Atlantic cranch squid drift through the twilight and midnight zones of the North Atlantic, maintaining neutral buoyancy by filling their spacious body cavities with a light ammonium chloride solution. Their nearly transparent tissues make them almost invisible to predators, while specialized photophores beneath their enormous eyes produce downward-directed bioluminescence to match faint downwelling sunlight. When startled, they can pull their head and arms into the swollen mantle chamber, transforming into a defensive, balloon-like sphere.
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