Invertebrates

Nudibranch

Nudibranchia · also called nudibranchs, sea slugs

Not Evaluated carnivore Ocean Coasts & shorelines

A nudibranch is a sea snail that threw away its shell and became a walking display of colour instead. It is defenceless on its own — soft, slow, no armour — so it steals its defences: the stinging cells of jellyfish relatives, the poisons of sponges, sometimes even working chloroplasts, all lifted from its prey and repurposed in its own skin.

Overview

Nudibranchs are the order Nudibranchia, over 3,000 species of shell-less marine gastropods in every ocean, most diverse on warm Indo-Pacific reefs. The name means “naked gills” — one group breathes through a feathery rosette on the back, another through finger-like projections called cerata. Every species is a specialist carnivore, and most eat just one or a few kinds of sponge, hydroid, anemone, coral, or other sea slug.

Stolen weapons

Nudibranchs that eat hydroids and jellyfish relatives swallow the prey’s stinging cells without setting them off, move them intact to the tips of the cerata, and fire them at attackers — armed with weapons they did not make. Sponge-eaters concentrate the sponge’s toxins in their skin, and a few keep stolen chloroplasts and feed off sunlight.

Colour, and no eyes to speak of

The bright colours are either warnings of toxicity or exact camouflage against a specific sponge or coral. The animal itself barely sees — a pair of tiny pigment-cup eyes — and navigates the reef almost entirely by smell, using the sensory rhinophores on its head. Every nudibranch is a hermaphrodite, and most live less than a year.

Status

Not Evaluated and globally secure across thousands of species, but because each is tied to a single prey species and narrow habitat, reef degradation, marine heatwaves, and the loss of a particular sponge or hydroid can make a nudibranch vanish locally.

Nudibranch: common questions

What is the difference between a nudibranch and a sea slug?

Sea slug is a loose everyday term for any shell-less or nearly shell-less marine gastropod, and it covers several unrelated groups, including sea hares, headshield slugs, sap-sucking slugs, and sea angels, as well as nudibranchs. A nudibranch is one specific and very large branch within that assortment, the order Nudibranchia, defined by having lost the shell completely as an adult, breathing through exposed gills or through the cerata on the back, and carrying a pair of sensory tentacles called rhinophores on the head. So every nudibranch is a sea slug, but not every sea slug is a nudibranch. The blue dragon, for example, is often called a nudibranch but is technically in a related group.

How do nudibranchs use stinging cells they did not make?

Species that feed on hydroids, siphonophores, and other stinging relatives of jellyfish have evolved a way to eat the prey without setting off its stinging cells, called nematocysts. The undischarged cells pass through the slug's digestive system unharmed, are sorted out, and are carried to storage sacs at the tips of the cerata on the animal's back. There they sit ready to fire, and when a fish or crab bites the nudibranch the stolen nematocysts discharge into the attacker exactly as they would have in the original jellyfish. Researchers still do not fully understand how the slug moves the delicate cells intact or keeps them from firing inside its own body.

Why are nudibranchs so brightly coloured?

For two opposite reasons, depending on the species. Many are toxic, either from chemicals taken up from the sponges they eat or from stolen stinging cells, and their bold contrasting colours are a warning signal to visually hunting fish, which learn to leave that pattern alone. Other nudibranchs are not advertising at all but hiding, and their colours precisely match the particular sponge, hydroid, or coral they live on and eat, making them extremely hard to see against that background. A few combine tactics, being camouflaged at rest and flashing bright colour when disturbed. Because a nudibranch has no shell and moves slowly, chemical defence and colour are most of what it has.

Do nudibranchs have eyes, and how do they sense the world?

They have eyes, but very simple ones, a pair of tiny pigment-cup eyes near the base of the rhinophores that can probably do little more than tell light from dark and detect movement. Nudibranchs rely mainly on chemical senses. The club-shaped or feathery rhinophores on the head are highly sensitive smell and taste organs used to track down the specific prey species the animal eats and to find mates, and a second pair of tentacles near the mouth is used for touch and taste as the slug crawls. In effect a nudibranch navigates a reef almost entirely by smell and touch rather than by sight.

Sources & further reading

Written by HideShark Editorial Team · reviewed by HideShark Science Desk. Published September 10, 2026. Last updated September 10, 2026.

Quick facts

Scientific name: Nudibranchia
Diet: Almost every nudibranch is a specialist carnivore, with different species eating only sponges, hydroids, anemones, corals, sea pens, bryozoans, tunicates, barnacles, fish eggs, or other sea slugs, and many feed on just one or a few prey species, so their distribution tracks their food
Naked gills: The name nudibranch means naked gills, and in one large group the breathing structures form an exposed feathery rosette on the back, while in another group the back is covered in finger-like projections called cerata that handle both respiration and defence
Stolen weapons: Nudibranchs that eat hydroids and jellyfish relatives can swallow the prey's stinging cells without triggering them, move them intact through the gut, and store them at the tips of the cerata, where they fire at anything that attacks the slug, so the animal is armed with weapons it did not make
Borrowed poison and borrowed food: Sponge-eating species concentrate the sponge's toxic chemicals in their skin as a deterrent, and some, like the blue dragon and a few others, also keep living algae or stolen chloroplasts in their tissues and gain food from the sunlight, a trick called kleptoplasty
Colour as a warning, or as camouflage: The spectacular colours of many nudibranchs are aposematic, advertising that the animal is toxic or stinging, while other species are near-perfect colour matches for the specific sponge or coral they live and feed on and are almost invisible until they move
Two sexes in one body: Every nudibranch is a simultaneous hermaphrodite with both male and female organs, and when two meet they line up side by side facing opposite directions and fertilise each other, then each lays a coiled ribbon of eggs, sometimes numbering in the millions
Short, fast lives: Most nudibranchs live less than a year, some only a few weeks, growing quickly, breeding, and dying, which is one reason a dive site can be covered in a species one month and have almost none the next
Range: Every ocean on Earth, from tropical coral reefs to polar seas and from tide pools and kelp forests to hydrothermal vents and the deep sea floor, with by far the greatest diversity in shallow warm water on Indo-Pacific reefs, and a few species that leave the bottom entirely to drift or swim in the open ocean.
Conservation status: Not Evaluated ≈ Not assessed as a group, and with well over 3000 described species and many more undescribed, nudibranchs as a whole are not considered threatened, but because most are tied to a single prey species and a narrow habitat they are highly sensitive to local change, and reef degradation, warming and marine heatwaves, pollution, and the loss of specific sponges or hydroids can make a nudibranch disappear from an area even though the group is globally secure in the wild