Vertebrate vision is made possible by photoreceptor cells in the back of the eye. These cells – called rods and cones – include pigment proteins that detect various types of light and convey that information to the brain.
Typical vertebrate eyes have many types of cones that work in bright conditions – each capable of feeling a certain range of colors – and one type of rod that feels light when the environment is dim. Stems cannot distinguish between colors because they all have the same pigment protein, which is why humans and most other animals are said to be color blind at night.
Cortesi and his colleagues wondered if they could find a few exceptions among fish that live in a continuously dark environment. Their question was triggered by a 2015 study of most shallow water fish that produced several species with more genes for cone pigment proteins than scientists thought.
"We just think if other fish are more varied in their visual systems than previously thought, we should see deep-sea fish," said Walter Salzburger, an evolutionary biologist at the University of Basel in Switzerland who oversees both 2015 and new studies. After all, if there are fish that benefit by having more ways to see in dark conditions, it will become fish that live in water so deep that light barely reaches them.
Very little is known about fish that are more than 1000 meters below sea level. Some develop large pupils and very long stems that help them capture whatever light is around them. (At that depth, most of the light is produced by the fish themselves through bioluminescence.)
For the new study, the researchers began by calculating the number of genes for cone and cone protein in the genome of 101 species of fish that live in various habitats. Although they found a dozen species with up to seven cone pigment genes, what really surprised them was the discovery of 13 species that had more than one gene pigment stem.
Four of these species stand out with five or more genes: eye-tube (Stylephorus chordatus), lanternfish glacier (Benthosema glaciale), long spinyfin (Diretmoides pauciradiatus) and silver spinyfin (Diretmus argenteus).
The four fish live 1,000 meters to 2,000 meters below sea level. Their most recent ancestors were around 100 million years ago, so researchers think that additional genes evolved independently in each lineage.
"Is it to see prey species? Or to find a partner in a truly dark, or almost dark environment? Or to avoid predators?" Salzburger asks. "These are the three main evolutionary advantages that we can think of."
But do these fish really use their extra pigment protein? To answer that question, the team examined specimens representing 36 different species of fish. Some tissue samples have been preserved in the laboratory, and others have been obtained on fishing expeditions.
Cortesi and other researchers dragged nets through the ocean from Perth to Sri Lanka. They trawl at night so the fish will not find sunlight that can damage their eyes. Cortesi can take six hours to fill only one small bucket of fish.
Most of the 36 species only have one active gene to produce the rod pigment protein. Species with at least five stem pigment genes have at least three active ones.
The star is silver spinyfin. It has 38 genes for rod pigment proteins, and 14 of them actually work inside the eye. (For comparison, most humans only use three types of cone pigment proteins to see the world in color.)
It's not clear how silver spinyfin uses all of these stem pigments, but scientists suspect they can increase their sensitivity to light, Salzburger said.
To get an idea of what color silver spinyfin might look, the researchers asked the bacteria to reproduce some of its stem pigment proteins in a petri dish. Then they illuminate each one to see what part of the spectrum of protein pigments can absorb. They found that they could detect light throughout the bioluminescence spectrum – from various blue and green to yellow.
Finally, they used the results to predict the colors that other deep-sea fish could see with several rod pigment proteins. The shape of the protein is the key, because different shapes are sensitive to different wavelengths of light.
Their work shows that lantern fish, eye-tubes and old spinyfin may be able to detect blue light, as well as shades of green and yellow-green. But they will not have the reach as wide as silver spinyfish.
Without behavioral experiments, scientists cannot know for certain whether these fish actually use their sticks to see colors. Experiments will be difficult because fish are not only hard to come by, they don't live long as they are brought to the surface, Salzburger said. (The water pressure at sea level is much lower than is usually the case in the deep ocean.)
However, scientists who were not involved in the study agreed that identifying fish with many protein stem pigments was new.
Biologist David Hunt, a professor emeritus at the University of Western Australia who specializes in the evolution of vertebrate vision, calls his findings "quite astonishing."
"It is something unknown and completely unexpected," he said. "I'm still trying to make my head really understand what that means."
Los Angeles Times