If you heard about a tiny, funny-looking animal that spends its life floating upside-down on the surface of the Pacific, Atlantic, or Indian Ocean thanks to an air bubble which it swallows and keeps inside its belly, going wherever the currents and the wind take it, you would probably think it was just a harmless creature that likes to relax in the water. But this slender, up-to-3-centimeter-long animal, which is called the blue glaucus, blue sea slug, or blue ocean slug, is not nearly as innocent as it seems.
The first trick it’s got up its sleeve is a form of camouflage called countershading that protects it from both flying and swimming predators while it floats. The underside of the blue glaucus, which faces upward, is blue, helping it blend into the water’s surface when seen from above, while its back, which faces downward, is a more grayish color, helping it blend into the ocean when seen from below. The second tricky feature of the blue glaucus is even more amazing. It feeds on hydrozoans (a group of animals in the same phylum as jellyfish), especially the highly poisonous Portuguese Man-O’-War. Although a sting by a Portuguese Man-O’-War is very painful to a human, the blue glaucus, like some other sea slugs, can swallow its prey’s stinging cells (known as nematocysts) without hurting itself. It may keep itself safe from the poison by releasing protective mucus and by hard barrier-like discs inside its skin. But the blue glaucus does more than simply protect itself against these stings. It stores the swallowed poison inside the up to 84 finger-like structures or cerata sticking out of its body, and then uses this poison to defend itself against other predators!
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.
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.
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 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.
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.
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 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.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.