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Overview

If you heard about a tiny, funny-looking animal that spends its life floating upside-down on the surface (1,2,3) of the Pacific, Atlantic, or Indian Ocean(2,3,4,5) thanks to an air bubble which it swallows and keeps inside its belly,(1,2,3,5) going wherever the currents and the wind take it,(3) you would probably think it was just a harmless creature that likes to relax in the water. But this slender,(5) up-to-3-centimeter-long(4,5,6) animal, which is called the blue glaucus,(3) blue sea slug,(1) or blue ocean slug,(2) is not nearly as innocent as it seems. The first trick it’s got up its sleeve is a form of camouflage called countershading that protects it from both flying and swimming predators while it floats.(2) The underside of the blue glaucus, which faces upward, is blue, helping it blend into the water’s surface when seen from above, while its back, which faces downward, is a more grayish color, helping it blend into the ocean when seen from below.(2,4) The second tricky feature of the blue glaucus is even more amazing. It feeds on hydrozoans (a group of animals in the same phylum as jellyfish), especially the highly poisonous Portuguese Man-O’-War.(1,2,3,4) Although a sting by a Portuguese Man-O’-War is very painful to a human,(3) the blue glaucus, like some other sea slugs, can swallow its prey’s stinging cells (known as nematocysts) without hurting itself.(1,2,7,8) It may keep itself safe from the poison by releasing protective mucus and by hard barrier-like discs inside its skin.(7,8) But the blue glaucus does more than simply protect itself against these stings. It stores the swallowed poison inside the up to 84 finger-like structures or cerata(4,6) sticking out of its body, and then uses this poison to defend itself against other predators!(1,2,3,4,6)

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