Lighting up life, bonus story
Some "lantern sharks" luminesce for the same reasons as anglerfish, to attract food sources, and some use their light to camouflage themselves against aquatic predators. The sky is bright, and a dark outline is easily seem from below. (See also Creaturecast on Squid Iridescence.) -- KMP
Claes JM, Ho H-C, Mallefet J. 2012. Control of luminescence from pygmy shark (Squaliolus aliae) photophores. J Exp Biol 215:1691-1699.