< All molluscs

blue dragon / blue sea slug

Scientific name Glaucus atlanticus
Descriptor Forster
Year of description 1777
IUCN category (World) NE
Family Glaucidae
Genus Glaucus
Glaucus atlanticus Glaucus atlanticus
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Introduction

Glaucus atlanticus, commonly known as the blue dragon, is a pelagic nudibranch noted for its striking appearance and unique characteristics. This species is distinguished by its potentially dangerous venom, which comes from the cnidocytes it accumulates while feeding on hydrozoans like Physalia physalis. It is predominantly found in the Atlantic and Indian Oceans, often seen floating at the water's surface, and occasionally stranded on beaches, particularly after meteorological disturbances.

Who is it?

Morphology

  • Type
    nudibranch
  • Size
    3 - 4 cm
  • Colours
    vivid
  • Chromatophores
    No
  • Motif
    horizontal stripes
  • Type
    nudibranch
  • Size
    3 - 4 cm
  • Colours
    vivid
  • Chromatophores
    No
  • Motif
    horizontal stripes

How to recognize the blue dragon / blue sea slug ?

The blue dragon has an elongated body that can reach up to 4 cm in length. Its coloration is striking: the underside is striped with horizontal blue and silver-gray lines, with a lighter central band, while the back is gray. The head, located at the front, features a dark blue mouth and two oral appendages. Conical, gray-silver rhinophores are situated dorsally. The body is surrounded laterally by cerata arranged in three pairs.

Behaviour & Life cycle

  • Sociability
    solitary
  • Way of living
    diurnal
  • Venomous
    No
  • Diet
    predator

Glaucus atlanticus is a pelagic species that floats at the ocean's surface, moving in a dorsal orientation. Its buoyancy is maintained by ingesting air, which forms a bubble in its stomach. This nudibranch is relatively passive, drifting in search of food. It primarily preys on hydrozoans, using its jaws and radular teeth to grasp and tear the soft tissues and tentacles of its prey.

Reproduction

  • Reproduction
    ovipare qui pond en eau libre
Glaucus atlanticus is a simultaneous hermaphrodite, possessing both male and female reproductive organs. During mating, two individuals meet face-to-face, intertwining their penises to exchange sperm. The mating process lasts about an hour, after which eggs are laid in strings of 10 to 30. The eggs hatch in approximately three days, with the veliger larvae developing into juvenile blue dragons that float to the water's surface.

Risks for humans

  • Venomous
    No

The blue dragon possesses stinging cells called cnidocytes contained in cnidosacs at the tips of its cerata. The nematocysts are ingested when feeding on prey, primarily hydrozoans such as Physalia physalis.

These cells make it extremely venomous. Its sting is painful and can cause anaphylactic shock, fainting, or infections.

As a pelagic species floating at the water's surface, the blue dragon may occasionally end up on a beach, carried by winds and tides.

Where to find it?

Conservation status of populations (IUCN)

World : NE

Geographic distribution & Conservation

Glaucus atlanticus is found primarily in the tropical and subtropical waters of the Atlantic and Indian Oceans. Its distribution is influenced by ocean currents and seasonal environmental changes. It is commonly observed along the southwestern Atlantic coast, from Brazil to Uruguay, and recent strandings have been reported in the Canary Islands in 2024. The species' occurrence is affected by meteorological events and seasonal shifts in oceanographic conditions.

What is its habitat?

Natural environment characteristics

  • Depth
    0 - 1 m

Biotope presentation

The blue dragon inhabits the surface waters of the oceans, often found in open pelagic environments above reefs and areas where currents converge. This pelagic species is associated with tropical and subtropical regions, where it drifts at the water's surface. Its biotope is closely related to ocean currents and meteorological conditions, which can cause passive displacement and occasional beach strandings.

Species of the same biotope

Fishkeeping

Not recommended

We do not recommend keeping this species in an aquarium. It has unpredictable needs which, if not met, generate significant stress, potentially leading to a shorter life expectancy, an interruption of its growth or the development of pathogens.

To go further

To read on the web

EOL

Sources & Contributions

Participation & Validation

The Fishipedia team and specialist contributors are committed to providing high-quality content. However, although the information comes from scientific sources or testimonials from specialists, the cards may contain inaccuracies.

Adrien Falzon

Adrien Falzon

Benoit Chartrer

Benoit Chartrer

Translation

Translation done with the valuable contribution of our translators, who make this information available to a wider audience. We sincerely thank them for their commitment.

Bibliographic references

On the occurrence and ecology of Glaucus atlanticus Forster, 1777 (Mollusca: Nudibranchia) along the Southwestern Atlantic coast - RAPHAEL M. PINOTTI - FABIO C. BOM - ERIK MUXAGATA - Anais da Academia Brasileira de Ciências - 2019.

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Scientific partners

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Tags

#Glaucidae

#Glaucus

#plein eau

#nudibranch

#Gulf of Mexico

#Mediterranean Sea

#Temperate Eastern Atlantic Ocean

#Eastern tropical Atlantic Ocean

#western Indian Ocean

#Tropical Eastern Pacific

#Western Tropical Pacific Ocean

Species of the same biotope

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