Special eyes for the ocean depths The retina of the silver spinyfin (Diretmus argenteus) has an unusual arrangement of low light–sensing rod cells, which house diverse photoreceptor proteins (right). 'At first it seems paradoxical-this is where there's the least amount of light,' Salzburger says. Multiple RH1 genes were active in the deep-sea species, and the total was 14 in an adult silver spinyfin, which thrives down to 2000 meters. To make sure the extra genes weren't just nonfunctional duplicates, the team measured gene activity in 36 species, including specimens of 11 deep-sea fish. Kristian Donner, a sensory biologist at the University of Helsinki. 'This is unheard of in vertebrate vision,' says K. Those fish-the lantern-fish, a tube-eye fish, and two spinyfins-all had at least five RH1 genes, and one, the silver spinyfin ( Diretmus argenteus), had 38. Most fish have one or two RH1 opsins, like many other vertebrates, but four of the deep-sea species stood apart, the researchers report this week in Science. Salzburger and his colleagues searched for opsin genes in 101 fish species, including seven Atlantic Ocean deep-sea fish whose genomes they fully sequenced. Found in the eye's rod cells, it enables humans to see in the dark-but only in black and white. Variation in the opsins' amino acid sequences changes the wavelength of light detected, so multiple opsins make color vision possible. To learn how fish can see it, a team led by evolutionary biologist Walter Salzburger from the University of Basel in Switzerland studied deep-sea fishes' opsin proteins. Most vertebrate eyes could barely detect this subtle shimmer. But over the past 15 years, researchers have realized that the depths are pervaded by a faint bioluminescence from flashing shrimp, octopus, bacteria, and even fish. 'That have all these opsins means there's a lot more complexity in the interplay between light and evolution in the deep sea than we realized,' Porter says.Īt a depth of 1000 meters, the last glimmer of sunlight is gone. Researchers had observed that the deeper a fish lives, the simpler its visual system is, a trend they assumed would continue to the bottom. The finding 'really shakes up the dogma of deep-sea vision,' says Megan Porter, an evolutionary biologist studying vision at the University of Hawaii in Honolulu who was not involved in the work.

Those extra genes have diversified to produce proteins capable of capturing every possible photon at multiple wavelengths-which could mean that despite the darkness, the fish roaming the deep ocean actually see in color.

They owe this power, evolutionary biologists have learned, to an extraordinary increase in the number of genes for rod opsins, retinal proteins that detect dim light. But fish that ply the sea at depths greater than sunlight can penetrate have developed super-vision, highly attuned to the faint glow and twinkle given off by other creatures. When the ancestors of cave fish and certain crickets moved into pitchblack caverns, their eyes virtually disappeared over generations.

Adaptations of deep sea creatures