Children of sea see clearly underwater. (Gypsy Secret).

For hundreds of years, small nomadic tribes called sea gypsies have lived among the islands of Southeast Asia, earning fame for their swimming and diving skills. Sea-gypsy children regularly collect food such as clams and sea cucumbers off the ocean floor. A research team studying one sea-gypsy tribe has now found that its children have better-than-normal underwater vision because their eyes adapt to the liquid environment.

While some animals such as frogs can see equally well on land and in water, the human eye has evolved to work best in air. Underwater, its focusing capability significantly deteriorates. That's why people typically need goggles to see clearly when diving.

Intrigued by tales from a colleague who had observed the food-collecting prowess of sea-gypsy children, vision researcher Anna Gislen of Lund University in Sweden decided to investigate how such kids can pick out small objects while diving without goggles. Since many sea-gypsy tribes live on boats in remote areas and dislike strangers, Gislen and her colleagues had to find a tribe willing to be studied. The researchers eventually worked with the Moken, a tribe living in the archipelago along the west coasts of Burma and Thailand.

Initially, the researchers compared the underwater vision of 6 Moken children with that of 28 European children visiting the region. In eye exams conducted in local waters, the sea-gypsy kids had superior resolving power and better perception of contrast, Gislen's team reports in the May 13 Current Biology.

Using an infrared video camera to film the eyes of both groups of children underwater, the investigators found that Moken kids constrict their pupils while European children don't. And the sea-gypsy children could also change their visual focus--in what researchers call accommodation-more than the European kids typically could.

"The small pupil and the accommodation both would serve to increase visual acuity, says David Guyton of Johns Hopkins Medical Institutions in Baltimore. Despite this improvement, the underwater vision of the sea-gypsy children is still impaired compared with their above-water eyesight, he adds.

Since sea-gypsy tribes have depended on the ocean for hundreds of years, it's possible that the Moken children have inherited genetic variations that enable them to see more clearly underwater, the researchers note. Or it may simply be that with regular diving, the eye learns to adapt to the underwater environment.

"My guess is that it's learning," says Gislen. She and her colleagues have preliminary evidence that with training, European children can develop better underwater vision in a few months. They still don't match the sea-gypsy children, who spend years diving for food, Gislen notes.