Vertebral vision is possible thanks to photoreceptor cells at the back of the eye. These cells – called rods and cones – include pigment proteins that detect different types of light and transmit this information to the brain.
Typical spinal eye has many kinds of cones that work in bright conditions – everyone can sense a certain range of colors – and a kind of stick that feels light when the environment is unclear. Sticks can not distinguish colors because they all have the same pigment protein, which is why people and most other animals are said to be stained at night.
Cortez and his colleagues wondered if they could find some exceptions among the fish that live in a darker environment. Their question was dictated by a 2015 study of predominantly shallow water fish that showed several species with more cone pigmentary protein genes than scientists expected.
"We just thought that if other fish are more variable in their visual system than previously thought, we should look at deep-sea fish," said Walter Salzburg, an evolutionary biologist at the University of Basel in Switzerland, who led the study of 2015, and the new one. After all, if some fish take advantage of more ways to see them in dark conditions, the fish will live in water so deep that the light barely reaches them.
Little is known about fish that are more than 1,000 meters below sea level. Some develop large pupils and very long sticks that help them capture any light. (At these depths, most of the light is produced by the fish themselves by bioluminescence.)
For the new study, researchers began counting the number of genes for both rod and cone pigment proteins in the genomes of 101 species living in diverse habitats. Although they have discovered a dozen species with up to seven cone pigment genes, what really hit them is the discovery of 13 species that have more than one gene pigment.
Four of these species stand out with five or more genes: the Stylephorus chordatus, the Benthosema glaciale, the Diretmoides pauciradiatus, and the Diretmus argenteus.
All four of these fish live at 1000 meters up to 2000 meters below sea level. Their last common ancestor dates back more than 100 million years ago, so researchers think that additional genes develop independently in each genus.
"Do you see a kind of booty, or find friends in a totally dark or almost dark environment, or avoid predators?" – asked Salzburg. "These are the three major evolutionary advantages we can think of."
But whether these fish actually use their extra pigment proteins? To answer this question, the team explores samples representing 36 different fish species. Some tissue samples were already stored in laboratories, others were acquired on fishing expeditions.
Corteses and other researchers pulled a network across the ocean from Perth to Sri Lanka. They wore night time, so the fish could not see sunlight that could hurt their eyes. It may take six hours to fill just one small bucket of thumb-sized fish, Cortesi said.
Most of the 36 species had only one active gene for the production of protein from rods. The species with at least five genetic pigment genes have at least three active ones.
The star was silver spinin. There were 38 gene for pigment protein from rods and 14 of these proteins were actually in working order in the eye. (By comparison, most people use only three types of cone pigment proteins to see the world by color.)
It is unclear how silver spinphine uses all these pigments, but scientists suspect that they can increase their sensitivity to light, says Salzburger.
In order to get an idea of what colors the silver spinphins can see, researchers have included bacteria to reproduce some of their pigment proteins in a petri dish. They then illuminated each of them to see how much of the spectrum they could absorb the pigment proteins. They found they could find light across the spectrum of bioluminescence – from different shades of blue to green to yellow.
Finally, they use these results to predict the colors that other deep-water fish with many protein pigment proteins can see. The forms of these proteins are key because the different forms are sensitive to different wavelengths of light.
Their work suggests that the fish of the lantern, the tubular eye, and the long spininphine would probably find blue light, as well as shades of green and yellow-green. But they would not have as wide a range as silver fish.
Without behavioral experiments, scientists can not know for sure whether these fish really use their sticks to see the color. Experiments would be difficult to retrieve because the fish are not difficult to obtain, they do not live long after they are brought to the surface, said Salzburg. (Water pressure at sea level is much lower than what they are accustomed to in the deep ocean.)
Nonetheless, scientists who did not participate in the study agree that identifying fish with multiple proteins from pigment rods is in itself a novelty.
Biologist David Hunt, an honorary professor at the University of Western Australia who specializes in the evolution of vertebral vision, called his findings "quite astounding."
"This is something that is unknown and really completely unexpected," he said. "I'm still trying to figure out what it means."
Los Angeles Times