Glaucus atlanticus

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Glaucus atlanticus
Glaucus atlanticus 1 cropped.jpg
Glaucus atlanticus
Scientific classification
Kingdom: Animalia
Phylum: Mollusca
Class: Gastropoda
Superfamily: Aeolidioidea
Family: Glaucidae
Genus: Glaucus
Forster, 1777
Species: G. atlanticus
Binomial name
Glaucus atlanticus
Forster, 1777

Glaucus atlanticus (commonly known as the sea swallow, blue angel, blue glaucus, blue dragon, blue sea slug and blue ocean slug) is a species of small-sized blue sea slug, a pelagic aeolid nudibranch, gastropod mollusk in the family Glaucidae.[1] It is closely related to Glaucus marginatus, which is sometimes included in Glaucus.[2]

These sea slugs feed on other pelagic creatures including the venomous cnidarian, the Portuguese Man o' War. Because the sea slug stores stinging nematocysts from the cnidarian within its own tissues, a human picking up the sea slug may receive a very painful sting.


The blue sea slug (here shown out of water, and thus collapsed) is one of the smallest members of its biological family, Glaucidae

At maturity Glaucus atlanticus can be up to 3 centimetres (1.2 in) in length.[3] 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 tapering body which is flattened, and has six appendages which branch out into rayed, finger-like cerata.[4]

The radula of this species bears serrated teeth.[5]

Studies suggest that the rich dark blue color of Glaucus atlanticus does not only protect it from being spotted by potential predators, but also provides it with protection from ultraviolet light. G. atlanticus floats upside down on the upper surface of the ocean, where it is exposed to an abundance of sunlight. The blue-violet pigments help it to reflect harmful UV rays.[6]

Distribution and habitat[edit]

This nudibranch is pelagic, and occurs throughout the world's oceans, in temperate and tropical waters. This slug is found in temperate and tropical waters in regions of East and South Coast of South Africa, European waters, the east coast of Australia and Mozambique.[6] This species floats upside down on the surface tension of the water letting itself be carried by the winds and currents. Due to their aptitude to stored air in their gastric cavity they are able to make slower swimming movements. This slower swimming capacity enhances their skill to move towards prey or approach a potential mate.[7] Glaucus atlanticus is camouflaged because their blue and white side faces upwards, so that when birds look down at them, they blend in with the water. Their silver/grey side is down so when fish look up, they blend with the surface of the water[8]

Glaucus atlanticus was recently found in the Humboldt Current ecosystem in Peru in 2013, and in Andhra Pradesh in India in 2012. This matches the habitat characteristics that they live in warm temperate climates in the Southern Pacific, and in Circumtropical and Lusitanian environments off the western Atlantic coast. Before finding Glaucus atlanticus in Andhra Pradesh, these nudibranchs were documented as having been seen in the Bay of Bengal and on the coast of Tamil Nadu, India, which is over 677 kilometers apart.[9]

Life history and behavior[edit]

G. atlanticus preys on other, larger pelagic organisms by floating; this is partly by means of an air bubble that they have swallowed and stored in their gastric cavity. They are able to move toward prey or mates by using their cerata to make slow swimming movements.[10] They have been know 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, individual Glaucus become cannibals, given the opportunity.

G. atlanticus is rarely seen on the shore due to the fact that they live in between the ocean floor and the ocean surface, but they can be found floating in coastal waters where they are sometimes washed up onto the shore.[11]

G. atlanticus is able to feed on Physalia physalis due to its immunity to the venomous nematocysts. The slug consumes the entire organism and appears to select and store the most venomous nematocysts for its own use. The nematocysts are collected in specialized sacs (cnidosacs) at the tip of the animal's cerata, the thin feather-like "fingers" on its body.[12] Because Glaucus concentrates the venom, it can produce a more powerful and deadly sting than the Man o' War upon which it feeds.[12]

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, also known as the sea swallow, floats upside down. The upper surface is actually the foot (the underside in other 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 counter shading, which helps protect it from predators that might attack from below and above.

Like almost all heterobranchs, Glaucus is a hermaphrodite, having both male and female reproductive organs. Unlike most nudibranchs, which mate with their right sides facing, sea swallows mate with ventral sides facing.[13] After mating, both animals produce egg strings.

Venomous nematocysts[edit]

Glaucus atlanticus is able to swallow the venomous nematocysts from the Portuguese Man o' War, and store them in the extremities of its finger-like cerata.[14] This venom has been shown, in stings from the Portuguese Man o' War, to cause fever, shock and cause problems with the heart and lungs. In very rare cases this venom has even lead to death.[15] Humans do not necessarily pose a threat or resemble a meal to the Glaucus atlanticus, but sometimes humans are stung by accident or as a result of people trying to pick up the sea slugs.


  1. ^ 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. Retrieved 13 Jan 2010. 
  2. ^ WoRMS. "Glaucus". World Register of Marine Species. Retrieved 5 August 2012. 
  3. ^ "Glaucus atlanticus (blue sea slug)". The Natural History Museum. Retrieved 2013-04-13. 
  4. ^ Piper, R. (2007). Extraordinary Animals: An Encyclopedia of Curious and Unusual Animals. Greenwood Publishing Group. pp. 42–43. ISBN 978-0-313-33922-6. 
  5. ^ 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.  edit
  6. ^ Warneke, Alex, et al. "Even corals heart fluid dynamics
  7. ^ Srinivasulu, Bhargavi, C. Srinivasulu, and G. Chethan Kumar. "First record of the blue sea slug (Glaucus atlanticus) from Andhra Pradesh–India." TAPROBANICA: The Journal of Asian Biodiversity 4.1 (2012): 52-53.
  8. ^
  9. ^ uribe, Roberto; Nakamura, Katia; Indacochea, Aldo; Pacheco, Aldo; 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" (0341-8391). pp. 43–60. Retrieved 24 October 2014. 
  10. ^ MacLellan, Amelia "Glaucus atlanticus (blue sea slug)". The Natural History Museum. Retrieved 2013-04-13
  11. ^ TAPROBANICA. Taprobanica Private Limited. April 2012. pp. 52–53. Retrieved 24 October 2014. 
  12. ^ a b Rudman, W. B. (6 November 1998). "Glaucus atlanticus Forster, 1777". Sea Slug Forum. Retrieved 26 February 2011. 
  13. ^ Debelius, H.; Kuiter, R. H. (2007). Nudibranchs of the world. IKAN-Unterwasserarchiv. ISBN 978-3-939767-06-0. 
  14. ^ Rudman, W. B. (6 November 1998). "Glaucus atlanticus Forster, 1777". Sea Slug Forum. [1] Retrieved 26 February 2011.
  15. ^ Stein, Mark R.; Marraccini, John V.; Rothschild, Neal E.; Burnett, Joseph W. (March 1989). "Fatal Portuguese man-o'-war (Physalia physalis) envenomation". Ann Emerg Med 18 (3): 312–315. doi:10.1016/S0196-0644(89)80421-4. PMID 2564268

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