If you heard about a tiny, funny-looking animal that spends its life floating upside-down on the surface (1,2,3) of the Pacific, Atlantic, or Indian Ocean(2,3,4,5) thanks to an air bubble which it swallows and keeps inside its belly,(1,2,3,5) going wherever the currents and the wind take it,(3) you would probably think it was just a harmless creature that likes to relax in the water. But this slender,(5) up-to-3-centimeter-long(4,5,6) animal, which is called the blue glaucus,(3) blue sea slug,(1) or blue ocean slug,(2) 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.(2) 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.(2,4) 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.(1,2,3,4) Although a sting by a Portuguese Man-O’-War is very painful to a human,(3) the blue glaucus, like some other sea slugs, can swallow its prey’s stinging cells (known as nematocysts) without hurting itself.(1,2,7,8) It may keep itself safe from the poison by releasing protective mucus and by hard barrier-like discs inside its skin.(7,8) 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(4,6) sticking out of its body, and then uses this poison to defend itself against other predators!(1,2,3,4,6)
The blue glaucus (Glaucus atlanticus), sometimes called the blue sea slug(1) or blue ocean slug,(2) is a bizarre-looking marine creature in the group of sea slugs known as nudibranchs.(2,3) Found in the temperate and tropical waters of the Pacific, Atlantic, and Indian Oceans,(2,3,4,5) this slender,(5) up-to-3-centimeter-long(3,5,6) slug lives its life floating upside-down on the surface of the ocean thanks to an air bubble which it swallows and keeps inside its stomach.(1,2,4,5) Its color pattern, an example of a phenomenon known as countershading,(2) helps it avoid both flying and swimming predators while floating wherever the wind and the currents take it(4): its underside, 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.(2,3) This camouflage, however, is not the blue glaucus’ only form of self-defense. It feeds on animals known as hydrozoans (in the same phylum as jellyfish), especially the highly venomous Portuguese Man-O’-War.(1,2,3,4) Although a sting by a Portuguese Man-O’-War is very painful to a human,(4) the blue glaucus, like some other nudibranchs, can swallow its prey’s stinging cells (known as nematocysts) without hurting itself.(1,2,7,8) It may be able to protect itself from the sting both by secreting mucus and by hard discs in its skin.(7,8) Far from being harmed by the poison, the blue glaucus stores it in the up to 84 finger-like structures or cerata(3,6) sticking out of its body, and uses it to defend itself against predators.(1,2,3,4,6)
- can be up to 3cm in size
- the species has up to 84 cerata - outgrowths on the upper surfaces of the body
- the cerata hold the stinging cells (nematocysts) which are taken from the jellyfish they eat and are stored in special sacs called cnidosacs
- Glaucilla marginata has more cerata than Glaucus atlanticus, up to 137 in total
- the tail (metapodium) of Glaucus atlanticus is much longer than that of Glaucilla marginata
- Glaucus atlanticus is the larger of the two species - Glaucilla marginata only reaches 12mm in length
Distribution and ecology
- the blue glaucus feeds mainly on hydrozoans although they are also known to be cannibalistic
- one of the specialities of their diet is the Portugese Man-O-War, Physalia physalis (Linnaeus, 1758), best known for the very painful stings they can give if you are unlucky enough to get caught in their tentacles
- the glaucus will eat the tentacles, both the fired and unfired stinging cells (nematocysts) and will pass these stings into special pouches in their cerata which they then use for defence
- They are found throughout the tropical Atlantic, Pacific and Indians oceans
- Glaucus live a pelagic life - this means they go where the winds and currents take them
- keeping air in their stomachs helps them to stay afloat on the surface of the oceans
Molecular Biology and Genetics
Statistics of barcoding coverage: Glaucus atlanticus
Public Records: 0
Specimens with Barcodes: 11
Species With Barcodes: 1
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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