In the depths of Earth’s oceans, researchers have uncovered fish equipped with a previously undescribed visual system that allows them to see color in the inky blackness, a finding in contrast to the idea that these organisms are essentially color-blind in their dark home. An analysis of 101 fish genomes revealed the novel visual system, which – instead of using a single rod opsin to see in the dark, as most other vertebrates do – relies on multiple, distinct rod opsin (RH1) photopigments tuned to cover a vast range of bioluminescence emitted from deep-sea organisms. We can see the world around us thanks to specialized light-sensitive photopigments (visual opsins), which convert the light entering our eyes into electrochemical signals for our brain to interpret. Vertebrates possess up to five types of visual opsins; four cone opsins and one rod opsin. While cones allow for color vision in bright conditions, rods – far more sensitive to light and dark changes, but not good for color vision – are used in dim-light conditions. Vertebrates that reside in Earth’s dark places, thought to be color-blind, rely upon their single rod photopigment for sight. Deep-sea fishes, many of whom spend their lives in an environment where bioluminescence is the sole source of light, have evolved a variety of biological adaptations to maximize their visual acuity in the dark. Here, Zuzana Musilova and colleagues evaluated the molecular adaptations in these fish’s visual systems by looking across more than 100 deep-sea fish genomes and discovered a previously unknown proliferation of RH1 genes that produce a diverse collection of rod opsin photopigments each tuned to various wavelengths of light present in the deep sea. Of the 13 species identified with more than a single RH1, the silver spinyfin (Diretmus argenteus) possesses 38 rod opsins – the highest number yet known in vertebrates. According to Musilova et al., the purely rod-based color vision described here may be helpful in recognizing the spectrally diverse bioluminescent light from predators, conspecifics or prey, though they are currently unable to behaviorally test this.
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