Glaucus atlanticus (common names 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.
The normal size of this species is up to about 3 cm, depending on the animal's age. 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. Its radula bears serrated teeth on their blades.
Glaucus atlanticus and its close relative, Glaucilla marginata, live in close association with what Sir Alistair Hardy described many years ago as "The Blue Fleet" - the siphonophores such as Physalia, Velella, Porpita and the other associated animals including the "Violet snails" of the genus Janthina. All these animals float on the surface of the ocean being carried by the currents and the winds. Most of us are only aware of their existence when days of onshore winds blow great fleets of them on to the beaches, causing pain and angst for swimmers.
Both species spend their life floating upside down in the water, partially bouyed by a gas bubble in their stomachs.
The two nudibranchs feed almost exclusively on Physalia, and as Tom Thompson and Isobel Bennett reported some years ago, it appears that they are able to select the most venomous of Physalia's stinging nematocysts for their own use. Like most aeolids, they store the nematocysts in special sacs (cnidosacs) at the tip of their cerata .
There are a number of reports in Australia of kids engaged in "Bluebottle" fights - where they throw stranded Physalia at each other - being badly stung by inadvertently playing with Glaucus and Glaucilla, both of which, by concentrating the most venomous of Physalia's nematocysts, are much more deadly.
Another interesting feature of the two species is their colouration. They both exhibit a textbook example of colour countershading. Their foot and undersides of the cerata, (which because they float upside down is effectively their dorsal surface), is blue or blue and white which helps to camouflage them from predation (sea birds) from above. Their true dorsal surface, which faces down in the water, is silvery grey to effectively camouflage them from fish looking up from below.
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 P. 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.
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- WoRMS. "Glaucus". World Register of Marine Species. http://www.marinespecies.org/aphia.php?p=taxdetails&id=138033. Retrieved 5 August 2012.
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- Glaucus atlanticus Forster, 1777. World Register of Marine Species, Retrieved 3 April 2010.
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- 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. http://www.ingentaconnect.com/content/umrsmas/bullmar/2004/00000075/00000003/art00003. Retrieved 2008-03-04.