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Flashy, Splashy Surprises

By Kathy Kranking 

Some people think "ugh" when they hear the word "slug." But they don't know sea slugs. Do you?

Sea Slugs - October 2017 RR

Like slow-moving splashes of color, sea slugs glide and slide through oceans around the world. The sea slugs in this story are beauties called nudibranchs (NOO-duh-branks). They’re the flashy cousins of land slugs and are basically sea snails without shells. Their soft, squishy bodies come in every color of the rainbow and are dazzlingly decorated with spots, stripes, bumps, and frills.

As you get to know nudibranchs, you’ll see that their looks aren’t the only surprising thing about them. Read on to be amazed!

Chipmunks - October 2017 RR

It might be hard to believe that there are more than 6,000 species of nudibranchs. Yet with all those colorful creatures out there, many people have never heard of them. Even for people who know about nudibranchs, finding them in the ocean can be tricky. Most are no bigger than your thumb, and some are smaller than a grain of rice. They’re often difficult to see as they move among colorful corals, seaweed, or sponges.

When you do see a nudibranch, the fleshy “fingers” on its back are hard to miss. Some nudibranchs have  just a few of them. Others have them all over their backs. Though they look like they’re only fancy decorations—surprise!—a nudibranch uses them to breathe!

The other things you might notice on a nudibranch are the “horns” on its head (see photo at top right). These act as a pair of “noses,” sensing chemicals in the water given off by other animals. The nudibranch’s eyes, which are very tiny, can sense only light and dark. So the nudibranch depends on the horns to “sniff” its way along.


Some animals have four feet; others have two. But a nudibranch has only one, and that’s all it needs. Like its snail cousins, a nudibranch has a big, flat muscle called a foot on the underside of its body. It ripples the muscle to slide slowly along whatever it’s on. Most nudibranchs spend their lives crawling, but some kinds drift in the water or wave their frilly sides to swim.

Here’s another surprise about nudibranchs: Each one is both a male and a female! So that means that all nudibranchs are able to lay eggs. Nudibranch eggs are pretty surprising, too. A nudibranch lays a delicate, ruffled ribbon of eggs that are stuck together with mucus. The beautiful ribbons come in different colors,  and each can contain up to a million eggs! 

Chipmunks - October 2017 RR

One thing almost all nudibranchs have in common is that they’re picky eaters. Each kind has a favorite food and sticks with it. A nudibranch doesn’t eat by using teeth to chew, as you do. Instead, it has a rough, file-like tongue that it uses to scrape at its food. Nudibranchs are carnivores. They eat small animals such as sponges, anemones, barnacles, and corals.

Different kinds of nudibranchs have different ways of getting their food. Some are sit-and-wait predators. They stay in one place and catch tiny creatures that come drifting by. Others eat food that can’t get away, such as sponges, which grow attached to things. But still others are hunters, creeping up on anemones and even other sea slugs to attack and eat them.

Some kinds of animals, such as fish, crabs, and sea turtles, would happily slurp up a nudibranch. To keep themselves safe, nudibranchs have a few surprising tricks up their slimy sleeves. One thing that some of them do is to take on the coloring of the food they eat. Then they are camouflaged while eating or hanging out on their food. But even more surprising is that some nudibranchs use a secret weapon. They can eat animals that are poisonous or have stingers without being harmed. Then they store the poisons or stingers in their fleshy “fingers” or other parts of their bodies. Their flashy colors are a warning to enemies that they are poisonous. And any enemy that ignores the warning will be sorry.


"Flashy, Splashy Surprises" originally appeared in the October 2017 issue of Ranger Rick magazine.
Click here for a closer view of the photos.

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