Sea Slugs (Nudibranch) - Facts for Kids

Basic Information

Sea slugs are actually called nudibranchs (NEW-dee-branks), which means “naked gills” because their breathing organs are exposed on their backs!

Scientists have discovered over 3,000 different species of nudibranchs living in our oceans today!

Nudibranchs are mostly solitary sea slugs — they don’t have a special group name because they usually live alone!

One of the smallest nudibranch species, Trapania goslineri, is only about 8 millimeters long — a bit slimmer than your finger!

One of the largest nudibranch, the Spanish Dancer (Hexabranchus sanguineus), can grow up to 40 centimeters long — bigger than a ruler!

Most nudibranchs live only a few weeks to about a year in the wild, which means every day of their bright, colorful lives is an adventure!

Baby nudibranchs are called veligers, and they start their life swimming freely in the ocean before settling down on the seafloor!

All nudibranchs are hermaphrodites, meaning each one has both male and female reproductive organs!

Nudibranchs can be found in every ocean on Earth, from the freezing waters of Antarctica to the warm tropical seas!

Most nudibranchs like to glide along the ocean floor using a flat ‘foot,’ but some can swim for short bursts by flexing their bodies when needed.

Physical Characteristics

Many nudibranchs are brightly colored to warn predators that they taste bad or are poisonous!

Nudibranchs breathe through feathery gills on their backs, which look like beautiful tiny branches waving in the water!

Some nudibranchs can steal the stinging cells from sea anemones they eat and use them for their own defense!

Most nudibranchs have two pairs of tentacles up front: one pair (rhinophores) to ‘smell’ the water, and another pair (oral tentacles) by the mouth to feel and taste what’s around them!

Some nudibranchs can kind of ‘wear’ the colours of what they eat — their food gives them pigments, so they may look different if they snack on one coral vs. another!

Nudibranchs don’t have a protective shell like other mollusks — they rely on chemical defenses instead!

In some nudibranchs, parts of their body are so see-through that you can glimpse their guts or other organs — especially in thin, transparent species!

Many nudibranchs have finger-like projections called cerata that help them breathe and digest food!

Many nudibranchs have finger-like projections called cerata that help them breathe and digest food!

Some nudibranchs can dropp parts of their bodies — like their cerata — to escape danger. Then they grow them back over several days or weeks.

Behavior & Intelligence

Nudibranchs use their rhinophores like tiny noses to follow chemical trails to food, mates, or tell when danger is nearby!

When two nudibranchs meet and are ready to mate, they sometimes do a little courtship dance — waving, touching with their tentacles, and circling around each other before mating!

Some nudibranchs can lay millions of eggs at once — in gorgeous spiral or ribbon-shaped egg masses — while others lay just hundreds or thousands, depending on how big they are!

Some nudibranchs are out and about in the daytime, others do their exploring at night — many follow their internal clocks so they move more in one part of the day than the other, depending on the species!

Some nudibranchs can drop one or more of their cerata (those finger-like bits on their backs) when attacked. The dropped cerata wriggle and distract predators — letting the nudibranch swim away!

Nudibranchs communicate with each other using chemical signals that we can’t see or smell!

Some sea slugs steal chloroplasts from algae they eat. The slugs then use these “stolen” chloroplasts to photosynthesize, producing their own food.

Nudibranchs can remember and learn from their experiences, showing they have simple forms of memory!

Habitat & Survival

Nudibranchs are super flexible about where they live — some live in splashy tide pools near land, others down in the dark deep sea more than 2,000 meters below the surface!

Some nudibranchs live their whole life in one spot on a reef — right where their food is. Others, like the ‘drifters,’ float or get carried by currents to new places.

Certain nudibranchs can survive in water temperatures as cold as -1.8°C (28.8°F) in Antarctic seas!

Nudibranchs can squeeze into tiny cracks and crevices to hide from predators or rest!

Some nudibranchs climb up seaweed or eelgrass to lay their egg ribbons there. Being up on plants keeps the eggs safer from things hiding on the sea floor, like crabs or hungry fish.

Certain nudibranchs can survive in extremely deep waters where there’s almost no light!

Nudibranchs make slime (mucus) that helps them glide smoothly over rocks and coral, and in some species this mucus also acts like a shield — it can stop stinging cells (nematocysts) from hurting them or include bitter or poisonous molecules that push predators away.

Nudibranchs can be found living on almost every type of ocean bottom, from sandy floors to rocky walls!

Life Cycle

Nudibranch eggs hatch after a few days to many weeks, depending on the species and how warm the water is. Warmer water = faster hatching; cooler water = slower development.

Baby nudibranchs start life with a tiny shell, which they lose as they grow up!

When nudibranch eggs hatch, the babies are very small — often just a few hundred micrometers (a fraction of a millimeter). At that size, they’re often hard to see without magnification.

Young nudibranchs can completely change their appearance as they grow, sometimes looking nothing like their parents!

Some nudibranchs can lay egg batches really often — even almost every single day — if they have lots of food and everything in their home is just right.

A single nudibranch egg ribbon sometimes holds hundreds of thousands to millions of eggs, depending on the species, how big the ribbon is, and how many little egg capsules it has.

Some nudibranchs can delay their development if conditions aren’t right for growing!

Diet & Feeding

Nudibranchs are carnivores that eat many different things including algae, sponges, anemones, corals, barnacles, and even other nudibranchs!

Many nudibranchs use their radula (a tongue-like organ) covered in tiny teeth to scrape food!

Certain nudibranchs can eat poisonous animals and store their toxins for their own defense!

Some nudibranchs are so specialized they only eat one type of food their entire lives!

Certain nudibranchs can eat fire coral without getting stung!

Nudibranchs can eat creatures many times their own size by slowly nibbling away at them!

Fun & Unique Facts

Some nudibranchs glow in the dark through a process called bioluminescence!