The Glaucus atlanticus is a mollusc that belongs to the ocean-dwelling group called nudibranchs. They have muscular feet and rhinophores, or the tentacles. The Blue Sea Slug has adapted physically to suit its needs. One great adaptation is the sac located in its stomach filled with gas, helping it stay afloat as drifting along the ocean waves is its only real mode of transportation.
The Sea Swallow also feeds on animals with deadly nematocysts (stinging cells) and some of these are dangerous suckers. These dangerous prey are called Blue Bottles or, sometimes, Portuguese Man O' Wars. The Glaucus atlanticus is immune to nematocysts, thus giving it the ability to consume its prey, as well as store its dinner's venom for its own defense! The venom is collected by the tips of the slug's 'fingers', called cerata, and stored in specialized sacs. This is why the Blue Sea Slug is feared more than the Portuguese Man O' War and can leave you with a much deadlier sting, depending on the amount of venom it has stored.
One of the most distinctive characteristics of this animal is the way it lies in the water. Glaucus atlanticus floats upside-down! Its dorsal surface (backside) is actually its 'foot' and belly and is either blue or blueish-white in color. The true dorsal surface of the Sea Swallow is a silvery-greyish color. Both coloration on each side help camouflage and protect it from both underwater and aerial predators. The ocean's surface from above looks blue and white because of the waves, but underwater, darker colors like silver and grey make it harder for prey to be spotted.
When out of water, they tend to roll into little balls and open up again once they are placed back into water. They also slowly turn themselves over in water if their silver-grey surface is facing skyward.
As in the case of most sea slugs, Glaucus atlanticus too are hermaphodite, allowing both slugs to produce egg strings after mating. Glaucus atlanticus can grow up to 40-50mm and are found all over the world in temperate to tropical waters. Glaucuses lay their eggs on driftwood or even the skeletons of their victims or on objects or animals that remain afloat to allow their young to develop until they have their own air sacs.
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