Scientists at the Queensland Brain Institute (QBI), in collaboration with an international team of researchers, have discovered that certain species of deep sea fish may have highly sensitive colour vision.

In particular, they have shown that the visual system of the silver spinyfin (Diretmus argenteus) has the highest number of light-sensitive proteins, called opsins, of any vertebrate known to date, even though it lives at depths where most sunlight has been filtered out.

Two types of photoreceptor cells — rods and cones — enable vision in vertebrates, and both contain specialised opsins.

“Vertebrates possess up to five types of opsins, and most of these tend to be used in cones,” said Dr Fabio Cortesi, a postdoctoral research fellow at QBI and co-first author of the study.

“Cones generally operate in bright-light conditions and using a variety of opsins allows sensitivity to a broad range of colours.”

“However, in dim-light, vertebrates generally rely on rod cells, which are sensitive to faint light, and 99% of all vertebrates have just one opsin protein in their rods.”

“Consequently, most vertebrates are colour-blind in dim-light conditions because they rely only on that single rod opsin,” he said.

Deep sea fish that live at 200 to 1500 meters below the surface tend to be no exception.

“At that depth, the water has filtered out most light other than blue wavelengths of 450 to 500 nm,” said QBI’s Dr Fanny de Busserolles, a deep-sea specialist on the team.

“Down there most of the world is faint blue light — it’s very monochromatic — so most fish have just one rod opsin to perceive blue light.”

“Like nocturnality, ‘deep sea-ality’ tends to push animals to reduce their colour sensitivity.”

Now Dr Cortesi, Dr de Busserolles, and Professor Justin Marshall at QBI, along with an international research consortium, including Assistant Professor Zuzana Musilova at Charles University in the Czech Republic and Professor Walter Salzburger at University of Basel in Switzerland, have discovered some spectacular exceptions.

“We looked at the genomes of 101 species of fish, and most have only one rod opsin gene,” said Dr Cortesi. “However, 13 species have more than one rod opsin gene, and 4 of those, all from the deep-sea, were found to have 5 or more rod opsin genes.”

Specifically, glacier lanternfish (Benthosema glacial) have 5 rod opsins, tube-eye fish (Stylephorus chordatus) have 6, and longwing spinyfin (Diretmoides pauciradiatus) have 18.

Remarkably, silver spinyfin (Diretmus argenteus) have 38 rod opsins.

By analysing gene activity patterns, the researchers determined these fish are indeed using their expanded repertoire of rod opsins. In particular, the silver spineyfin uses seven of its rod opsins as larvae, and uses 14 during adulthood.

New vision system found in the dark

Most deep-sea fish can’t survive at surface pressure and temperature, so it’s difficult to do behavioural studies, but it’s still possible to infer which colours an animal might see using genetic data, Dr Cortesi explained.

In particular, rod opsin proteins contain a cluster of 27 key-amino acids that directly affect which wavelengths of light are absorbed. A change in any one of these can influence which colour light the protein is sensitive to.

“24 of the possible 27 amino acid changes observed across all vertebrates re-appear in the rod opsins of the silver spinyfin fish,” said Dr Cortesi. “This suggests they’re picking up a wide range of different wavelengths of light.”

“We also reconstructed some of the proteins to measure their light sensitivity and then used protein simulations to verify our results, this showed they are indeed diversified across the visible light spectrum.”

The findings support the existence of purely rod based colour vision systems.

Why would they need this trait in such dim, monochromatic light?

“A likely explanation is for detecting bioluminescence,” said Dr Cortesi.

Predator or prey?

At depths below 200 meters, bioluminescence increasingly replaces surface illumination as the primary source of light.

“There are many colours of bioluminescence down there, and it mainly appears in flashes,” he said.

“So they may be able to distinguish between different bioluminescent flashes or might have evolved specific behaviours hard-wired to different colours of bioluminescent light.

“If you want to survive down there you need to quickly decide if you want to avoid being eaten or eat what you see.”

Intriguingly, it seems that deep sea fish with multiple rod opsins are not closely related.

“They’re separated by more than 100 million years of evolution,” said Dr Cortesi. “It looks like they have evolved this attribute independently.”

Dr Cortesi, Dr de Busserolles, Professor Marshall and their colleagues are now broadening the study to include other deep sea fish and are also looking for shallow water relatives of spiny silverfish that may have evolved a large repertoire of rod opsins.

The study is published in the journal Science.