Glaucus atlanticus (common names sea swallow, blue glaucus, blue sea slug and blue ocean slug) is a species of small-sized blue sea slug, a pelagic aeolid nudibranch, a marine gastropod mollusk in the family Glaucidae. This is the only species in the genus Glaucus,but is closely related to Glaucilla marginata, which sometimes is included in Glaucus The normal size of this species is up to 3 cm. It is silvery grey on its dorsal side and dark and pale blue ventrally. It has dark blue stripes along the edge of its foot. It has a tapering body which is flattened and has six appendages which branch out into rayed cerata. Its radular teeth bear serrated teeth on their blades.
Distribution and habitat This nudibranch is pelagic, and is distributed throughout the world’s oceans, in temperate and tropical waters. Regions where this slug is found include the East and South Coast of South Africa, European waters, the east coast of Australia and Mozambique. This species floats upside down on thesurface tension of the ocean. Life history and behavior G. atlanticus preys on other, larger pelagic organisms: the dangerously venomous Portuguese Man o’ War Physalia physalis; the by-the-wind-sailorVelella velella; the blue button Porpita porpita; and the violet snail, Janthina janthina. Occasionally, individual Glaucus become cannibals given the opportunity. G. atlanticus is able to feed on P. 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 venom is collected in specialized sacs (cnidosacs), on the tip of their cerata, the thin feather-like “fingers” on its body. Because Glaucus stores 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, Glaucus atlanticus floats at the surface. Due to the location of the gas sac the sea swallow floats upside down. The dorsal surface, actually the foot and underside, has either a blue or blue-white coloration. The true dorsal surface is completely silver-grey. This coloration is an example of counter shading, which helps protect it from predators from below, sides and above. Scientists have often argued over whether Glaucus atlanticus moves on its own or depends on wind for locomotion Glaucus is, like most sea slugs, a hermaphrodite, containing 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.