In the ocean, invisibility trumps ultra-vision
The mirror-like scales on some fish provide a built-in invisibility cloak. But have predators evolved better vision to outsmart them? Amy Middleton reports.
In an oceanic battleground, which underwater superpower would win – extreme camouflage or polarisation-enhanced ultra-vision?
That effectively is the question addressed in a new study into the combative and defensive traits of underwater species on the Great Barrier Reef. And camouflage seems to trump super-sight.
The work, by researchers from the US and Australia, was published in Current Biology.
Polarisation vision appears on the CVs of both land and marine animals including insects, octopus and squid, shrimp and other crustaceans, and some fish, including trout and salmon. It’s the ability to see light waves travelling at a certain angle, such as vertical or horizontal, while filtering out the rest.
Polarised glare-reducing sunglasses, for instance, block horizontally polarised light waves, which are produced when light bounces off water's surface.
In the depths of the ocean, though, this inbuilt ultra-vision's role is unclear. Some biologists guessed the trait might serve as a counter-attack to another underwater superpower – that is, how some silvery fish to camouflage in the open water, mirroring their surrounds with their reflective scales, acting as a kind of invisibility cloak.
Does that mirror camouflage reflect polarised light which is then detected by predators?
A team of Australian and US researchers led by Sönke Johnsen at Duke University in North Carolina wanted to answer this question by seeing whether polarisation vision made reflective, mirror-scaled fish more visible at a distance.
"Sighting distance is important because hunting and avoiding being eaten in the open ocean is about seeing other animals before they see you," says Johnsen.
"Once you're seen, you're dead. It's over."
Johnsen and his team used custom-built underwater cameras with tiny polarising filters to photograph 12 species of reflective fish in the Great Barrier Reef, including tuna, amberjack, barracuda and queenfish.
To analyse the images as though they were being processed by a marine creature’s eyes, the team measured both the polarisation and the brightness of the light reflected from each fish and its watery backdrop. The data was combined with a mathematical model to see how those images were visually perceived.
Among the images, each fish was predictably difficult to see against its reflected backdrop. And they found a slight change in polarisation between the fish and its surrounds, suggesting they could be visible in some contexts.
But when the visual perception model was applied, the results suggested brightness, rather than polarisation, gave fish away to their onlookers.
This suggests polarisation vision does not help animals spot a reflective fish from a distance – rather the fish would be just as visible to animals with regular vision.
The team is now keen to repeat the study at close range to these fish. The mystery of the ultra-vision continues.