It’s All Done With Mirrors
Les yeux sont le miroir de l’âme (The eyes are the mirror of the soul.) I am not sure what the late James Brown would have said about that, but a recent discovery that a unique adaptation for vision evolved in the brownsnout spookfish has captured the fascination of evolutionary biologists.
This the first time that a vertebrate has been found to use crystals to reflect light onto the retina of the eye in order to aid vision. It illustrates the wonder of the eye and the multitude of ways in which evolution has solved two issues critical to the survival of metazoan creatures:
- What’s available for me to eat?
- What wants to eat me?
Among all the senses, vision was the trickiest question that Charles Darwin sought to resolve in developing the theory of natural selection for evolution. While he did allow that it is difficult to imagine a giant leap from sightlessness to clear vision in a single bound, he was able to propose processes by which sight developed in minute stages to the eyes that humans, raptors, dogs, cephalopods and, um, lots and lots of animals use from the humble beginnings of eyespots in planaria.
To suppose that the eye, with all its inimitable contrivances for adjusting the focus to different distances, for admitting different amounts of light, and for the correction of spherical and chromatic aberration, could have been formed by natural selection, seems, I freely confess, absurd in the highest possible degree. (Darwin 1872)
It does seem absurd to think that nature can do what man was unable to do until the invention of the camera: to interpret changes in light in ways that provide meaningful data to the beholder. The above passage from Darwin has been used in quotemines to cry triumphant defeat of evolution, admitted to by the scientist most closely associated with natural processes of biological evolution. Darwin admitted no such thing, and it is important to understand his method of presentation.
- Present an insurmountable problem.
- Explain its resolution.
From the talkorigins.org Index to Creationist Claims (CA113.1, Isaak, 2004):
- The quote is taken out of context. Darwin answered the seeming problem he introduced. The paragraph continues,
Yet reason tells me, that if numerous gradations from a perfect and complex eye to one very imperfect and simple, each grade being useful to its possessor, can be shown to exist; if further, the eye does vary ever so slightly, and the variations be inherited, which is certainly the case; and if any variation or modification in the organ be ever useful to an animal under changing conditions of life, then the difficulty of believing that a perfect and complex eye could be formed by natural selection, though insuperable by our imagination, can hardly be considered real. How a nerve comes to be sensitive to light, hardly concerns us more than how life itself first originated; but I may remark that several facts make me suspect that any sensitive nerve may be rendered sensitive to light, and likewise to those coarser vibrations of the air which produce sound. (Darwin 1872, 143-144)
Darwin continues with three more pages describing a sequence of plausible intermediate stages between eyelessness and human eyes, giving examples from existing organisms to show that the intermediates are viable.
The process by which the brownsnout spookfish solves the food-finding and prey-avoidance problem is not yet fully understood, but the point is that there are many separate ways for vision to evolve. Spookfish use guanine crystals, layered into a mirror, to reflect din light from below. Their eyes look above them to see shadows and light streaming down from the surface to their deep habitat, in the depths to which light barely penetrates. The mirrors help them to avoid predators who may be attacking their vulnerable bellies from below. It’s this downward vision that may save them.
The spookfish were discovered and named in 1888, but the specimens captured were all dead at the time of recovery and had been mangled badly enough that until recently their eyes didn’t reveal their special adaptation. A new article in Current Biology1
describes the process by which the fish were netted and then examined carefully using microscopes.
Tamara Frank took a series of pictures of the eyes and discovered that the flash from the camera reflected through the
retina of the true eye in the way that she would expect. The lower, “diverticular eye,” which appears as a “bump” on the head, instead reflected into the retina. Professor Julian Partridge was curious as to why this happened, so he prepared slides of the lower eye and found the guanine crystals. He discovered that the precisely layered crystals aimed light into the retina, giving the spookfish vision into the depths below.
The spookfish inhabit depths between 500 and 1,000 meters (1,500 to 3,000 feet.) As I mentioned above, very little sunlight can penetrate this far down. Many fish are clear, lacking the pigment in their skin to create color. Bioluminescence is rather common at this depth, creating an additional source of light.
Any advantage that evolution can give a fish to see its predators and its prey is likely to survive. As this is so far the only known vertebrate to possess this sort of eye (some crustaceans carry the trait). I am curious as to why fortune favored the fish in this way but the trait hasn’t been spread through other fish.
I would think it would pass an energy and mass cost advantage to the fish that possess the mirrored eye, because it reduces the need for muscular control of the eyes. They needn’t pivot nor rotate their eyes to increase their field of vision. Right there, we have an adaptive advantage.
This would seem to be such a plus for fish that we would expect to see more of them with this trait. I am curious as to why they are unique, but this also illustrates another point of evolution. Evolution doesn’t create optimal solutions to survival problems; it creates solutions that work. It can only use the tools available to it. Where this adaptation is found, its roots are still a mystery. What were the first creatures in the spookfish’s ancestry to carry the genes to create the crystals? What was the developmental pathway that led to those genes’ expressions? What was the process that led to their development to mirrored eyes?
The fossil ancestors of the brownsnout spookfish may or may not be able to yield clues, if we can find them. The remaining fossils could be buried deep below the ocean floor, and we may not have the technology to recover them for many years to come. Genome sequencing of this fish and its cousins may help discover the genetic and mutational roots of this fabulous eye.
This is one solution to the vision problem. Darwin would have loved it had he known, and likely would have included a grand description of it in his discussion on the evolution of the eye. I venture to say that as marine biologists and divers continue to explore the depths, they will find more and stranger eyes.
Yahoo News India: “4-eyed Spookfish, first vertebrate to use mirrors for seeing.”
Zipcodezoo.com “Dolichopteryx longipes”
Phsyorg.com “Spookfish uses mirrors for eyes.“
- subscription required: A Novel Vertebrate Eye Using Both Refractive and Reflective Optics, by Hans-Joachim Wagner, Ron H. Douglas, Tamara M. Frank, Nicholas W. Roberts and Julian C. Partridge. Current Biology 19, 1-7, January 27, 2009. DOI 10.1016/j.cub.2008.11.061 [↩]
This entry was posted on Monday, February 2nd, 2009 at 7:11 pm and is filed under Mike Haubrich, Science. You can follow any responses to this entry through the RSS 2.0 feed. Both comments and pings are currently closed.