Glaucus atlanticus and its close relative, Glaucilla marginata, live in close association with what Sir Alistair Hardy described many years ago as "The Blue Fleet" - the siphonophores such as Physalia, Velella, Porpita and the other associated animals including the "Violet snails" of the genus Janthina. All these animals float on the surface of the ocean being carried by the currents and the winds. Most of us are only aware of their existence when days of onshore winds blow great fleets of them on to the beaches, causing pain and angst for swimmers.
The normal size of this species is between 5 and 8 cm in length. 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.
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 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.
The Violet sea snail inhabits the ocean surface by secreting a raft of mucus bubbles to keep it afloat. It spends its whole life drifting on the ocean surface in warm seas at the mercy of the winds. It is blind and has a paper-thin shell. It feeds on by-the-wind sailors and other colonial hydroids that drift through the oceans. This gastropod molluscs grows up to 4 cm.. It manages to float by producing its own mucus-covered bubble raft, and feeding on floating cnidarians such as Blue Bottles (Physalia physalis) and By-the-wind Sailors (Velella velella).
Originally posted by Solasis
What a beautiful creature! So delightfully incomprehensible, and what coloration! It calls to mind some of the creatures from the writings of HP Lovecraft -- though significantly less prone to causing madness, if this thread is any indication