Deep-sea fishes’ eye chemistry might let them see colors in near darkness

Some fishes in the deep, dark sea may see their world in more than just shades of gray.

A survey of 101 fish species reveals that four from the deep sea had a surprising number of genes for light-sensitive eye proteins called rod opsins, researchers report in the May 10 Science. Depending on how the animals use those light catchers, the discovery might challenge the widespread idea that deep-sea fishes don’t see color, says coauthor Zuzana Musilová, an evolutionary biologist at Charles University in Prague.

To see, many fishes, humans and most other vertebrates rely on two types of light-detecting cells in the eye known as rods and cones. Cone cells use two or more kinds of opsins and need decent amounts of light to work. Rods generally use only one opsin called RH1, which works in dim light. That variety in opsins in cones, but not in rods, lets vertebrates see a range of colors in well-lit conditions but be color-blind in the near dark.

In the new study, Musilová and Fabio Cortesi of the University of Queensland in Brisbane, Australia sailed on research ships equipped to reach into the ocean depths for fish. The deep–sea specimens came from the “twilight” zone 200 to 1,000 meters below the surface, where sunlight becomes only a subtle lessening of darkness. The most colorful things to look at would be bioluminescent spots on animals’ bodies.

The four deep-sea fishes with the special eyes came from three different lineages that had independently evolved genes for more than one kind of RH1 rod opsin, Musilová, Cortesi and their colleagues report. A glacier lantern fish (Benthosema glaciale) had genes for five different forms of RH1, and a tube-eye (Stylephorus chordatus) had six. Two kinds of spinyfin had even more, 18 genes for the longwing spinyfin (Diretmoides pauciradiatus) and a stunning 38 for the silver spinyfin (Diretmus argenteus).

Finding even two rod opsins would have been notable, but the silver spinyfin’s tally is “astounding,” says evolutionary biologist Megan Porter at the University of Hawaii at Manoa, who was not involved in the new research. But she and others warn against jumping to conclusions about how fishes use all this variety, because there are no tests with fish behavior.

Considering where these fishes live, such tests may not even be possible, Musilová says. When brought to the surface, “most of them simply die due to the pressures changes,” she says. “Even getting them to the surface alive doesn’t guarantee they would behave the same way they do in the depths.”

The fish that the researchers caught let them check which opsin genes were actually turned on in the animals’ retinas. That work confirmed that the silver spinyfin actually uses at least 14 of its 38 RH1 genes to make proteins.

The researchers also put the silver spinyfin’s various RH1 genes into bacteria, which manufactured fish opsins. Tests of those opsins’ function showed they have the potential to capture both very faint daylight and a wide range of blue and green light from living bioluminescent creatures, the scientists found.

Overall, the authors are rightly “cautious” in not claiming that deep-sea fish can see color, says Almut Kelber of Lund University in Sweden, who has studied low-light color vision in frogs.

The new fish results, for example, don’t say whether different RH1 opsins cluster in individual rod cells or are scattered, with different rod cells carrying different opsins. To differentiate colors, the rod opsins would need to be in different cells. But if the proteins clump in each rod, then the fish probably just have enhanced sensitivity to light and could pick out fainter objects in shades of black and white.

Even with the uncertainties, finding all these unexpected opsins “is still exciting,” Kelber says.