Vision technology adds colour to life in the ocean

 

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Marine creatures can see their environment in ways that are beyond the natural limits of human perception. For them, it’s a matter of survival; for us, it’s a source of wonder and innovation.

When David Attenborough set out to film his recent BBC series Life in Colour, he ran into a problem with how to illustrate the complex colour vision of marine animals, such as clownfish, which can see UV light in addition to the red, blue and green wavelengths of light that we can detect.

One of the experts the broadcaster approached for help was the Queensland Brain Institute’s Professor Justin Marshall, who has been making discoveries about the vision of marine animals for 30 years.

“How do you illustrate something that we can’t see?” said Professor Marshall, who heads up the Institute’s Sensory Neurobiology Lab. “We designed cameras to simulate how other animals see in ways that can’t even imagine. We filmed both in colour and UV and overlapped the two, using a beam-splitter mirror, so you can clearly see what we normally can’t.”

In the marine environment some species can see UV light or detect polarised light, which allows for very different colour vision to humans. “Many animals have much better colour vision,” he said.

For a scientist interested in studying the evolutionary diversity of vision systems, the ocean is the place to go, said Professor Marshall. “Light in the marine environment is a lot more variable than it is on land, so there are many more amazing visual adaptations in the sea. There are some pretty cool ones on land, but the ocean and its light environment have led to more weird things.”

Vision key to life under the sea

While human eyes are only sensitive to light of red, blue and green wavelengths, lobster-like  creatures called mantis shrimps have a remarkable 12 different types of colour receptors in their retinas. As Professor Marshall’s team has discovered, other crustaceans and many fish also have unique vision systems and can differentiate many types of light, allowing them to collect more information with their eyes and see greater contrast in the environment.

Some invertebrates, such as mantis shrimp and fiddler crabs, can also detect the polarisation state of light. They “use it like colour, but we can't see this at all,” he said. Polarisation can be thought of as the direction of vibration of light waves – up and down, for example, or side to side. Polarised sunglasses help us to reduce glare by filtering out reflected light, but mantis shrimp use the polarisation of light to detect objects and make decisions about what to do with them.

Illustrating a world seen through polarised light for television presented yet another challenge. The Sensory Neurobiology Lab’s Dr Sam Powell has developed “underwater cameras which show polarisation contrast, by converting it into colour so we can see it,” Professor Marshall explains. “We helped Attenborough’s team translate polarisation into colour and provided a sort of a heat map revealing it, a bit like how heat is revealed by an infrared camera.”

Colour and survival

All the visual complexity in the ocean has led to an incredible diversity of colours. Reef fish, for example, are among the most colourful of all creatures. “It’s the most colourful environment on Earth,” he said. “It’s the reef where you find a lot of colours all packed in together.”

This adaptation allows reef fish – which are often similar in size and shape – to recognise their own species, Professor Marshall said. Colour helps them determine whether they want to eat, flee from or mate with other fish they encounter.

“They have these obvious colourations so they can tell each other apart. Colours are often about sex – they’re about male fish advertising to females,” he adds. “But then of course there’s a problem with camouflage as well…and you end up with this wonderful mixture of colour; a combination of conspicuousness and camouflage.”

Mantis shrimp can detect the polarisation state of light - an ability that doesn't exist for humans.

As David Attenborough has said: “We’re only just beginning to understand the many ways in which animals use colour, particularly those colours we can’t even see. For us colour in the natural world is a source of beauty, of wonder – but for animals, it’s a tool for survival.” 

While Professor Marshall’s research on colour visions systems in the marine environment has led to new ways of detecting cancer, improved satellite designs and novels ways of storing information digitally, he said it’s not practical applications that motivate him. What he’s most interested in is fundamental discovery and how fascinating discoveries about the natural world can instil the wonder of nature in people and encourage them to care more about the environment.

“We’ve taken so much nature away from the Earth that we’re killing the planet and in doing so, of course, we’re killing ourselves,” he said. “I want to enthuse people about nature. As David Attenborough said, instead of being apart from nature, we should become a part of nature again. We need to re-wild the world and return animals and environments that ultimately keep us alive.”