Unique genetic adaptation lets deep-sea fish see color in the darkness

[May 11, 2019]  By Will Dunham

WASHINGTON (Reuters) - While people and other vertebrates are color blind in dim light, some deep-sea fish may possess keen color vision to thrive in the near total darkness of their extreme environment thanks to a unique genetic adaptation, scientists said on Thursday.

Researchers analyzed the genomes of 101 fish species and found that three lineages of deep-sea fish, living up to about a mile (1,500 meters) below the surface, boast a specialized visual system to allow for color vision in inky blackness.

Having 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, the planet'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 colors. The rods are used in dim light, not geared to detect colors.

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 colors. One species, the silver spinyfin, had 38 copies of the rhodopsin gene, rather than the usual one.

The deep-sea lanternfish (Myctophiformes) is seen in this undated image released in Prague, Czech Republic on May 9, 2019. Zuzana Musilova/Charles University/Handout via REUTERS

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 color purely by rods, which is unique among vertebrates," Musilova said.

These fish are smallish, up to a foot (30 cm) long, eating plankton and shrimps at depths mostly between one-quarter to three-quarters of a mile (400-1,200 meters).

Residual surface light reaches down to about six-tenths of a mile (1,000 meters). 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.

(Reporting by Will Dunham; Editing by James Dalgleish)

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