FaunaHub

Nudibranch (order Nudibranchia)

Marine invertebrateMolluscSea slug

A brightly coloured Chromodoris nudibranch (sea slug) on the reef.

Nudibranch (Chromodoris willani), Philippines.

Image: Diego Delso, CC BY-SA 4.0, via Wikimedia Commons.

Overview

Nudibranchs (order Nudibranchia) are soft-bodied marine molluscs — a kind of shell-less sea slug — famous for their extraordinary colours and shapes. Despite being gastropods (relatives of snails), adult nudibranchs have no shell, and many are jewel-like in appearance. The reference shown here is a Chromodoris nudibranch.

Their dazzling colours are usually a warning: many nudibranchs are distasteful or defended, and some perform a remarkable trick — eating stinging or toxic animals and recycling those defences for their own protection.

Note: nudibranchs are hugely diverse (thousands of species), so habits and colours vary enormously. Treat general statements as approximate and verify against authoritative sources.

Habitat & Range

Nudibranchs are found in seas all over the world, from warm coral reefs to cold and even polar waters, and from the intertidal zone to the deep sea. Most live on the seabed, crawling over reefs, rocks, sand, and the very organisms they feed on.

Diet

Nudibranchs are carnivores, and many are specialists: different species feed on sponges, hydroids, anemones, corals, bryozoans, or even other sea slugs. Remarkably, some that eat stinging cnidarians can store the prey's undischarged stinging cells in their own bodies for defence, while others retain toxins or even photosynthetic algae from their food.

Behavior

Nudibranchs glide slowly on a muscular foot, sensing their surroundings with a pair of head tentacles (rhinophores) that detect chemicals in the water. Many breathe through feathery gills or other projections on the back. They are hermaphrodites — each individual has both male and female organs — and a pair will exchange sperm when they mate.

Human Interaction & Conservation

Nudibranchs are among the most photographed animals in the sea and are a favourite of divers for their colour and variety. Scientists also study their chemistry for natural compounds. They are sensitive to reef and ocean health, so the broad concerns relate to habitat condition. Consult authoritative sources for specifics.

More photos of the nudibranch

A striped Chromodoris nudibranch crawling over the seabed.

Nudibranch (Chromodoris strigata).

Image: Diego Delso, CC BY-SA 4.0, via Wikimedia Commons.

Frequently Asked Questions — Nudibranch

Is a nudibranch a slug or a snail?
Both, in a sense. Nudibranchs are gastropod molluscs — the same class as snails and land slugs — but adults have lost the shell, so they are a type of "sea slug." The name nudibranch means "naked gills," referring to the exposed breathing structures many of them carry on the back.
Why are nudibranchs so brightly coloured?
Their vivid colours are usually a warning (aposematic) signal that the animal is distasteful or defended. Bright, conspicuous patterns tell predators to keep away, which works best when the nudibranch really does carry toxins or stings.
Can nudibranchs really use other animals' stings?
Some can. Certain nudibranchs eat stinging cnidarians (like hydroids or anemones) and store the prey's undischarged stinging cells in projections on their own backs, effectively borrowing those defences. Others retain toxins from their food, or even keep working algae from what they eat.
What do nudibranchs eat?
They are carnivores, and many are highly specialised — feeding on sponges, hydroids, anemones, corals, bryozoans, or other sea slugs depending on the species. This specialised diet is closely tied to their defences and sometimes their colours.

Sources and further reading

Authoritative wildlife references used for general educational context. Conservation status should always be verified against current IUCN Red List data. External links open in a new tab.