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No. 101: Sep-Oct 1995

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Dragon fish see red

Most fish that make a living in deep, dark ocean waters have eyes that are most sensitive to the blue part of the sun's rays (470-490 nanometers). These are the rays that penetrate to the greatest depths in the sea. This adaptation to blue light means that deepsea fish have evolved visual pigments different from those of surface fish and land animals. Visual pigments are complex chemical compounds, and one must suppose that many, many random mutations took place before deepsea fish were able to manufacture visual pigments different from their relatives living near the surface. (Or did deepsea fish come first?) But there is more to this story. Many dwellers in the black abysses generate their own light. They sport bioluminescent organs so they can be seen by others of their own species and, in addition, illuminate prey for easier capture. In another remarkable example of evolutionary convergence, these bioluminescent organs emit light spectrally matching the eye sensitivity of deepsea fish! So far, though, this story is not any more amazing that many others woven into evolution's fabric.

But suppose that a deviant species of deepsea fish upset this cosy status quo by evolving visual pigment and bioluminescent organs operating in a part of the electromagnetic spectrum that other deepsea fish could not perceive. It would be as if this species had radar but the others did not!

Well, three genera of dragon fish do have organs (photophores) that emit far-red light, and their eyes are correspondingly red-shifted by new visual pigments. Thus, these dragon fish can communicate with each other without being detected by other species. When hunting, they can prowl the depths with lights on, illuminating prey surreptitiously. Lab tests indicate that these dragon fish can detect a meal much father away than possible with their lateralline sense. (Partridge, Julian C., and Douglas, Ron H.; "FarRed Sensitivity of Dragon Fish," Nature, 375:21, 1995)

Comment. Note that the innovative spectral shift had to occur synchronously in both eyes and bioluminescent organs to be useful; i.e., have survival value.

From Science Frontiers #101 Sep-Oct 1995. 1995-2000 William R. Corliss