Environment

More than 1000 metres beneath the ocean’s surface, these fish may have a unique way of seeing colours

26Views

Scientists have long presumed that the creatures in the deep ocean experienced a dark, colourless world. But some of the fish who live there may be able to see colours thanks to a newly discovered visual system that has never been seen in vertebrates before.

The find, reported in the journal Science, challenges long-held assumptions about how these animals perceive colours.

A deep-sea lanternfish. Some members of the lanternfish family were found to use multiple rod opsins within their eyes, allowing them to see colours in dark waters.

A deep-sea lanternfish. Some members of the lanternfish family were found to use multiple rod opsins within their eyes, allowing them to see colours in dark waters.Credit:LA Times

"We didn't expect this at all," said study co-leader Fabio Cortesi, a marine biologist at the University of Queensland in Brisbane. "We were like, 'Whoa, what's happening?'"

Christopher Kenaley, a biologist at Boston College who wasn't involved in the study, said the report should prompt scientists to reconsider the conventional wisdom that deep-sea fish have very limited vision. "There's some important questions in the deep sea about how animals communicate down there," he said. "This gives us insight about how they might be able to detect one another."

Advertisement

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 different types of light and relay that information to the brain.

A typical vertebrate eye has multiple types of cones that work in bright conditions – each capable of sensing a certain range of colours – and one type of rod that senses light when the environment is dim. The rods can't distinguish between colours because they all have the same pigment protein, which is why humans and most other animals are said to be colourblind at night.

Cortesi and his colleagues wondered if they could find some exceptions among fish who lived in perpetually dark environments. Their question was prompted by a 2015 study of mostly shallow-water fish that turned up several species with more genes for cone pigment proteins than scientists had expected.

"We just thought if other fish are more variable in their visual system than previously thought, we should look at the deep-sea fishes," said Walter Salzburger, an evolutionary biologist at the University of Basel in Switzerland who oversaw both the 2015 study and the new one. After all, if any fish stood to benefit by having more ways to see in dark conditions, it would be fish that live in water so deep that light barely reaches them.

Very little is known about fish that reside more than 1000 metres below sea level. Some developed large pupils and very long rods that help them catch whatever light is around. (At those depths, most of the light is produced by fish themselves through bioluminescence.)

For the new study, the researchers started by counting the number of genes for both rod and cone pigment proteins in the genomes of 101 species of fish living in a diverse array of habitats. Although they found a dozen species with up to seven cone pigment genes, what really struck them was the discovery of 13 species that had more than one rod pigment gene.

Four of those species stood out with five or more of the genes: the tube-eye (Stylephorus chordatus), the glacier lanternfish (Benthosema glaciale), the longwing spinyfin (Diretmoides pauciradiatus) and the silver spinyfin (Diretmus argenteus).

All four of these fish live 1000 metres to 2000 metres below sea level. Their most recent common ancestor dates to more than 100 million years ago, so the researchers think the additional genes evolved independently in each lineage.

"Is it to see prey species? Or to find mates in a completely dark, or almost dark, environment? Or to avoid predators?" Salzburger asked. "These are the three main evolutionary advantages we can think of."

But were these fish actually using their extra pigment proteins? To answer that question, the team examined specimens representing 36 different fish species. Some tissue samples were already preserved in laboratories, and others were acquired on fishing expeditions.

Cortesi and other researchers dragged a net through the ocean from Perth to Sri Lanka. They trawled at night so the fish wouldn't encounter sunlight that might damage their eyes. It could take six hours to fill just one little bucket of the thumb-size fish, Cortesi said.

Most of the 36 species had only one active gene for producing rod pigment proteins. The species with at least five rod pigment genes had at least three that were active.

The star was the silver spinyfin. It had 38 genes for rod pigment proteins, and 14 of those proteins were actually at work inside the eye. (For the sake of comparison, most humans use only three types of cone pigment proteins to see the world in colouRead More – Source