Glaucus atlanticus

Glaucus atlanticus (common names include the blue sea dragon, sea swallow, blue angel, blue glaucus, dragon slug, blue dragon, blue sea slug, and blue ocean slug) is a species of sea slug in the family Glaucidae.[2]

Glaucus atlanticus
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Mollusca
Class: Gastropoda
Subclass: Heterobranchia
Order: Nudibranchia
Suborder: Cladobranchia
Family: Glaucidae
Genus: Glaucus
G. atlanticus
Binomial name
Glaucus atlanticus
Forster, 1777
  • Doris radiata Gmelin, 1791 (synonym)
  • Glaucus distichoicus d'Orbigny, 1837
  • Glaucus flagellum Blumenblach, 1803 (synonym)
  • Glaucus hexapterigius Cuvier, 1805 (synonym)
  • Glaucus lineatus Reinhardt & Bergh, 1864
  • Glaucus longicirrhus Reinhardt & Bergh, 1864

These sea slugs live in the pelagic zone (open ocean), where they float upside-down by using the surface tension of the water to stay afloat. They are carried along by the winds and ocean currents. G. atlanticus makes use of countershading; the blue side of their bodies faces upwards, blending in with the blue of the water. The silver/grey side of the sea slug faces downwards, blending in with the sunlight reflecting on the ocean's surface when viewed from below the surface of the water.

G. atlanticus feeds on other pelagic creatures, including the Portuguese man o' war and other venomous siphonophores. This sea slug stores stinging nematocysts from the siphonophores within its own tissues as defence against predators. Humans handling the slug may receive a very painful and potentially dangerous sting.



This species looks similar to, and is closely related to, G. marginatus, which is now understood to be not one species, but a cryptic species complex of four separate species that live in the Indian and Pacific Oceans.[1][3] It shares the common name "blue dragon" with Pteraeolidia ianthina[4] and G. marginatus.[5]



At maturity, G. atlanticus is usually around 3 cm (1.2 in) in length,[6] though larger specimens have been found.[7] It can live up to a year under the right conditions.[8] It is silvery grey on its dorsal side and dark and pale blue ventrally. It has dark blue stripes on its head. It has a flat, tapering body and six appendages that branch out into rayed, finger-like cerata.[9]

Cerata, also known as papillae, extend laterally from three different pairs of peduncles. The papillae are placed in a single row (uniseriate) and may number up to 84 in total.[10]

G. atlanticus is usually found in tropical/subtropical areas, floating at the ocean's surface due to the stored gulped air inside its stomach. It usually feeds on cnidarians, which can be noisy due to air escaping its stomach as it feeds.[11][12][13]

The radula of this species bears serrated teeth,[14] which paired with a strong jaw and denticles, allows it to grasp and "chip down" parts of its prey.[8]

Buoyancy and coloration


With the aid of a gas-filled sac in its stomach, G. atlanticus floats at the surface. Due to the location of the gas sac, this species floats upside down. The upper surface is actually the foot (the underside in other slugs and snails), and this has either a blue or blue-white coloration. The true dorsal surface (carried downwards in G. atlanticus) is completely silver-grey. This coloration is an example of countershading, which helps protect it from predators that might attack from below and from above.[15] The blue coloration is also thought to reflect harmful ultraviolet sunlight.

Distribution and habitat

Glaucus atlanticus is the blue sea slug shown here out of water on a beach, and thus collapsed; however, touching the animal directly with your skin can result in a painful sting, with symptoms similar to those caused by the Portuguese man o' war
The slug in the water

This nudibranch is pelagic, and some evidence indicates that it occurs throughout the world's oceans, in temperate and tropical waters. It has been recorded from the east and south coasts of South Africa, European waters, the east coast of Australia, and Mozambique.[3] Observations in 2015 and 2016 suggested that the G. atlanticus species' geographical range had increased northward by 150 km in the Gulf of California compared with previous sightings.[16]

Since the middle of the 19th century, records of this species have been reported on the Azores.[7]

G. atlanticus was recently found in the Humboldt Current ecosystem in Peru in 2013, and off Andhra Pradesh in India in 2012. This is in line with the known habitat characteristics of the species; they thrive in warm, temperate climates in the Southern Pacific, and in circumtropical and Lusitanian environments. Before finding G. atlanticus off Andhra Pradesh, these nudibranchs were documented as having been seen in the Bay of Bengal and off the coast of Tamil Nadu, India, over 677 km apart.[17] G. atlanticus was also recently found off Bermuda in January 2016,[18] and uncommonly washes ashore on east coast beaches at Barbados, Lesser Antilles.

