What's a Nudibranch? (And Why Divers Get Obsessed)

Rick Harkins   Sep 24, 2026

scuba diving courses nudibranch ecology

You're drifting along a reef and you spot something about the size of your thumb. It's bright orange, with frilly tufts on its back, crawling across a rock.

Congratulations. You just met a nudibranch.

So what is it?

A nudibranch (say NEW-dih-brank) is a sea slug. It's a mollusk, in the same big family as snails and clams, but adults don't have a shell.

The name means "naked gills." That's because many of them wear their gills right out in the open, as feathery tufts on their backs, according to National Geographic.

How big are they, and how many are there?

Sizes run from about a quarter inch to around a foot long. There are thousands of known species, and new ones are still being found.

Where do you find them?

In every ocean. They're most common in shallow, warm water, so a reef trip is where most divers first get hooked.

Why are they so colorful?

Some use bright colors as a warning to predators. Others blend in with their surroundings. The Living Planet Aquarium notes that many get their color from what they eat.

Five things that make them weird

• They smell with their heads. Those two horns are called rhinophores. They pick up chemical signals to find food and mates.
• Each one is both male and female. They still need a partner to reproduce. They can't fertilize themselves.
• Some steal weapons. Certain nudibranchs eat stinging animals, then store the stinging cells in their own bodies for defense. Biology Dictionary has the details.
• They're picky eaters. They're carnivores that graze on things like sponges, anemones, corals, and barnacles. Some eat only one type of prey.
• They're slow. They crawl along the bottom. That's good news for anyone holding a camera.

Want to go deeper?

If you're curious about all of this, our SSI Nudibranch Ecology specialty is for you. It runs seven sessions and covers classification, habitat, how they move and sense the world, reproduction, feeding, defense, and conservation.

It's a great fit for marine biology fans, students, and anyone who's just curious. Check the course page for the next date, and contact us if you have questions about prerequisites.

One more tip. Good buoyancy skills help you get close to a nudibranch without touching the reef. Nice and easy does it.

Questions? Call us at 833-PA-DIVER or stop by the shop.