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Colour vision in complete darkness? Scientists made a surprising discovery in the deep ocean

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Posted May 11, 2019

There is no colour without light. Different things in your environment reflect light differently and appear to be different colours – that is all. In the dark all things are colourless. However, now scientists are noticing that some fish living at the depths of 1500 meters below the surface level have developed surprisingly diverse vision. This is surprising, knowing that they live almost in complete darkness.

Ailver spinyfin (Diretmus argenteus) is one of those deep-ocean fish that has an enhanced colour vision. Image credit: Emma Kissling via Wikimedia

Evolution is actually pretty simple – you lose everything you don’t need. This is why those animals that live in the deep ocean or underground are usually almost completely blind. But now an international research team involving University of Queensland scientists made a surprising discovery – some fish in the deep ocean can actually see colour. This could be a way for them to determine predator from prey in the dimly-lit depths of the ocean. Interestingly, scientists say that it is not all about those fish – this discovery can uncover a lot of truth about the evolution of vision in vertebrates, including humans.

Vertebrates that can see have rods and cones in their eyeballs. Cones are used in bright light conditions and rods – in the darker environments, but both types of photoreceptor cells contain light-sensitive proteins called opsins. You can find several types of opsins in your eyes, which makes us see colour. However, 99 % of vertebrates have only one type of opsin, which is why they do not possess a coloured vision. Seeing colour is simply not important for many animals and you would assume that deep ocean fish is one of them.  Water filters most of the light out anyway, so why would they see colour?

Scientists analysed the genomes of 101 species of fish and found 13 species that had more than one rod opsin gene. Quite spectacular, having in mind that most of fish at that depth perceive blue light only – it is very monochromatic down there anyway. Silver spinyfin (Diretmus argenteus) has 38 of these opsins, which would allow it to see a lot of colour. But the question stands – why evolution would create this ability?

Dr Fabio Cortesi, one of the authors of the study, said: “There are many colours of bioluminescence – light produced and emitted by living organisms – down there, and it mainly appears in flashes coming from other fish. If you want to survive down there you need to quickly decide if you are seeing a potential predator or potential prey”.

That’s right – some creatures in the deep ocean produce their own flashes of light, which is why seeing colour is actually a pretty useful trait. While there is very little light down there, it is important to differentiate it. In this case it helps some fish to survive in a hostile environment.

 

Source: University of Queensland

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