Although these sea slugs live on the open ocean, they sometimes accidentally wash up onto the shore, so they may be found on beaches.[10] In April 2022, specimens were found in the Gulf of Mexico along the Texas coast.[19] On August 31, 2023, blue sea slugs were reported to be found along Karon Beach, Phuket, Thailand.[20][21]

Life history and behavior


G. atlanticus preys on other larger pelagic organisms. The sea slugs can move toward prey or mates by using their cerata, the thin feather-like "fingers" on its body, to make slow swimming movements.[10][22] They are known to prey on the dangerously venomous Portuguese man o' war (Physalia physalis), the by-the-wind-sailor (Velella velella), the blue button (Porpita porpita), and the violet snail, Janthina janthina. Occasionally, individuals attack and eat other individuals in captivity.

The species is able to feed on the Portuguese man o' war due to its immunity to the venomous nematocysts. The slug consumes chunks of the organism and appears to select and store the most venomous nematocysts for its own use against future prey.[23] The nematocysts are collected in specialized sacs (cnidosacs) at the tip of the animal's cerata. Because G. atlanticus concentrates the venom, it can produce a more powerful and deadly sting than the man o' war on which it feeds.[24]

Like almost all heterobranchs, blue dragons are hermaphrodites and their male reproductive organs have evolved to be especially large and hooked to avoid their partner's venomous cerata.[8] Unlike most nudibranchs, which mate with their right sides facing, sea swallows mate with ventral sides facing.[25] After mating, both individuals are able to lay eggs and can release up to 20 on an egg string, often laying them in wood pieces or carcasses.[8] On average, G. atlanticus can lay 55 egg strings per hour.[12] G. atlanticus is not globally panmictic, but is localized within ocean basins. Gene flow among Afro-Eurasian and American populations is thus hindered by physical obstructions and water temperatures in the Arctic and Southern Oceans.[11]



G. atlanticus is able to swallow the venomous nematocysts from siphonophores, such as the Portuguese man o' war, and store them in the extremities of its finger-like cerata.[24] Picking up the animal can result in a painful sting, with symptoms similar to those caused by the Portuguese man o' war.[26] The symptoms that may appear after being stung are nausea, pain, vomiting, acute allergic contact dermatitis, erythema, urticarial papules, potential vesicle formation and postinflammatory hyperpigmentation.[13]


