Sea Gypsies of Asia Boast "Incredible" Underwater Vision

Brian Handwerk
for National Geographic Ultimate Explorer
May 14, 2004
For centuries the seminomadic Moken people have lived as
hunter-gatherers, dwelling on boats or stilted dwellings along the
coasts of Myanmar (Burma) and Thailand.

They harvest the sea's bounty by traditional methods. Without masks or scuba gear, they are able to gather tiny shellfish and other food on the ocean floor at depths as low as 75 feet (23 meters).

It's a difficult way to survive, but scientists have learned that these sea gypsies have an important edge. Studies of Moken children have shown them to have incredible underwater vision—twice as good as that of European children of the same age.

Anna Gislén, of Sweden's Lund University, did the study after hearing of the Moken's talents from a colleague.

"Another scientist, Erika Schagatay, was in the south of China working with sea nomads and their diving response," Gislén recalled. "She noticed that the children were picking out small brown clams from among brown stones. To her, this was incomprehensible, as she could hardly see them with goggles, and the children used no such thing. It was not her area of science, so eventually it ended up on my desk."

Gislén ventured to Thailand's Surin islands where she conducted underwater tests on Moken children and compared their scores with those of European kids vacationing in the area. Her results were first published in the May 13, 2003 issue of Current Biology.

Gislén found no differences in the children's respective eye structures or in their vision on land. Underwater, however, it was a different story. The Moken children displayed underwater vision twice as sharp as their European counterparts.

Their secret lies in the way their eyes adapt to the underwater environment.

Underwater Adaptations

The refractive power of the eye's corneal surface, a key to clear vision, is greatly reduced underwater. The different densities of air and water cause the problem. Water has similar density to fluids inside the eye, so refraction is limited as light passes into the eye.

But the Moken are able to accommodate, or muscularly change the shape of the eye's lense, in order to increase light refraction.

"It seems they have learned to control their accommodative response, such that they can voluntarily accommodate even in the blurry underwater environment," Gislén explained.

"Normally, severe blur does not elicit accommodation, and no accommodative response can be found in untrained European children." The Moken's pupils also adapt, constricting to a mere 0.08 inch (1.96 millimeters). The European children's pupils constricted to only a tenth of an inch (2.5 millimeters).

"Their constricting pupils improve vision further," Gislén said. "It's the same process that improves focal depth if using a camera with a smaller aperture."

The Moken children use these adaptations to forage for small clams and sea cucumbers at depths of 10 to 13 feet (3 to 4 meters). It's a key to survival, but is it learned, or might there be a genetic component?

"I think that in general this is very hard to know," Gislén said. "Genes and environment are so intertwined it's hard to separate them. What I do know is that we have [more recently] trained European children to become as good at underwater tasks as the Moken children. So training seems to do the trick."

"However," she continued, "I cannot rule out that genes may influence the speed of learning, or that the Moken children may be better at things we did not test underwater, due to some genetic component."

Gislén hopes to continue her research further afield, testing and comparing the underwater vision of other sea nomads who dive even more than the Moken. Research is expensive, however, and further limited by the shy and reclusive nature of many sea peoples.

Other subjects may be similarly elusive.

"I have also heard about monkeys that forage in the waters around Sri Lanka," Gislén said. "It would be interesting to see whether they use the same strategies as humans apparently do to see food items on the seafloor."

As fascinating as her study has been, Gislén stresses that her research is just a single example of the incredible adaptive powers of the human body.

"I think that the human body is extremely flexible, much more than we may be aware of," she said.

"The diving response is another good example of adaptation," she continued. "Some tribes of sea nomads in the Philippines can dive down 200 to 230 feet (60 to 70 meters), pick some pearls and then go up again, holding their breath for about six to seven minutes., Gislén added, "Europeans told to do the same thing would just shake their heads and say it was impossible. But clearly it's not."

Neither is clear underwater vision, apparently, if you have the right training—or if you happen to be a Moken.

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