Some deep-sea fish may possess keen colour vision to thrive in the near total darkness of their extreme environment thanks to a unique genetic adaptation, scientists say.
Researchers analysed the genomes of 101 fish species and found that three lineages of deep-sea fish, living up to about 1500 metres below the surface, boast a specialised visual system to allow for colour vision in inky blackness.
Acute vision could provide tremendous advantages to these fish as they search for food and mates and try to avoid becoming another creature's dinner in the exotic dark world of the ocean depths, earth's largest habitat.
"Their eyes are certainly much more sensitive, so we believe their vision in the depths would be very good," said evolutionary biologist Zuzana Musilova of Charles University in Prague, one of the researchers in the study published in the journal Science.
Vertebrates use two types of photoreceptor cells in the retina to see: light-sensitive so-called rods and cones. The cones are employed in bright-light conditions and perceive colours. The rods are used in dim light, not geared to detect colours.
Rod cells contain a single type of photopigment - pigments that react to a certain wavelength of light - called rhodopsin.
The researchers found 13 species from the three lineages of deep-sea fish that had a proliferation of genes controlling rhodopsin, apparently letting the fish use rods to detect colours. One species, the silver spinyfin, had 38 copies of the rhodopsin gene, rather than the usual one.
The spinyfin, with a bright silver body, has an almost circular body shape and large eyes. Other fish with this visual system include the extremely elongated tube-eye fish and the bioluminescent lanternfish.
"They very likely are able to see colour purely by rods, which is unique among vertebrates," Musilova said.
These fish are smallish, up to 30 cm long, eating plankton and shrimps at depths mostly between 400 and 1200 metres.
Residual surface light reaches down to about one kilometre. Light also emanates from bioluminescent creatures common in the deep ocean including the anglerfish, which has a glowing lure attached to its head to attract prey.
