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)
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