Vision of the vertebrate is made possible by photoreceptor cells inside the eye. These cells, called rods and cones, contain pigment proteins that detect a variety of light and transmit that information to the brain.
Typical vertebrae have several types of cones that operate in bright conditions. Each condition is a bar type that can detect a specific range of colors and detect light when the environment is dark. Humans and most other animals are said to be color blind at night because they can not distinguish color because they all have the same pigment protein.
Cortesi and his colleagues wondered if there was an exception among the fish that lived in an eternal dark environment. Their question was triggered by a 2015 study of mostly shallow water fish that found several species with more genes for cone pigment proteins than scientists expected.
Walter Salzburger, an evolutionary biologist at Basel University in Switzerland, said, "We thought we should observe deep-sea fish if the other fish were more variable in the visual system than previously thought." Anyway, if any fish stood to benefit from more ways to see in a dark environment, a fish that lives too deep in the water will barely reach the light.
Little is known about fish over 1000 meters above sea level. Some have developed large eyes and very long bars to help catch all the light around them. (At that depth, most light is produced by the fish itself through bioluminescence.)
For a new study, researchers began counting the number of genes for both rod and cone pigment proteins in 101 species of fish genomes in various habitats. Twelve species with seven primary pigment genes were found, but in fact, it was the discovery of thirteen species with more than one species of pigment gene.
Four of them have been identified with five or more genes, such as Stylephorus chordatus, smelt lanternfish (Benthosema glaciale), longwing spinyfin (Diretmoides pauciradiatus) and silver spinney pin (Diretmus argenteus).
All four of these fish live at altitudes of 1000m to 2000m above sea level. The most recent common ancestor goes back 100 million years. So researchers think that additional genes have evolved independently in each lineage.
"Do you look at the food species, looking for colleagues in a completely dark or nearly dark environment, or avoiding predators?" Salzburger asked. "This is the three major evolutionary benefits we can think of."
But were these fish actually using extra pigmented proteins? To answer this question, the team investigated a sample representing 36 different species of fish. Some tissue samples were already preserved in the laboratory, and other tissue samples were collected at the fishing expeditions.
Cortesi and other researchers carried the net through the ocean from Perth to Sri Lanka. A fish walks at night because it does not meet sunlight and can damage your eyes. Cortesi said it would take six hours to fill a thumb-sized fish.
Most of the 36 species have only one active gene that produces a rod pigment protein. At least three species with at least five rod pigment genes were active.
The star was a silver spinyfin. It had 38 genes for the rod pigment protein, and 14 of those proteins were actually working inside. (For comparison, most humans use only three types of cone pigment proteins to see the world using color.)
It is not clear how the silver spinyfin uses these rod pigments, but Salzburger says scientists think they can increase their sensitivity to light.
The researchers enlisted bacteria to reproduce the cultured pigment protein in a culture dish to see what color of silver spinyfin is. They then illuminated each light to identify the portion of the spectrum that the pigment proteins could absorb. They have found that light can be detected in the entire spectrum of bioluminescence from different shades of blue and green to yellow.
Finally, they used these results to predict the color that other deep sea fish with multiple rod coloring proteins can see. The shape of the protein was the key. Because the shape is sensitive to the wavelength of the other light.
Their work suggested that lantern fish, tube eyes and longwing spinyfin can detect shades of green and yellowish green as well as blue light. However, it will not have as wide a range as a spinalfish.
Without behavioral experiments, scientists can not tell whether these fish can actually see the color using the rod. Professor Salzberger said it would be difficult to start the experiment because fish can not survive if it is not difficult to grow and is buried on the surface. (Water pressure at sea level is much lower than that used in deep water.)
Nonetheless, scientists who did not participate in the study agreed that identifying fish with multi-rod pigment proteins is novel in itself.
"The results are amazing," said David Hunt, an honorary professor at the University of Western Australia and biologist at David Hunt who has evolved the vertebrate vision.
"It is not known and it is completely unexpected," he said. "I am still trying to look back on what it means."
Los Angeles Times