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SURPRISINGLY, if a fish swimming in the ocean wants to be invisible to
predators, the clever colour to be is not sea blue—it’s red. In a
mathematical model of how light travels through a tropical ocean, Sönke
Johnsen of Duke University in North Carolina has shown that red light scatters
most evenly in the ocean at depths greater than 20 metres. So although fish
nearest the surface are hard to spot from above if they’re blue, any swimming
deeper than 20 metres are less likely to be seen by predators if they’re red
(Proceedings of the Royal Society…