Fish create mobile red light districts


Fish with red fluorescence get brighter, the deeper in the ocean they live.

Many fish species glow red and can see red light, even at depths where the red part of the sunlight spectrum has been absorbed by the water above. It was long assumed that red light did not play a role at depths of 10m or more, but researchers have discovered a much stronger fluorescence at greater depths in six out of eight species.

The tropical Striped triplefin, Helcogramma striata, (pictured at the top of the page) is one of the most strongly fluorescent fish in the tropics. Individuals at a depth of 20m glow more than six times brighter than their relatives living above them at a depth of 5m.
 
They tested the popular hypothesis that fluorescing protects the fish from ultraviolet rays, as it does for corals which grow close to the surface and are exposed to strong sunlight. UV radiation is much greater in shallow water, and if protection from the ultraviolet was important, the researchers would have expected fish living closer to the surface to emit greater red fluorescence — the opposite of what they found.
 
This suggests that the fish illuminate their surroundings with red light to strengthen contrasts and see better. The researchers will now investigate exactly how the fish use the light, in particular whether fluorescence around the eyes could help them navigate and find food in dark water.

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