Glaucus atlanticus (common names include the sea swallow, blue angel, blue glaucus, dragon slug, blue dragon, blue sea slug and blue ocean slug) is a species of small, blue sea slug, a pelagic aeolid nudibranch, a shell-less gastropod mollusk in the family Glaucidae.
These sea slugs are pelagic: they float upside down by using the surface tension of the water to stay up, where they are carried along by the winds and ocean currents. Glaucus atlanticus makes use of countershading: the blue side of their body faces upwards, blending in with the blue of the water. The silver/grey side of the sea slugs faces downwards, blending in with the sunlight reflecting on the ocean's surface when viewed facing upwards underwater.
Glaucus atlanticus feed 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, Glaucus marginatus, which is now understood to be not one species, but a cryptic species complex of four separate species that live in the Indian Ocean and Pacific Ocean. It shares the common name "Blue Dragon" with Pteraeolidia ianthina.
At maturity Glaucus atlanticus can be up to 3 centimetres (1.2 in) in length. 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.
Cerata, also known as papillae, extend laterally from three different pairs of peduncles. The papillae are placed in a single row (uniseriate) and may be up to 84 inches total, (Forster, 1777).
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 snail), 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. The blue coloration is also thought to reflect harmful UV sunlight.
Distribution and habitat
This nudibranch is pelagic, and there is some evidence 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. The G. atlanticus species geographical range increased northward by 150 km in the Gulf of California.
Glaucus 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 live in warm temperate climates in the Southern Pacific, and in circumtropical and Lusitanian environments. Before finding Glaucus 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 kilometers apart. Glaucus atlanticus was also recently found off Bermuda in January 2016.
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 to make slow swimming movements.  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.
G. atlanticus is able to feed on the Portuguese man o' war 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 against future prey. The nematocysts are collected in specialized sacs (cnidosacs) at the tip of the animal's 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 on which it feeds.
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. Studies have indicated that the 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.
The Glaucus 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. Picking up the animal can result in a painful sting, with symptoms similar to those caused by the Portuguese man o' war. The symptoms that may appear after being stung are nausea, pain, vomiting, acute allergic contact dermatitis, erythema, urticarial papules, potential vesicle formation and post-inflammatory hyperpigmentation.
- "Glaucus". WoRMS. World Register of Marine Species. Retrieved 5 August 2012.
- 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 January 2010.
- Churchill, Celia K. C.; Valdés, Ángel; ó Foighil, Diarmaid (2014). "Churchill, C. K. C.; Valdés, Á; Ó Foighil, D. (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.
- "Glaucus atlanticus (blue sea slug)". The Natural History Museum. Archived from the original on 27 June 2015. Retrieved 13 April 2013.
- Piper, R. (2007). Extraordinary Animals: An Encyclopedia of Curious and Unusual Animals. Greenwood Publishing Group. pp. 42–43. ISBN 978-0-313-33922-6.
- Holland, Brenden (March 2012). "First record of the blue sea slug (Glaucus atlanticus) from Andhra Pradesh – India". Taprobanica: The Journal of Asian Biodiversity. 4 (1): 52. doi:10.4038/tapro.v4i1.4386. S2CID 130162921.
- 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.
- "Habitat - Glaucus Atlanticus". Bluedragonslug.weebly.com. Retrieved 14 March 2018.
- Hernández, Luis, et al. "Occurrence of Glaucus Atlanticus in the Midriff Islands Region, Gulf of California, Mexico". American Malacological Bulletin, vol. 36, no. 1, 2018, pp. 145–149.
- 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". pp. 43–60. Retrieved 24 October 2014.
- Johnston-Barnes, Owain (25 January 2016). "Diver finds 'blue dragons' at Spittal Pond". The Royal Gazette.
- Taprobanica. Taprobanica Private Limited. April 2012. pp. 52–53. Retrieved 24 October 2014.
- 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.
- MacLellan, Amelia "Glaucus atlanticus (blue sea slug)". The Natural History Museum. Retrieved 2013-04-13
- Asmelash, Leah. "Rare blue dragons are washing up on the Padre Island National Seashore". CNN. Retrieved 11 May 2020.
- 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.
- 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. doi:10.1007/s00227-014-2389-7. ISSN 1432-1793.
- Ottuso, Patrick Thomas (May 2009). "Aquatic antagonists: Indirect nematocyst envenomation and acute allergic contact dermatitis due to nudibranchs" (PDF). Cutis. 83.
- Pinotti, Raphael M.; Bom, Fabio C.; Muxagata, Erik; Pinotti, Raphael M.; Bom, Fabio C.; Muxagata, Erik (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. ISSN 0001-3765. PMID 30994760.
- Churchill, Celia K. C.; Valdés, Ángel; Foighil, Diarmaid Ó. (April 2014). "Afro-Eurasia and the Americas present barriers to gene flow for the cosmopolitan neustonic nudibranch Glaucus atlanticus". Marine Biology (Berlin). 161 (4): 899–910. doi:10.1007/s00227-014-2389-7.
- 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. Retrieved 4 March 2008.
- 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.
|Wikimedia Commons has media related to Glaucus atlanticus.|
|Wikispecies has information related to Glaucidae|
- Sea Slug Forum
- "Glaucus atlanticus" at the Encyclopedia of Life
- "Blue Sea slug: Glaucus Atlanticus, Gold Coast, Australia". YouTube. 17 November 2017.