  1. ^ a b "Glaucus". WoRMS. World Register of Marine Species. Retrieved 5 August 2012.
  2. ^ Lalli, C. M.; Gilmer, R. W. (1989). Pelagic snails: the biology of holoplanktonic gastropod mollusks. Stanford University Press. p. 224. ISBN 978-0-8047-1490-7.
  3. ^ a b Churchill, Celia K. C.; Valdés, Ángel; ó Foighil, Diarmaid (2014). "Molecular and morphological systematics of neustonic nudibranchs (Mollusca: Gastropoda: Glaucidae: Glaucus), with descriptions of three new cryptic species". Invertebrate Systematics. 28 (2): 174. doi:10.1071/IS13038. S2CID 84010907.
  4. ^ Rudman, W. B. (15 July 2010). "Pteraeolidia ianthina (Angas,1864)". The Sea Slug Forum. Australian Museum. Retrieved 13 February 2021.
  5. ^ Salleh, Anna (12 February 2021). "Bizarre 'blue fleet' blows onto Australia's east coast". ABC News. Australian Broadcasting Corporation. Retrieved 13 February 2021.
  6. ^ "Glaucus atlanticus (blue sea slug)". The Natural History Museum. Archived from the original on 27 June 2015. Retrieved 13 April 2013.
  7. ^ a b "Dragão Azul do Mar". RTP. Archived from the original on 17 November 2021. Retrieved 14 November 2020.
  8. ^ a b c d "Blue Dragons of the Sea". Smithsonian Museum of Natural History. Retrieved 14 November 2020.
  9. ^ Piper, R. (2007). Extraordinary Animals: An Encyclopedia of Curious and Unusual Animals. Greenwood Publishing Group. pp. 42–43. ISBN 978-0-313-33922-6.
  10. ^ a b c Srinivasulu, Bhargavi; Srinivasulu, C.; Kumar, G. Chethan (2012). "First record of the blue sea slug (Glaucus atlanticus) from Andhra Pradesh–India". Taprobanica: The Journal of Asian Biodiversity. 4 (1): 52–53. doi:10.4038/tapro.v4i1.4386.
  11. ^ a b Churchill, Celia K. C.; Valdés, Ángel; Ó Foighil, Diarmaid (1 April 2014). "Afro-Eurasia and the Americas present barriers to gene flow for the cosmopolitan neustonic nudibranch Glaucus atlanticus". Marine Biology. 161 (4): 899–910. Bibcode:2014MarBi.161..899C. doi:10.1007/s00227-014-2389-7. S2CID 84153330.
  12. ^ a b Helm, Rebecca R. (14 December 2021). "Natural history of neustonic animals in the Sargasso Sea: reproduction, predation, and behavior of Glaucus atlanticus, Velella velella, and Janthina spp". Marine Biodiversity. 51 (6): 99. Bibcode:2021MarBd..51...99H. doi:10.1007/s12526-021-01233-5. S2CID 245186096.
  13. ^ a b Pinotti, Raphael M.; Bom, Fabio C.; Muxagata, Erik (8 April 2019). "On the occurrence and ecology of Glaucus atlanticus Forster, 1777 (Mollusca: Nudibranchia) along the Southwestern Atlantic coast". Anais da Academia Brasileira de Ciências. 91 (1): e20180154. doi:10.1590/0001-3765201920180154. PMID 30994760. S2CID 119501907.
  14. ^ Thompson, T. E.; McFarlane, I. D. (2008). "Observations on a collection of Glaucus from the Gulf of Aden with a critical review of published records of Glaucidae (Gastropoda, Opisthobranchia)". Proceedings of the Linnean Society of London. 178 (2): 107–123. doi:10.1111/j.1095-8312.1967.tb00967.x.
  15. ^ "Habitat – Glaucus Atlanticus". Retrieved 14 March 2018.
  16. ^ Hernández, Luis; Munguía-Vega, Adrián; Pérez-Alarcón, Fernanda; Fernández-Rivera-Melo, Francisco J.; Angulo-Campillo, Orso (2018). "Occurrence of Glaucus atlanticus in the Midriff Islands Region, Gulf of California, Mexico". American Malacological Bulletin. 36: 145–149. doi:10.4003/006.036.0113. S2CID 89794645.
  17. ^ Uribe, Roberto A.; Nakamura, Katia; Indacochea, Aldo; Pacheco, Aldo S.; Hooker, Yuri; Schrödl, Michael (September 2013). "A review on the diversity and distribution of opisthobranch gastropods from Peru, with the addition of three new records". Spixiana. 36 (341–8391): 43–60. Retrieved 24 October 2014.
  18. ^ Johnston-Barnes, Owain (25 January 2016). "Diver finds 'blue dragons' at Spittal Pond". The Royal Gazette.
  19. ^ Patton, Mary Claire (11 April 2022). "Don't Touch: Rare blue dragons are showing up on Texas beaches again". KSAT 12.
  20. ^ "Venomous 'blue dragon' found on Phuket beach". Bangkok Post. Retrieved 1 September 2023.
  21. ^ Petpailin, Petch (31 August 2023). "Beach warning: Venomous blue dragon sea slug spotted in Phuket". Thaiger. Retrieved 1 September 2023.
  22. ^ MacLellan, Amelia "Glaucus atlanticus (blue sea slug)". The Natural History Museum. Retrieved 2013-04-13
  23. ^ Asmelash, Leah (9 May 2020). "Rare blue dragons are washing up on the Padre Island National Seashore". CNN. Retrieved 11 May 2020.
  24. ^ a b Rudman, W. B. (6 November 1998). "Glaucus atlanticus Forster, 1777". Sea Slug Forum. Retrieved 26 February 2011.
  25. ^ Debelius, H.; Kuiter, R. H. (2007). Nudibranchs of the world. IKAN-Unterwasserarchiv. ISBN 978-3-939767-06-0.
  26. ^ Ottuso, Patrick Thomas (May 2009). "Aquatic antagonists: Indirect nematocyst envenomation and acute allergic contact dermatitis due to nudibranchs" (PDF). Cutis. Vol. 83. Archived from the original (PDF) on 23 July 2021. Retrieved 27 April 2015.

Further reading

  • Valdés, Ángel; Orso Angulo Campillo (November 2004). "Systematics of Pelagic Aeolid Nudibranchs of the Family Glaucidae (Mollusca, Gastropoda)". Bulletin of Marine Science. 75 (3): 381–389.
  • MILLER, M. C. (January 1974). "Aeolid nudibranchs (Gastropoda: Opisthobranchia) of the family Glaucidae from New Zealand waters". Zoological Journal of the Linnean Society. 54 (1): 31–61. doi:10.1111/j.1096-3642.1974.tb00792.x.