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You are what you eat. You may have heard this idea before. Food gives you the energy you need to move and grow. Well, little sea creatures called nudibranchs take this to a whole new level. These colorful, squishy blobs are masters of their meals. The food they eat holds the secrets to their survival.

Before we get to all the food facts, here are some basics. Nudibranchs are a type of sea slug. They belong to the group of animals known as mollusks, along with clams, oysters, and snails. But unlike those well-armored creatures, these sea slugs have no shell for protection. They are just blobs of muscle, skin, and organs. Most are about the size of your finger, though some can grow as long as a computer keyboard.

Animals that live in the ocean usually breathe with gills that are tucked neatly inside their bodies. Not nudibranchs. Most have gills that stick out of their backs in colorful, feathery tufts. When in danger, they can quickly pull the important gills inside their bodies.

Nudibranchs crawl through the world's oceans, from frigid seas to warm tropical waters. Some species even live near the boiling vents of underwater volcanoes.

Nudibranchs are rarely in a hurry. To inch along over coral and rocks, they use a flat muscle on their underside, called a sticky foot. The foot ripples from back to front, slowly pulling the slug forward. True to its name, the foot leaves behind a trail of sticky slime.

Some nudibranchs are able to leave the ocean floor and swim short distances. One is the Spanish dancer. It moves by tightening and releasing its muscles. As it swims, it looks like the swishing red skirt of a flamenco dancer.

Nudibranchs are as colorful as a candy store. One type sports pink polka dots and flashy red gills. Another is royal blue with yellow and black stripes that line its body. Some nudibranchs are round and chubby. Others are long and sleek. If you imagine a shape and color, there is probably a nudibranch to match.

Now that you have the basics, are you ready for the first important food fact? Nudibranchs get their amazing colors from the food they eat. For example, when one eats a sponge, it absorbs the sponge's color pigments. Eating an orange sponge gives a slug tinges of orange. Eating a red sponge can turn a slug red—and so on.

For these shell-less little creatures, matching the colors of their food is about more than just looking good. It's about defense. A hungry sea turtle may have a hard time spotting a red slug eating a red sponge.

Nudibranchs can't see their own rainbow colors. That's because their tiny eyes can only see white, black, and shades of gray. They also lack a nose, ears, and tongue. Instead, they use two tentacles, called rhinophores to smell, feel, and taste things around them.

Nudibranchs eat things like coral, sponges, and anemones. Also on the menu are other sea slugs! That's right—nudibranchs will sometimes eat other nudibranchs. Maybe that's a food fact better left untold.

Without a shell, how do these slow-moving sea slugs stand a chance against predators? It's time for the second food fact: Nudibranchs can turn their food into a weapon.

Stinging sea anemones are a popular nudibranch meal. They eat the anemone—stingers and all. They don't digest the stingers, though. The stingers settle in the nudibranchs' cerata. These are finger-like parts some slugs have on their backs for breathing, digestion, and in this case, defense. Should something try to chew on the slugs, they can use the stingers as weapons. Ouch!

Stingers aren't the only weapons these sea slugs steal from their meals. Some nudibranchs dine on poisonous sponges. The toxins don't hurt nudibranchs—they help. The sea slugs store the poisons to use later. When they are threatened, the nudibranchs release the toxins into the water around them. The cloud of poison helps keep predators away.

If a sea turtle decides to go ahead and snack on the slug, it will get a mouthful of poison. Spanish dancers even put the borrowed toxins in their eggs for protection until they hatch. Nontoxic nudibranchs can also benefit from this poison power. Some harmless species mimic their toxic relatives. Predators can't tell the difference. So they leave both species alone.

Nudibranchs certainly make the most of their meals, but they are fussy eaters. Many eat only one type of coral or sea anemone. That can make it hard for them to find food.

Unfortunately, the slow slugs can't just dash off to dine someplace else. They sometimes have to wait a long time between meals. This brings up the last, and possibly coolest, food fact. Some of these slugs farm food inside their bodies. You could even say these special sea slugs are solar-powered!

The blue dragon is a sun-powered slug. It eats coral filled with tiny plants called algae. After eating the coral, the nudibranch keeps the algae in its cerata. There, the algae get to work. Algae, like other plants, use sunlight to make food through photosynthesis. Inside the slug's cerata, the algae keep using sunlight to make food. That food then fuels the nudibranch.

To keep their algae farms going, blue dragons must soak up a lot of sun. Hanging out in shallow water during the day helps them get the sun they need. After eating algae, the sea slug can go for days without another meal. It lives off the food being farmed inside its body!

Nudibranchs have short lives. Some live only a few weeks. Others live up to a year. Since they don't leave a shell behind, they don't give scientists much to study. Still, scientists have found over 3,000 kinds of nudibranchs. They are finding more almost daily. With each new discovery, scientists uncover more facts.

Nudibranchs' bodies are simple to study. Some scientists use them as models for learning and memory studies. Figuring out how nudibranchs learn and remember could help scientists better understand how the human brain works. The chemicals slugs use may help scientists find new ways of healing human organs. Who knows what other secrets are still locked inside their squishy bodies? Not bad for a little blob at the bottom of the ocean.

Article by Rebecca Paley. Top-of-page photograph © Borut Furlan/Peter Arnold Inc. "Sea Slugs" appears on page 2 of the Nov.-Dec. 2009 issue.