Blue Dragon

The blue glaucus (Glaucus atlanticus), sometimes called the blue sea slug(1) or blue ocean slug,(2) is a bizarre-looking marine creature in the group of sea slugs known as nudibranchs.(2,3) Found in the temperate and tropical waters of the Pacific, Atlantic, and Indian Oceans,(2,3,4,5) this slender,(5) up-to-3-centimeter-long(3,5,6) slug lives its life floating upside-down on the surface of the ocean thanks to an air bubble which it swallows and keeps inside its stomach.(1,2,4,5) Its color pattern, an example of a phenomenon known as countershading,(2) helps it avoid both flying and swimming predators while floating wherever the wind and the currents take it(4): its underside, 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.(2,3) This camouflage, however, is not the blue glaucus’ only form of self-defense. It feeds on animals known as hydrozoans (in the same phylum as jellyfish), especially the highly venomous Portuguese Man-O’-War.(1,2,3,4) Although a sting by a Portuguese Man-O’-War is very painful to a human,(4) the blue glaucus, like some other nudibranchs, can swallow its prey’s stinging cells (known as nematocysts) without hurting itself.(1,2,7,8) It may be able to protect itself from the sting both by secreting mucus and by hard discs in its skin.(7,8) Far from being harmed by the poison, the blue glaucus stores it in the up to 84 finger-like structures or cerata(3,6) sticking out of its body, and uses it to defend itself against predators.(1,2,3,4,6)

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)