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Seeing red

点击量:   时间:2019-02-26 09:03:01

By Lou Bergeron TWO species of dagger-toothed fish that lurk in the deep sea can scan their gloomy world with bursts of light that only they can see. The fish have an extra pigment in their eyes that allows them to pick up reflections of the light from their prey, say researchers in Britain. Bioluminescence, the light produced by living organisms, is common in deep-sea fish, and is produced by organs called photophores. Most species emit blue light with a wavelength of around 470 to 490 nanometres, and most fish eyes are tuned to this wavelength, which matches the visible light that penetrates hundreds of metres into the deep ocean. But two genera of stomiid fish, Aristostomias and Pachystomias, are equipped with two types of photophores. One emits blue light, but the other produces red light with a wavelength of around 700 nanometres—almost in the infrared region of the spectrum and invisible to other fish. Scientists suspected that the red light was used to illuminate potential prey, which would be unaware of the danger they were in. “Most fish will be able to see that there’s blue bioluminescence going on, but they’ll be completely unaware that there’s red bioluminescence,” says Ronald Douglas of City University in London. For this ploy to work, however, the stomiid fish would have to have a pigment in their eyes that would allow them to detect the red wavelengths, in addition to their two known pigments that detect shorter wavelengths. At last week’s Eighth Deep Sea Biology Symposium in Monterey, California, Douglas reported that he and his colleague Julian Partridge of the University of Bristol have discovered this third pigment. Douglas and Partridge took the intact retinas of the stomiid fish, sandwiched them between pieces of wire gauze, and scanned them using a spectrophotometer. This revealed a pigment that is still sensitive to light at wavelengths of about 700 nanometres. Douglas suspects that the pigment has not been detected before because earlier studies relied on mashing the retina before extracting the pigments. This process may have destroyed the newly discovered pigment, he says. Douglas believes the pigment does indeed pick up reflected red light from prey. “It’s equivalent to an infrared sniper scope,” he says. But he also thinks the red bioluminescence might be used to attract mates in the sparsely populated depths without alerting predators. “They really have a secret waveband of communication,” he says. “It’s a fascinating picture of these animals cruising along down there, illuminating prey and talking to each other without anyone else knowing that they’re doing it.” The researchers add that there may eventually be more to the story. Although the third visual pigment can pick up the 700-nanometre light that the fish emit, it would not give good vision in that range because it is most sensitive to wavelengths around 600 nanometres. Douglas suspects there is a fourth pigment in the retinas that experiments have yet to pick up. “I’m almost willing to lay my life down, but not quite,