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Asian child divers see better underwater

South East Asian children who dive for their livelihood can see twice as clearly underwater than European children of the same age, new research has found.

The findings are reported by Anna Gislén from Lund University in Sweden and colleagues, in a recent issue of the journal Current Biology.

The researchers noticed that children from the Moken people - who live along the Burmese archipelago and Thailand's western coast - were capable of seeing small objects underwater without the aid of goggles. In the clear waters of Thailand's Ko Surin National Park, they set out to test whether the childrens' eyes were specially adapted by comparing the underwater vision of 17 local Moken children with 18 holidaying European children.

As nomadic seafarers, the Moken people are renowned for their swimming and diving skills. Traditionally, they have depended on marine resources - male adults spear fish, and children dive for clams and sea cucumbers.

"When I first saw them [the children], I was instantly struck by their familiarity with water and - though it may sound like a cliché - they looked a bit like a school of little fish swimming around," Gislén told ABC Science Online.

The researchers tested the children's sharpness of vision by asking them to look at different black and white patterns underwater - which required different degrees of focussing power - and to identify the same pattern once they surfaced. The results showed that the Moken children could see fine detail more than twice as well as the Europeans.

The human eye uses its curved cornea and internal lens to bend light and focus sharp images on the retina. However two-thirds of this refractive power is lost in water, which is why we are generally unable to focus without the aid of a mask or goggles.

To explain the Moken childrens' improved underwater vision, the researchers performed several experiments to examine the features of the eye that affect its ability to focus. They measured the curvature of the cornea, the ability of the lens to change shape, and the size of the pupil, which regulates the amount of light that comes into the eye.

On land, they found that there were no significant differences between the eyesight of the two groups of children and both were found to have the same pupil size. Underwater, however, two important differences became clear. Moken children are able to constrict their pupils to the smallest point possible - 1.96mm across, 20% smaller than the Europeans' pupils. They can also change the their lens shape to the known limit of human performance.

"This extreme reaction - which is routine in Moken children - is completely absent in European children," the researchers write. "This behaviour is clearly an adaptive strategy."

Pupils usually widen underwater because there is less light. The Moken children's smaller underwater pupil significantly improves the resolution of images because it decreases the diameter of what is known as the "blur circle" on the retina.

Gislén believes the Moken children have learned to control the tiny muscles that shift their lenses and pupils: "Several people have asked me whether this could be genetic changes behind their ability. Of course I cannot rule them out. But I think it's mostly about learning."

"All children should theoretically be able to accommodate this much. The Moken do not do anything superhuman - they simply use the eye to its limits", she said. "I think it would be possible for all human beings to see better underwater, provided you practised...We are training Swedish children to see if they can learn."