On July 11th, 2016 we learned about

The kids who see clearly at the bottom of the sea

For the past four years, my second grader has taken swim lessons as the local pool. From the first lesson, the kids wore goggles in the pool, and at this point my daughter feels so dependent on them that she seems to forget all the strokes she knows if goggles aren’t available. I don’t dispute that seeing underwater makes swimming a lot easier, but it turns out the goggles may not have been strictly necessary. Children from a tribe in Thailand have been found to have amazing visual acuity underwater, spotting details deep underwater where most people would be left with a dark blur. The trick can be achieved with practice, but it’s also an ability that nobody can hold onto forever. At some point, we’re all doomed to goggles.

The kids in question are from the Moken tribe, and from a young age they contribute to their family’s fishing by diving for clams, shells and sea cucumbers. They’re quite adept at this thanks to their apparently superior vision deep in the water. Researchers were interested in just how their eyes were somehow coping with the optics of seeing underwater. Adding to the mystery was that only the kids could see this well— adults from the same families seemed to have more average vision, and switched to spear fishing as they aged.

Light moving through liquids

The physiological issue is how our eyes had to change to work on land, and that that messes up our vision when we go back in the water. Very roughly, every time light switches what material it’s passing though, it’s going to be bent, or refracted, by some amount. In animals that live on land, our outer cornea has a layer of water that bends the light leaving the air, which is then bent again by the lens. Fish, on the other hand aren’t switching media from air to water, and thus accomplish all their focusing with their lens alone. But when you put an eye adapted to focus through air into water, the expected refraction doesn’t happen, and the eye can’t focus very well. Unless your traditional Moken upbringing has helped you train it to handle the water.

Assessing aquatic vision

Figuring out how Moken children were focusing underwater was done by eliminating possible explanations. First the kids were tested to see if they really were seeing better than other kids, and by how much, by identifying visual patterns placed on the sea floor. They did well on that test, plus a test of their vision on land to confirm that their eyes didn’t have any severe restructuring going on. This left options related to temporary adjustments in the water, like changes to their eyes’ lenses, or their pupils.

It was easy to measure that the Moken kids were tightly constricting their pupils, which would help their depth perception, but that change wasn’t enough to explain their aquatic vision entirely. This left lens accommodation, a muscular adjustment to the shape of the eye’s lens, to make up the difference. This explanation actually put a lot of pieces of the puzzle together, as it could also explain why adults couldn’t see as well underwater— as we age, our lenses harden and can’t be reshaped as well. What’s more this is the same way other mammals that once lived on land, like dolphins, have adapted to reentering the sea.

The last question was why this was turning up with Moken kids specifically, since there didn’t seem to be any genetic predisposition to lens accommodation. As an experiment, Swedish children were brought to Thailand to be taught to dive like the locals, and many of them were able to “learn” to see underwater in as little as 11 sessions. The one remaining catch was that the Swedish kids would turn up with red, irritated eyes, while the Moken kids seemed quite comfortable looking through salt water. This is where a real genetic advantage might be at play, but it might not last long enough to test. Tsunamis have pushed Moken villages inland, and younger kids aren’t able to spend as much time in the water as they once did.

Source: The 'sea-nomad' children who see like dolphins by Helen Thomson, BBC Future

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