BMC Ecol. 8, 16 (2008)

Credit: N. K. MICHIELS ET AL.

Water preferentially absorbs red light, so at sea it's not possible to make out red objects below a depth of around 10 metres. It was therefore thought that red was irrelevant in signalling between coral-reef fishes.

Not so. Scuba diving with filters on their masks to make red easier to pick out, Nico Michiels of the University of Tübingen in Germany and his colleagues have found at least 32 species of reef fishes that look red at depth.

These fish have crystals or pigments in their skin that fluoresce red under the incoming, mainly blue-green, light (as in the goby Eviota destai, pictured). The researchers say that the red markings, which are often mixed with other colours so that they appear pink, lilac or reddish brown, are most probably used for communication within each species. They add weight to this idea by showing that at least one fish, the goby E. pellucida, can see red.