The Strange Beauty of the Glaucus Atlanticus Sea Slug

Adriana Morales
Adriana Morales
Scribol Staff
Environment, January 24, 2011
  • It wasn’t Finding Nemo that turned us on to this! Honest!

    Seriously though, the Glaucus atlanticus, also known as the Blue Sea Slug, Sea Swallow and Blue Dragon Sea Slug, is an animal that raises many questions. Questions like:

    1. “Does it really float on its back?”
    2. “How can you tell whether it’s upside down?”
    3. “What does it do?”
    4. “Is it poisonous?”
    5. “Where does it come from?”

    Let’s start with the most important question of all:

    “What is the Glaucus atlanticus?”

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  • The Glaucus atlanticus is an animal that belongs to the ocean-dwelling group known as nudibranchs – sometimes thought of as an ‘unusual’
    groups of molluscs. Molluscs have muscular feet and rhinophores, the term for their tentacles. What is noticeably different about the Blue Glaucus and other nudibranchs compared to many molluscs is that they do not have shells (at least, not as adults).

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  • The Blue Sea Slug has adapted physically to suit its needs. One amazing adaptation is the sac located in its stomach which is filled with gas, helping it stay afloat. It is among the Blue Glaucus‘s most important attributes, bearing in mind the fact that drifting among the ocean’s waves is its only real form of transportation. The Sea Swallow also feeds on animals with deadly nematocysts (that is, stinging cells) – and some of these are dangerous suckers, too…

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  • These dangerous prey are called Blue Bottles or, more interestingly, Portuguese Man O’ Wars (Physalia physalis).

    The Glaucus atlanticus is immune to the nematocysts (stinging cells), thus giving it the ability to consume its prey, as well as store its dinner’s venom for its own use! The venom is collected by the tips of the slug’s feather-like ‘fingers’, called cerata, and stored in specialized sacs called cnidosacs. (The Glaucus actually has three pairs of cerata, used for respiration, defense and digestion.) This is why the Blue Sea Slug is feared more than the Portuguese Man O’ War; the Glaucus can have a much deadlier sting, depending on the amount of venom stored within its body.

    If there are not enough Portuguese Man O’ Wars or other larger prey species like the By-the-Wind-Sailor (Velella velella) or Blue Button (Porpita porpita) available, the Blue Sea Slug may take to cannibalism and feed on other Glaucuses.

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  • One of the most distinctive characteristics of this animal is the way it lies in the water. Glaucus atlanticus does indeed float 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 colorations on each side help camouflage and protect the Glaucus atlanticus from both underwater and aerial predators. Think about the color of the ocean’s surface from above. It looks blue and white because of the waves, whereas underwater, darker colors like silver and grey make it harder for prey to be spotted.

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  • Another interesting fact is that the Blue
    Glaucus is a hermaphodite. In actuality, most sea slugs are hermaphodites, allowing both slugs to produce egg strings after mating.

    Glaucus atlanticus can grow to be about 40-50mm and can be found all over the world in temperate to tropical waters. Glaucuses tend to lay their eggs on driftwood or even the skeletons of their victims (yuck!). They lay the eggs on objects or animals that remain afloat to allow their young to develop until they have their own air sacs.

    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 flip themselves over in water if their silver-grey surface is facing skyward.

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  • There is much more to learn about these strange looking creatures, and more and more people are becoming aware that they exist. Hopefully, animals like these may inspire people to take interest in the natural world around them. You never know what you may come across on our planet, so keep your eyes open!

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