Glaucus atlanticus (common names include the sea swallow, blue angel, blue glaucus, 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 on the surface tension of the water, where they are carried along by the winds and ocean currents. Glaucus atlanticus is camouflaged: 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 silvery surface of the sea.
Glaucus atlanticus feeds on other pelagic creatures, including the venomous cnidarian, the Portuguese Man o' War. This sea slug stores stinging nematocysts from the cnidarian within its own tissues, which is additional protection from predation attempts, but a human picking up one of these sea slugs that has accidentally washed up on the beach 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 which live in the Indian Ocean and Pacific Ocean.
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 tapering body which is flattened, and has six appendages which branch out into rayed, finger-like cerata.
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 counter shading, 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.
Glaucus atlanticus was recently found in the Humboldt Current ecosystem in Peru in 2013, and in 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 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, over 677 kilometers apart.
Although these sea slugs live on the open ocean, they sometimes accidentally wash up onto the shore, and therefore they may be found on beaches.
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 will attack and eat other individuals in captivity.
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. Because Glaucus concentrates the venom, it can produce a more powerful and deadly sting than the Man o' War upon 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.
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. This protects the sea slug from predation.
People sometimes pick up these unusual blue sea slugs after they wash up on beaches. When humans are stung by Glaucus atlanticus, the venom stored in the nematocysts is injected under the skin. In stings from the Portuguese Man o' War, this venom has been shown to cause fever, shock, and problems with the heart and lungs. In very rare cases this venom has even led to death.
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