SURPRISINGLY, if a fish swimming in the ocean wants to be invisible to
predators, the clever colour to be is not sea blue鈥攊t鈥檚 red. In a
mathematical model of how light travels through a tropical ocean, S枚nke
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鈥檙e blue, any swimming
deeper than 20 metres are less likely to be seen by predators if they鈥檙e red
(Proceedings of the Royal Society B, DOI 10.1098/rspb.2001.1855).
Johnsen says the finding makes sense when you look at marine creatures:
鈥淭here are really blue animals near the surface鈥攖hink of a tuna鈥攖hen
redder animals like shrimp further down.鈥