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Science & Nature • Marine Biology

Nudibranch

Nudibranchs are shell-less marine gastropod molluscs in the order Nudibranchia, commonly called sea slugs.

By Ethan Anderson| Reviewed by Olivia Bennett, Standards & Fact-Checking Lead| Updated July 26, 2026|2 min read| Fact-checked

Nudibranchs are shell-less marine gastropod molluscs in the order Nudibranchia, commonly called sea slugs. They are known for their bright colours, unusual body shapes, and diverse defensive adaptations, and more than 3,000 species have been described.

Description

Nudibranchs are soft-bodied, shell-less marine gastropods with external breathing structures or body appendages. Their appearance varies widely, and many species have vivid colours and ornate shapes.

Classification

They belong to the mollusc class Gastropoda and the order Nudibranchia. Modern references commonly treat them as a distinct order of marine opisthobranch molluscs.

Habitat and distribution

Nudibranchs live in marine environments around the world, from shallow tropical reefs and temperate waters to depths of several hundred metres or more.

Feeding and defence

Many nudibranchs feed on other invertebrates, especially cnidarians, sponges, and other small marine animals. Some species can sequester toxins or stinging cells from prey and use them for protection.

Diversity

The group includes more than 3,000 described species, with especially high diversity in warm, shallow marine habitats. Nudibranchs are often divided informally into dorid and aeolid types.

Key facts

  • Nudibranchs are marine gastropods that lose their shells after the larval stage.
  • They are commonly called sea slugs.
  • Many species are brightly coloured and have elaborate external appendages.
  • Some species can store stinging cells from prey for defence.
  • More than 3,000 nudibranch species are known.
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Canadian context

Nudibranchs occur in Canadian marine waters, including the Pacific and Atlantic coasts, and are of interest to divers, naturalists, and marine biologists studying local sea slug diversity.

Frequently asked questions

What is a nudibranch?
A nudibranch is a shell-less marine gastropod mollusc, commonly called a sea slug.
Why are nudibranchs called nudibranchs?
The name means “naked gill,” referring to exposed breathing structures on the body.
How many nudibranch species are there?
More than 3,000 species have been described.
Where do nudibranchs live?
They live in marine habitats worldwide, from shallow coastal waters to deep seafloors.
What do nudibranchs eat?
They feed on other invertebrates, including sea anemones, sponges, coral-associated prey, and sometimes other nudibranchs.
How do nudibranchs defend themselves?
Some species use bright warning colours, toxins from prey, or stinging cells taken from cnidarians.

References

  1. Wikipediahttps://en.wikipedia.org/wiki/Nudibranch
    Supports: Shell loss after larval stage, marine gastropod classification, colour, diversity, and global depth range.
  2. Encyclopaedia Britannicahttps://www.britannica.com/animal/nudibranch
    Supports: Definition, lack of shell and true gills, cerata, feeding on invertebrates, and marine distribution.
  3. Dictionary.comhttps://www.dictionary.com/browse/nudibranch
    Supports: Common name 'sea slug' and definition of nudibranch.
  4. Merriam-Websterhttps://www.merriam-webster.com/dictionary/nudibranch
    Supports: Definition of nudibranch as an order of marine mollusks without a shell in the adult state.
  5. GBIFhttps://www.gbif.org/species/144093729
    Supports: Shell shedding after larval stage, colourful forms, and dorid/aeolid grouping.
  6. The Wildlife Trustshttps://www.wildlifetrusts.org/wildlife-explorer/marine/sea-snails-and-sea-slugs/what-nudibranch
    Supports: UK species diversity, shell-less marine mollusc description, defence using prey stinging cells, and hermaphroditism.
  7. South Carolina Department of Natural Resourceshttps://www.dnr.sc.gov/marine/sertc/Nudibranchs.pdf
    Supports: Morphology, rhinophores, cerata, respiration, defence, carnivory, and worldwide seafloor habitat.
  8. Natural History Museumhttps://www.nhm.ac.uk/discover/nudibranchs-psychedelic-thieves-of-the-sea.html
    Supports: Meaning of the name as 'naked gill' and shell-less marine mollusc description.