Scientists have discovered that many shark species, living secretively in the dark depths of the ocean, actually glow to become more visible to each other, and presumably to make it easier to find a mate. The latest documentation of biofluorescence in marine life came in a 2016 study of two species, the chain catshark and the swell shark. Both are small fish -- about three feet (.9 m) long -- that hide in crevices at depths of 1,600 to 2,000 feet (488 to 610 m).
In the case of the swell shark, this biofluorescence is bright green and is produced by fluorescent proteins inside its skin. Humans can’t see it, but other sharks can, thanks to a higher density of light-sensitive cells in their retinas.
Finding biofluorescence in the sea: