Pinholes. This infrared image of a Moken child's eyes underwater shows how much pupils can constrict.

Sea Travelers See Better

Divers know that all sorts of equipment is necessary to function underwater, including goggles to correct water-warped vision. But researchers have found that some Southeast Asian seafaring people can see underwater very clearly without such corrective devices. They push their eyes to the very limit of human perception.

Human eyeballs are filled with a watery fluid, so they have almost the same refractive index as water itself. Thus light coming into the eye isn't bent in the same way when underwater as on land. The unfocused light rays reduce clarity by about two-thirds. But certain Southeast Asian nomads dive and swim without eye protection and still manage to harvest small camouflaged shells, clams, and sea cucumbers. In order to find out their secret, a team of researchers from Lund University and the Institute of Clinical Neuroscience in Mölndal, both in Sweden, went to Thailand and Burma to study the Moken tribe.

On land, Moken children had the same visual acuity as a group of vacationing European children. Underwater, the Moken children could see about two times better than the European children, according to tests using grids with varying fineness. The researchers observed that Moken children could control the size of their pupils, constricting them underwater to the smallest diameter humanly possible--about 2 millimeters--whereas the European children's pupils opened slightly, the researchers report in the 13 May issue of Current Biology. Smaller pupils improve resolution, focusing light rays like the tiny opening in the front of a pinhole camera. According to the researchers' calculations, the Moken children also change the shape of their lenses with their eye muscles to focus even better.

Such so-called accommodation may be a genetically inherited trait useful for people who rely on the sea for food. But more likely, the response is learned, says Anna Gislén of the Lund team. "It is a surprise," says Tom Cronin of the University of Maryland, Baltimore County. "You would expect ... that [human] vision wouldn't change much from place to place." The children, he says, are accommodating "as well as anyone can do it." Gislén and her co-workers are training European children now to see if they can learn the same techniques.

Related sites
Anna Gislén's Web page
Underwater eyes in animals
UNESCO Surin Islands Project: Indigenous Peoples and Parks
When underwater, Moken children can see grids more than three times finer than this one.