Glaucus atlanticus (commonly known as the sea swallow, blue angel, blue glaucus, blue dragon, blue sea slug and blue ocean slug) is a species of small-sized blue sea slug, a pelagic aeolid nudibranch, gastropod mollusk in the family Glaucidae. It is closely related to Glaucus marginatus, which is sometimes included in Glaucus.
These sea slugs feed on other pelagic creatures including the venomous cnidarian, the Portuguese Man o' War. Because the sea slug stores stinging nematocysts from the cnidarian within its own tissues, a human picking up the sea slug may receive a very painful sting.
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.
Studies suggest that the rich dark blue color of Glaucus atlanticus does not only protect it from being spotted by potential predators, but also provides it with protection from ultraviolet light. G. atlanticus floats upside down on the upper surface of the ocean, where it is exposed to an abundance of sunlight. The blue-violet pigments help it to reflect harmful UV rays.
Distribution and habitat
This nudibranch is pelagic, and occurs throughout the world's oceans, in temperate and tropical waters. This slug is found in temperate and tropical waters in regions of 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 water letting itself be carried by the winds and currents. Due to their aptitude to stored air in their gastric cavity they are able to make slower swimming movements. This slower swimming capacity enhances their skill to move towards prey or approach a potential mate. Glaucus atlanticus is camouflaged because their blue and white side faces upwards, so that when birds look down at them, they blend in with the water. Their silver/grey side is down so when fish look up, they blend with the surface of the water
Glaucus atlanticus was recently found in the Humboldt Current ecosystem in Peru in 2013, and in Andhra Pradesh in India in 2012. This matches the habitat characteristics that 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, which is over 677 kilometers apart.
Life history and behavior
G. atlanticus preys on other, larger pelagic organisms by floating; this is partly by means of an air bubble that they have swallowed and stored in their gastric cavity. They are able to move toward prey or mates by using their cerata to make slow swimming movements. They have been know 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, individual Glaucus become cannibals, given the opportunity.
G. atlanticus is rarely seen on the shore due to the fact that they live in between the ocean floor and the ocean surface, but they can be found floating in coastal waters where they are sometimes washed up onto the shore.
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.
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, also known as the sea swallow, floats upside down. The upper surface is actually the foot (the underside in other snails), 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 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.
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 venom has been shown, in stings from the Portuguese Man o' War, to cause fever, shock and cause problems with the heart and lungs. In very rare cases this venom has even lead to death. Humans do not necessarily pose a threat or resemble a meal to the Glaucus atlanticus, but sometimes humans are stung by accident or as a result of people trying to pick up the sea slugs.
- Cnidosacs, the anatomical structures that hold the stinging cells in aeolid nudibranchs
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