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.
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