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In this issue |
Catfish Wink for Cover
kathryn{at}biologists.com
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Ron Douglas almost got the fright of his life a few years back when he saw a fish wink at him! Everyone knows that fish eyes just stare coldly because of their fixed iris that keeps the pupil wide and dark. When Douglas took a closer look at the winking fish, he realised that its iris had contracted, closing the pupil to a tiny dark dot in the middle of the fish's eye. He began staring into other fish eyes, and discovered several species could wink in the blink of an eye, but the suckermouth armoured catfish was slower, taking several minutes to veil its pupil. Douglas reasoned that the few species that developed a contracting iris must have had a very good reason to go down this evolutionary path. After testing a variety of optical theories, he has come to the conclusion that the suckermouth armoured catfish's contracting pupil evolved to provide concealment (p. 3425).
Human eyes don't work so well when we are submerged, because we rely on the cornea and the lens to focus light onto the retina, but under water the cornea's ability to focus light is lost. The fish has overcome this optical disadvantage by evolving a high-powered lens, but does the lens produce a high quality image too? Douglas reasoned that maybe this catfish just had bad eyesight. If the large lens produced a fuzzy image, the focusing power of a small pupil might be able to compensate for the lenses aberrations, in the same way that a small camera aperture sharpens up a shot. He wondered if the cat fish's contracting pupil could have evolved to correct poor vision, so he decided take a look inside the fish's eye to see how good its lens was.
Douglas' went to his local tropical fish shop, and chose ten fish, thinking he'd chosen ten suckermouth armoured catfish. But when he showed them to specialists at London's Natural History Museum, he discovered that he had a complete mixture of species (some barely known to science) that included a few suckermouth armoured catfish.
Once they'd separated the impostors from the real armoured catfish, Douglas and his colleagues, Julie Corrigan and Shaun Collin, began tested the fish's eyesight by looking for optical aberrations in the lens. They also videoed the fishes' slowly contracting pupils with an infrared camera to see if the pupil's final size correlated with the quality of the lens. But the lens turned out to be as good as any other wide-eyed fish! The pupil wasn't correcting for poor eyesight, so what else was it doing?
When Douglas looked at the fish's life style, he realised that most winking fish are bottom dwelling creatures. He explains that when you see the catfish in its natural environment, the contracted pupil blends in well with the background, while a huge black pupil is a sure give-away. Having given the fish a thorough eye test, Douglas believes that the origin of the catfish's wink has more to do with camouflage than correction.
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