Vertebrate vision is made possible by photoreceptor cells in the eye. These cells – called rods and cones – contain pigment proteins that detect different types of light and transmit that information to the brain.
A typical spine eye has several types of cones that work under light conditions – each one can sense a certain number of colors – and a type of rod that feels light when the environment is dim. The rods cannot distinguish between colors because they all have the same pigment protein, why humans and most other animals are said to be color blind at night.
Cortesi and his colleagues wondered if they could find any exceptions among fish living in ever-dark environments. Their question was the subject of a 2015 study of mostly groundwater fish that showed several species with more genes for pigmentary proteins than researchers had expected.
"We just thought about other fish being more variable in their visual system than previously thought, we should look at the deep-sea fish," said Walter Salzburger, an evolutionary biologist at Basel University in Switzerland who monitored both the 2015 study and the new one. If all the fish could benefit from having more ways to see in dark conditions, it would be fish living in water so deep that the light barely reaches them.
Little is known about fish that live more than 1000 meters below sea level. Some developed large students and very long rods that help them capture the light that is around. (At these depths most of the light is produced by fish themselves through bioluminescence.)
For the new study, the researchers began to count the number of genes for both rod and pigment proteins in the genomes of 101 fish species living in a variety of habitats. Although they found a dozen species with up to seven cone pigmentary genes, what really struck them was the discovery of 13 species that had more than one rod pigment gene.
Four of these species had five or more of the genes: tube eyes (Stylephorus chordatus), glacier lantern (Benthosema glaciale), Longwing spinyfin (Diretmoides pauciradiatus) and Silver Spinyfin (Diretmus argenteus).
All four fishermen live 1000 meters to 2000 meters below sea level. Their latest common ancestor dates back more than 100 million years ago, so the researchers believe that the additional genes evolved independently in each family.
"Is it to see prey species? Or to find buddies in a completely dark or almost dark environment? Or to avoid predators?" Salzburger asked. "These are the three most important evolutionary benefits we can think of."
But were these fish actually using their extra pigment proteins? To answer that question, the team examined specimens that represented 36 different fish species. Some tissue samples were already preserved in laboratories, and others were acquired on fishing resources.
Cortesi and other researchers drew a net through the sea from Perth to Sri Lanka. They trawl at night so the fish would not encounter sunlight that could damage the eyes. It may take six hours to just fill a small bucket of thumb, Cortesi said.
Most of the 36 species had only one active gene to produce rod pigment proteins. The species with at least five rod pigment genes had at least three that were active.
The star was the silver spider. It had 38 genes for rod pigment proteins, and 14 of these proteins were actually at work inside the eye. (For the sake of comparison, most people use only three types of cone pigment proteins to see the world in color.)
It is not clear how silver spinphin uses all these rod pigments, but the researchers suspect that they can increase their sensitivity to light, said Salzburger.
In order to get an idea of what colors the silver spider can see, the scientists shot bacteria to reproduce some of their rod pigment proteins in a petri dish. Then they illuminated a light on each of them to see which part of the spectrum the pigment proteins could absorb. They found that they could detect light across the spectrum of bioluminescence – from different shades of blue and green to yellow.
Finally, they used the results to predict the colors that other deepwater fish with multiple rod pigment proteins could see. The forms of these proteins were the key, because different forms are sensitive to different wavelengths of light.
Their work suggested that the lantern fish, snake eye and longwing spinyfin could probably detect blue light, as well as shades of green and yellow green. But they would not have as wide a range as the silver spin fish.
Without behavioral testing, scientists may not know if these fish really use their rods to see color. The experiments would be difficult to pull off, as the fish are not only difficult to obtain, they do not live long when they are selected for the surface, said Salzburger. (Water pressure at sea level is much lower than what they are used to in the deep sea.)
Although researchers who were not involved in the study agreed to identify fish with multiple-pigment proteins, it was a novelty in itself.
Biologist David Hunt, a professor emeritus at the University of Western Australia, who specializes in the development of spinal vision, called his findings "quite amazing".
"It's something that is unknown and really quite unexpected," he said. "I'm still trying to get my head around what that means."
Los Angeles Times