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Unusual eyes help fish see in the murk

The elephantnose fish (Gnathonemus petersii).
[Image courtesy of Gerhard von der Emde]

Elephantnose fish are long-snouted, freshwater fish that live in dim, murky environments. Unlike other animals that are adapted to the dark, these fish do have eyes and rely partly on their vision to find their way around.

The fish are able to do this because their eyes are built in an unusual way, which researchers have just figured out. The findings appear in the 29 June 2012 issue of the journal Science.

Vertebrate eyes have two kinds of light-sensing cells: rods, which are very sensitive to light but don't detect color or fine details, and cones, which are less sensitive to light but can distinguish color and fine detail.

Most vertebrate eyes primarily use either rods or cones. But, Moritz Kreysing at the University of Cambridge in the United Kingdom and an international team of colleagues now show that the retina of the elephantnose fish is organized in a way that delivers light in just the right wavelength to both types of receptors, allowing them to work simultaneously.

With both rods and cones working, the fish can distinguish between different colors in dim light, which may help the fish spot predators.