A fish that can see more than red

Male Pelvicachromis about to prey upon an amhipod. Image credit: Denis MeuthenDogs can hear sounds we can't, and many other animals can see light beyond our human "visual spectrum," such as ultraviolet light waves to help find prey or avoid predators. But until now, the near infrared spectrum was considered off limits to animals.

New research, published in the October 21 online edition of NaturWissenschaften, and led by Dr. Sebastian Baldauf from the University of Bonn, reports on the first case of a fish with near infrared vision. The fish, called Pelvicachromis taeniatus, is native to Africa and lives in shallow streams. Usually, near infrared light signals are considered unsuitable for perception because of "signal noise" emanating from heat. 

"Physiologists thought that noise levels in the near-infrared range were too high to allow visual perception," Baldauf said.
Snakes can perceive prey using infrared signals, "but they don't perceive near infrared signals, and they don't use their eyes for this purpose, instead they have a heat-sensitive pit organ that perceives long-wave infrared signals". The West African fish, however, definitely used its eyes.

To test this capability, the researchers fed fish one of their favorite prey--a freshwater shrimp called Gammarus pulex that reflects in the near infrared spectrum. In a dark room in which only infrared lamps were installed each fish was offered a choice between two groups of shrimps. One group was presented behind a filter that allowed the near-infrared light to pass through, whereas the other prey was located behind a filter that blacked out all infrared light. The fish spent significantly more time hunting shrimp located in the near-infrared light-emitting area.

One reason why the fish might prefer near-infrared light lies in its habitat. It hunts in shallow, warm streams, where near-infrared light is scattered by plankton, suspended dirt and other non-living material. Thus, light transmission of near-infrared is facilitated by the fish's habitat. In addition, warmer temperatures also can increase near-infrared transmission. These all add up to near-infrared as a near-perfect way for the fish to detect prey in warm, murky water. However, it is not clear yet how the fish can detect light signals from other heat-generated signals, that are also in the near-infrared spectrum. 

"It is a clear selective advantage if you can perceive additional signals that others cannot perceive," Baldauf said. Now, scientists need to find out how this newly discovered use of near-infrared light occurs. How does the fish process near-infrared in the brain? Do animals use it to aid migration, or sexual selection? What genes and proteins control its use? Researchers are now mapping out a whole new system of perception in these animals.

Meuthen, et al. (2012). Visual prey detection by near-infrared cues in a fish. Naturwissenschaften. DOI 10.1007/s00114-012-0980-7.

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