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.
