Fish in the murky exploit infrared view

The cloudier the water, the better the infrared ability of fish

Mozambique Cichlid (Tilapia) © Destinationkho / CC-by-sa 3.0
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Only recently have researchers found that some fish species can see infrared. Now a comparative experiment shows that the higher the habitat of a species, the better the species are adapted to the infrared view. This proves that the infrared view is actually the result of an evolutionary adaptation - and could occur in more fish than previously thought.

The cloudier the water, the worse the visibility. For parts of the light visible to us are scattered and absorbed by the microparticles that cloud the water. The long-wave red and infrared light, on the other hand, is not so strongly scattered, so its relative proportion increases significantly in the murky water. The knowledge that fish are even able to see this light in the near infrared region is still very new in biology. Only last year, researchers from the University of Hohenheim discovered that mosaic cichlids react to infrared light. In the test, the fish swam towards the infrared light source.

Fish compared to clear and murky water

Denis Shcherbakov from the University of Hohenheim and his colleagues have now carried out a systematic comparative study to investigate the ecological significance of infrared vision. They chose five species of fish from different habitats - three species that prefer clear water and two from murky waters. "As clear-water fish, we chose two popular ornamental fish, guppy and swordtail, and the zebrafish, which is very popular as a research object, " explains Alexandra Knörzer. The Nile Cichlid and the Mozambique Cichlid, on the other hand, are two economically important edible fish from very murky waters.

For the experiments, the biologists each placed a juvenile in a small round water container and illuminated it with infrared light of different wavelengths. "A single light source affects fish like the light at the end of a tunnel, " explains the researcher. "When they notice it, they move right towards it."

Long-wave view especially in the murky

It showed that the fish species reacted very differently. "Most sensitive to light were Nile and Mozambique cichlids, the two species from the murky habitat. They perceived the infrared radiation up to a spectral range of over 930 nanometers ", summarizes Shcherbakov. On the other hand, the photosensitivity of the clear-water fish was much lower: "Guppies and zebrafish showed no reaction at wavelengths longer than 910 nanometers. The swordtail even only responded to wavelengths up to 845 nanometers. "Display

According to the researchers, these results point to a clear evolutionary adaptation: the different species of fish have adapted over time to the prevailing light conditions in their habitats. "Obviously, fish took advantage of this fact in the course of evolution, " says Shcherbakov.

The results also provide completely new observational opportunities for behavioral studies. "If we know the individual visual limits of the different species of fish, we can observe the fish in light waves that are visible to observation cameras, while the fish fears in complete darkness. This is important for all studies in which a disturbing effect of perceptible light must be safely excluded. Special LED lamps could then be used for such experiments: these radiate infrared Light from a well-defined spectrum, which the fish can certainly not perceive. (PLoS ONE, 2013; doi: 10.1371 / journal.pone.0064429)

(University of Hohenheim, 25.06.2013 - NPO)