For fish in blackness, colours abound

Contrary to expectations, some deep-water fish species see in colour, researchers have discovered.

The depths of the ocean are unimaginably dark. Any remaining light from the surface is mostly blue and thus, it has long been assumed, for the denizens of the deep the world is dim, drab and monochromatic. 

But now an international team of scientists have found the assumption to be wrong, after discovering that many deep-sea fish species come equipped with a range of previously unknown vision-related proteins.

Vertebrates use sight for just about everything: from foraging and avoiding being eaten by passing predators, to navigation and choosing a mate. This vision is based on two types of photoreceptor cells: rods and cones. The cells contain light-sensitive proteins called opsins, which come in several varieties.

Cones deal with bright light situations and have four kinds of opsins, while rods are more specialised for low light conditions and in 99% of vertebrates contain only one opsin type. This means that most vertebrates are near colour-blind in dim light.

It had been long thought that deep sea fish, living between 200 and 1500 metres below the surface, were in the same situation.

Research published in the journal Science reveals some remarkable exceptions. 

Lead author Zuzana Musilova of Charles University in the Czech Republic, senior author Walter Salzburger of the University of Basel in Switzerland, and a team that includes a trio of scientists from the Queensland Brain Institute (QBI) in Australia, have uncovered the makings of a vast array of new opsins hidden in the genomes of deep sea fish.

Because deep sea conditions are rather uncomfortable for the average scientist and surface conditions inevitably fatal to deep water fish, the team had little choice but to investigate deep sea vision indirectly, through genomics. {%recommended 7431%}

Fabio Cortesi, one of the authors from QBI, says researchers examined the genomes of 101 types of fish and “found 13 species had more than one rod opsin gene, and one – the silver spinyfin fish – had a remarkable 38 of these opsins”. Oddly, most of the 13 species were not closely related to each other.

What the researchers discovered was that there were genetic variations leading to changes in 27 amino-acids that are part of rod opsins. These are key, given that rods are the dominant component of deep-sea vision, because they are adapted for low-light conditions.

Changes in the amino acids correlate to changes in the rods’ ability to detect different wavelengths of light. They were most abundant in the silver spinyfish (Diretmus argenteus).

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

So, the ocean dark is actually a colourful place for it and a handful of other species. But, as Fanny de Busserolles, also from QBI, notes, “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.”

Why then does the spinyfish need the ability to see colour?

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

The light given off by various organisms, often in multicoloured flashes, is the main illumination source below 200 metres and it seems that these remarkable fish have adapted to this element of their environment. 

“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,” says Cortesi. 

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