Glaucus Atlanticus, Fantasy Slug

Glaucus atlanticus
Forster, 1777

Order:NUDIBRANCHIA
Suborder:AEOLIDINA
Family:Glaucidae

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.^[5] This species floats upside down on the surface tension of the ocean.

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.

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.^[7] After mating, both animals produce egg strings.

Dorsal surface, which faces downward as animal floats upside down on the surface of the ocean (Pleuston). This is the smaller of two similar pleustonic species which are blown ashore during periods of strong onshore winds.

Feeding on the "By-the-wind sailor" (Velella velella). Also feeds on the Portuguese man-o'-war, commonly
known as the Bluebottle (Physalia physalis) and, below, on Porpita porpita.

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Mating sequence shown below

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.^[6] Because Glaucus stores the venom, it can produce a more powerful and deadly sting than the Man o' War upon which it feeds.

man o war, a very poisonous jellyfish