Most bioluminescent shark

The world’s most bioluminescent shark is the cookie-cutter shark (Isistius brasiliensis), inhabiting the central regions of the Atlantic and Pacific. Up to 1.5 m (5 ft) long, its upper surface is a dull brown, but its entire undersurface is often covered with light-producing organs called photophores, which emit a very bright, ghostly green glow. The function of this shark’s vivid bioluminescence is currently unclear – it may serve to attract other sharks of its own species, or to attract prey, or even as camouflage if viewed from below. So brightly do the photophores on this species' undersurface glow that it is possible to read a book using only their intense green light as illumination, which persists for up to three hours after the shark is taken out of water.