Some deep-sea fishes have developed highly sensitive color vision that could help them determine predator from prey in the dimly-lit depths.
Color vision in vertebrates is usually achieved through the interaction of various photopigments in the cone cells found in the retina.
Each of these photopigments reacts to a certain wavelength of light. In humans, these wavelengths are the red, green and blue range of the light spectrum.
Color vision is only possible in daylight, however. In darkness, vertebrates detect the few available light particles with their light-sensitive rod cells, which contain only a single type of the photopigment rhodopsin â€” explaining why nearly all vertebrates are color-blind at night.
â€śVertebrates use two types of photoreceptor cells â€” rods and cones â€” in order to see. Cones are used in bright-light conditions, while rods are generally used in dim-light,â€ť said Dr. Fabio Cortesi, a researcher in the Zoological Institute at the University of Basel, Switzerland, and the Queensland Brain Institute at the University of Queensland, Australia.
â€śBoth rods and cones contain light-sensitive proteins called opsins that absorb light at specific wavelengths.â€ť
â€śColor vision in vertebrates is due to the fact that cones use around four different opsins. This variety allows sensitivity to a broad range of colors.â€ť
â€ś99% of all vertebrates have just one opsin protein â€” rhodopsin â€” in their rods, so most are color-blind in dim-light conditions because they rely only on that single rod opsin.â€ť
â€śDeep-sea fishes that live at about 650 to 5,000 feet (200-1,500 m) below the surface tend to be no exception.â€ť
â€śWater at that depth filters most light out,â€ť said Dr. Fanny de Busserolles, a deep-sea visual ecology specialist in the Queensland Brain Institute, Australia, and the Red Sea Research Center, Saudi Arabia.
â€śDown there itâ€™s very monochromatic, and most fish just perceive blue light. But we discovered some spectacular exceptions.â€ť
The scientists examined the genomes of 101 deep-sea fish species and found a previously unknown proliferation of rod opsin genes.
They identified 13 fish species with more than one rod opsin gene.
Notably, four deep-sea species had five or more rod opsin genes: the glacier lanternfish (Benthosema glaciale) with 5 genes; the tube-eye (Stylephorus chordatus) with 6 genes; and two species of the Diretmidae, the longwing spinyfin (Diretmoides pauciradiatus) with 18 genes and the silver spinyfin (Diretmus argenteus) with 38 genes.
â€śThis makes the darkness-dwelling silver spinyfin the vertebrate with the most photopigment genes by far,â€ť said Professor Walter Salzburger, from the University of Basel.
â€śGene sequence analysis and experiments on how those rod opsins function suggested silver spinyfins are able to pick up a wide range of wavelengths of light, meaning they probably see many colors. This ability could have evolved as a survival weapon,â€ť Dr. Cortesi added.
â€śThere are many colors of bioluminescence â€” light produced and emitted by living organisms â€” down there, and it mainly appears in flashes coming from other fish.â€ť
â€śIf you want to survive down there you need to quickly decide if you are seeing a potential predator or potential prey.â€ť
The findings were published in the journal Science.
Zuzana Musilova et al. 2019. Vision using multiple distinct rod opsins in deep-sea fishes. Science 364 (6440): 588-592; doi: 10.1126/science.aav4632