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Science & Nature • Marine Biology

Blue Dragon Sea Slug

The blue dragon sea slug is a small pelagic nudibranch, scientifically known as Glaucus atlanticus.

By Ethan Anderson| Reviewed by Olivia Bennett, Standards & Fact-Checking Lead| Updated July 27, 2026|2 min read| Fact-checked

The blue dragon sea slug is a small pelagic nudibranch, scientifically known as Glaucus atlanticus. It floats upside down on the ocean surface and is known for its blue-and-silver coloration and ability to store stinging cells from prey for defence. It occurs in temperate and tropical waters of the Atlantic, Pacific, and Indian Oceans.

Taxonomy and naming

Blue dragon sea slug is the common name for Glaucus atlanticus, a nudibranch in the family Glaucidae. It is also known by several other common names, including blue glaucus, sea swallow, and blue angel.

Description

This species is small, typically only a few centimetres long, and has a striking blue, silver, and white appearance. It floats on its back at the ocean surface, with body structures that help it stay buoyant.

Feeding and defence

Blue dragons eat cnidarians such as Portuguese man o' war and can retain their prey's stinging cells. These stored nematocysts can be used as a defensive sting against predators or handlers.

Distribution and habitat

The species occurs in temperate and tropical waters across the Atlantic, Pacific, and Indian Oceans. It is pelagic, meaning it lives in the open ocean rather than on the seafloor.

Key facts

  • Blue dragon sea slugs are shell-less marine gastropods in the nudibranch group.
  • They live at the surface of the open ocean and often float upside down.
  • They feed on venomous cnidarians, especially Portuguese man o' war.
  • They can store stolen stinging cells in their own body for defence.
  • Its blue colouring helps it blend with the ocean surface when viewed from above.
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Canadian context

The blue dragon sea slug is of interest to Canadian marine readers because it is part of globally distributed ocean biodiversity and is sometimes reported in marine science and natural history coverage relevant to Atlantic and Pacific waters.

Frequently asked questions

What is a blue dragon sea slug?
It is a small marine nudibranch, Glaucus atlanticus, that lives at the ocean surface and is known for its vivid blue colouring.
Where does it live?
It is found in temperate and tropical waters of the Atlantic, Pacific, and Indian Oceans.
What does it eat?
It feeds mainly on cnidarians, especially Portuguese man o' war.
Is it dangerous?
It can sting because it stores stinging cells from its prey in its body, which it can use defensively.
How big is it?
It usually grows to about 2.5–3 cm in length.

References

  1. Bureau of Ocean Energy Management — https://www.boem.gov/newsroom/ocean-science-news/blue-dragon-sea-slug
    Supports: Common name and broad distribution in temperate and tropical Atlantic, Pacific, and Indian Oceans
  2. Science and the Sea — https://www.scienceandthesea.org/program/blue-dragon
    Supports: Feeding on Portuguese man o' war and use of stolen stinging cells for defence
  3. Smithsonian Magazine — https://www.smithsonianmag.com/science-nature/glaucus-atlanticus-science-picture-of-the-week-65306082/
    Supports: Species identity, small size, and prey relationship with Portuguese man o' war
  4. PBS Nature — https://www.pbs.org/wnet/nature/blog/inside-nature-featured-creature-blue-dragon/
    Supports: Nudibranch classification, size, surface-drifting habit, and distribution
  5. Oceana — https://www.oceana.org/marine-life/blue-glaucus/
    Supports: Common names, floating upside down at the surface, size, habitat, and feeding on venomous prey