Glaucus atlanticus (commonly known as the sea swallow, blue glaucus, blue dragon, blue sea slug and blue ocean slug) is a species of small-sized blue sea slug, a pelagic aeolid nudibranch, a marine gastropod mollusk in the family Glaucidae. This is the only species in the genus Glaucus, but is closely related to Glaucilla marginata, which sometimes is included in Glaucus.
At maturity Glaucus atlanticus can grow to be up to 3cm in length.  It is silvery grey on its dorsal side and dark and pale blue ventrally. It has dark blue stripes on its face. It has a tapering body which is flattened and has six appendages which branch out into rayed cerata.
Distribution and habitat 
This nudibranch is pelagic, and is distributed throughout the world's oceans, in temperate and tropical waters. Regions where this slug is found include the East and South Coast of South Africa, European waters, the east coast of Australia and Mozambique. This species floats upside down on the surface tension of the ocean.
Life history and behavior 
G. atlanticus preys on other, larger pelagic organisms: 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 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 venom is collected in specialized sacs (cnidosacs), on the tip of their cerata, the thin feather-like "fingers" on its body. Because Glaucus concentrates the venom, it can produce a more powerful and deadly sting than the Man o' War upon which it feeds.
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, the sea swallow floats upside down. The upper surface, actually the foot (the underside in other snails), has either a blue or blue-white coloration. The true dorsal surface (downwards in G. atlanticus) is completely silver-grey. This coloration is an example of counter shading, which helps protect it from predators from below, sides, 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. After mating, both animals produce egg strings.
- 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.
- WoRMS. "Glaucus". World Register of Marine Species. Retrieved 5 August 2012.
- "Glaucus atlanticus (blue sea slug)". The Natural History Museum. Retrieved 2013-04-13.
- Piper, R. (2007). Extraordinary Animals: An Encyclopedia of Curious and Unusual Animals. Greenwood Publishing Group. pp. 42–43. ISBN 978-0-313-33922-6.
- 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.
- Glaucus atlanticus Forster, 1777. World Register of Marine Species, Retrieved 3 April 2010.
- Rudman, W. B. (6 November 1998). "Glaucus atlanticus Forster, 1777". Sea Slug Forum. Retrieved 26 February 2011.
- Debelius, H.; Kuiter, R. H. (2007). Nudibranchs of the world. IKAN-Unterwasserarchiv. ISBN 978-3-939767-06-0.
Further reading 
- Valdés, Ágel; Orso Angulo Campillo (November 2004). "Systematics of Pelagic Aeolid Nudibranchs Of The Family Glaucidae (Mollusca, Gastropoda)". Bulletin of Marine Science 75 (3): 381–389. Retrieved 2008-03-04.
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