There are tiny shark with glowing bellies

Funny dwarf shark (Squaliolus aliae) live in the coastal waters of the south-eastern and western Pacific Ocean. The size of such akulki reaches only 22 cm in length. Terrible predator of this size could become easy prey for larger fish, so the nature came up with the crumb disguise: glowing abdomens. Against the background of weak light near the surface of the water silhouettes of sharks invisible, if they include a "highlight". Photophores abdominal camouflage from predators sharks inhabiting deeper.

via factroom.ru