Fish See With Electric Eyes
Biologists knew that some electric fish shock their prey and others with weak electricity can navigate with it, but they didn’t know till recently just how much information these fish can detect with their unique sense. French and British scientists ran some experimental tests on weakly electric fish, the African elephantnose fish Gnathonemus petersii, which sends out pulses of electricity, and the Amazonian longtail knifefish Sternopygus macrurus, which sends out electrical waves. These fish feed on aquatic larvae at night where eyes are of no use. But they have eyes of a different sort: electric eyes.
Graff et al., in their paper in the May 4 issue of Current Biology,1 showed experimentally that these fish use their electric sense for much more than just location. Their electric organs effectively provide a map of their surroundings. The electric field they generate is modulated by the dielectric properties and impedances of objects in the area: grass, stones, other fish and larvae each have their own electrical signature. These signatures are picked up by numerous sensory organs in the skin of the fish. The skin thus acts like a retina, allowing the fish to see its way in the dark. The information is so rich and varied, you could even say it’s analogous to 3D color vision.
Through a series of clever experiments, the scientists demonstrated that these species of fish can detect the following information about objects: size, composition, distance to objects, distance between objects, spatial patterns, 3D orientations, and similarities between different objects. They can even memorize configurations in space to create a mental map. They can do all these relying solely on their electric sense, “which is alien to all other animals” (They do not mention one possible exception, the duck-billed platypus.)
The authors coined a new term, “electroperception,” to indicate that this special sense does more than just electro-location. They compared it to the echolocation of bats and dolphins who, similarly, can discern shapes, textures, motions and distances with their special senses. Needless to say, the fish’s electrical sensory apparatus requires processing for interpretation and response. They may not have the human cerebral cortex, but “evolution favored the valvula cerebelli, a hypertrophied part of the metencephalon (cerebellum), which is likely to be responsible for such functions.”
See also Science Now for a summary of the findings.
1Graff, Kaminski, Gresty and Ohlman, “Fish Perform Spatial Pattern Recognition and Abstraction by Exclusive Use of Active Electrolocation,” Current Biology Vol 14, 818-823, 4 May 2004.
This was such a great story, why did they have to mess it up with a stupid evolutionary personification at the end? Evolution can’t favor anything, nor can it build a brain or a hypertrophied (enlarged) valvula cerebellum. They presented no evolutionary pathways or missing links; the comment added nothing. *Sigh*
Let us forgive this brief indecorum (maybe they had to say the E word to get it past the Darwin Party guards of Current Biology). Let us just thank them for performing good experimental technique to bring another wonder of nature to our attention. Just a dumb fish at night, but with the coolest of gadgets: electric eyes. Stunning.