs sneak up on their poisonous prey; raise themselves above the anemone, carefully dodging tentacles; and then invert their mouths and dive into their dinner.

Once the anemone realizes what’s happening and withdraws its tentacles, it’s usually too late. The digestion process has already begun. What’s more, the sea slug is simultaneously stealing the anemone’s defense mechanism, storing the stinging tentacles underneath its own skin. Predators beware: This nudibranch is now armed and dangerous. 

For more on nudibranchs, check out this week’s A Blue View podcast:

Episode Transcript

What animals do you think exhibit the most kaleidoscopic variety of colors and patterns in the wildest diversity of forms in the animal kingdom? Tropical birds? Rain forest frogs?

Well, move over toucans; and hop aside, poison-dart frogs. Because the prize for the most flamboyant group of animals out there has to go to the 3,000 species that make up the sub-class called nudibranchs.

If you’ve never heard of nudibranchs, worry not. Most people haven’t. With a name derived from two Latin words meaning “naked gills,” nudibranchs are sea slugs, scientifically known as “shell-less marine gastropod mollusks.” And if that sounds about as much like an oxymoron as “jumbo shrimp,” it’s because they really do start out with shells and end up, well, shell-less.

After hatching from eggs as shelled larvae called veligers, nudibranchs lose their shell as they grow, eventually developing sinuous adult forms and colors that can be spectacular. Their common names give away their outrageous costumes. There's the Florida regal goddess, so named for its golden crown. The bright red Spanish dancer looks for all the world like a sinuous flamenco skirt flapping across a dance floor. There’s even the aptly named sea clown nudibranch, replete in a suit of orange polka dots.

Even the Latin names of nudibranchs are not immune to hyperbole. One pop-art-styled Indo-Pacific nudibranch’s electric blue mantle and neon yellow pinstripes led otherwise sensible scientists to name it Chromodoris magnifica—and it lives up to its name, well, magnificently. Now how often do slugs earn such adjectives?

Nudibranchs are so visually appealing that it's hard to believe they were not designed to show off. And in a way, they were. For some, their colors are a warning to predators, saying “Don’t eat me. I taste very bad.” Others are mimics, pretending to be as just unpalatable as their bitter-tasting cousins. As it is in the insect world, most predators simply choose not to find out the hard way.

Some nudibranchs aren’t just bad-tasting; they pack a serious sting, too. What’s truly fascinating is how these adaptive hunters do so. Preying on jellyfish, corals and anemones, they actually ingest the stinging cells of their prey and then co-opt them for their own use. Their colors serve as a warning that a painful sting awaits those who try to nibble at their skin. Very tricky.

And finally, there are nudibranchs that camouflage themselves by turning the same color as their prey, like the sponge-eating nudibranch that turns as red as its prey, the better to blend in.

In all cases, these thrifty drifters undulate through the water looking for food, or glide along the bottom on their muscular foot, called a gastropod. Though they may look delicate, all nudibranchs are highly adapted carnivorous predators, feeding on corals, sponges, worms and even other nudibranchs. Because their eyes are tiny and their sight limited, they can only distinguish between light and dark, so they hunt by smell, detecting their prey using tentacles on the top of their head called rhinophores.

Oblong, flattened, lobed, frondy and feathery, nudibranchs can sport tentacles, bulbs and horns, or be as smooth and drab as a garden slug. They range in size from a fraction of an inch to a foot long, and are found in every ocean, in all temperatures, from the sandy shallows to the deep sea.

All in all, not bad for an animal that’s just skin, muscle and organs sliding across the ocean floor.

To see the most colorful of the nudibranchs and learn more about their unusual ways of hunting, visit aqua.org/ablueview.

This is John Racanelli of the National Aquarium for WYPR, your NPR news station.

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