When the ancestors of cave fish and certain crickets moved into black-haired caverns, their eyes practically disappeared for generations. But fish that plow through the sea to depths beyond what sunlight can penetrate have developed a super vision, very much in tune with the glimmer and glitter of other creatures. They owe this power, as evolutionary biologists have discovered, to an extraordinary increase in the number of genes for rod opsins, retinal proteins that detect dim light. Those additional genes have been diversified to produce proteins capable of capturing every possible photon at multiple wavelengths, which could mean that despite the darkness, the fish that roam the depths of the ocean actually see in color.
The finding "really shakes the dogma of the vision of the deep sea," says Megan Porter, an evolutionary biologist who studies vision at the University of Hawaii in Honolulu who did not participate in the work. Researchers had observed that the deeper a fish lives, the simpler is its visual system, a trend that they badumed would continue to the bottom. "That [the deepest dwellers] having all these characteristics means that there is much more complexity in the interaction between light and evolution in the deep sea than we realized, "says Porter.
At a depth of 1000 meters, the last ray of sunlight disappears. But in the last 15 years, researchers have realized that the depths are impregnated by a slight bioluminescence of flashes of shrimp, octopus, bacteria and even fish. Most vertebrate eyes could barely detect this subtle glow. To learn how fish can see it, a team led by evolutionary biologist Walter Salzburger of the University of Basel in Switzerland studied the opsin proteins of deep-sea fish. The variation in the amino acid sequences of the opsins changes the wavelength of the detected light, so multiple opsins make color vision possible. An opsina, RH1, works well in low light. Found in the cells of the rod of the eye, it allows humans to see in the dark, but only in black and white.
Salzburger and his colleagues looked for opsin genes in 101 fish species, including seven deep-sea fish from the Atlantic Ocean whose genomes were completely sequenced. Most fish have one or two RH1 opsins, like many other vertebrates, but four of the deep-water species stood out, researchers report this week in Science. Those fish, the lantern fish, a pipe eye fish and two fine spines, all had at least five RH1 genes, and one, the silver thorn (Diretmus argenteus), he was 38. "This is not known in the vertebrate vision," says K. Kristian Donner, a sensory biologist at the University of Helsinki.
To ensure that the additional genes were not simply non-functional duplicates, the team measured the genetic activity in 36 species, including specimens from 11 deep-sea fish. Multiple RH1 genes were active in deep-water species, and the total was 14 in an adult silver spiny fin, which grows to 2000 meters. "At first it seems paradoxical: this is where there is the least amount of light," says Salzburger.
Researchers can predict the wavelengths at which an opsin protein is most sensitive from its amino acid sequence. The deep-water fish had a total of 24 mutations that alter the function of their RH1 proteins, tuning each to see a narrow range of blue and green wavelengths, the colors of bioluminescence. "Some of these opsins could be tuned to detect particular bioluminescent signals badociated with food, danger or social interactions," says Gil Rosenthal, a behavioral ecologist at the University of Texas A & M in College Station.
The four deep-water species belong to three different branches of the fish family tree, indicating that this monitoring evolved repeatedly. "This indicates that animals living in extreme light environments may be subject to extreme natural selective pressures to improve visual performance," says Eric Warrant, visual ecologist at Lund University in Sweden.
The abundant opsins also help explain the unusual anatomy of the spiny fin retina. Some of their rod cells are much longer than usual, and many are stacked one on top of the other instead of being arranged in a single layer. Enlarged cells and stacking help ensure that more incoming photons are detected, but researchers have long badumed that all these bars had the same opsin. Now, it seems that, like the layers of an old photographic film, bars of different sizes could capture different wavelengths of light. "Now we have to accept that our eyesight [of deep-sea vision] It has been too limited, "says Donner.
Due to the depths that these fish inhabit, it is impossible to collect live specimens to test their vision. But the opsins of multiple rods can allow them to distinguish the color, according to Salzburg and others. For these fish, the slight bioluminescence in the depths of the ink could be as vivid and varied as the bright world of the surface.