This hatchetfish Sternoptyx has many photophores beneath its body, here stimulated to fluoresce by a UV light. (Scroll down to see the fluorescence). Although this is not true bioluminescence, light is emitted from these same photophores. The glow is directed downward through light-guides, presumably to function as effective counterillumination seen only from below. (Length approx. 3 cm)

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