Asia's 'sea gypsies' show unusual underwater vision

By Linda Carroll

NEW YORK (Reuters Health) - The "sea gypsies" of Southeast Asia, who gather most of their food from the ocean, have adapted to this lifestyle by developing sharp and clear underwater vision, a new study shows.

When researchers compared sea gypsy and European children, they found that the sea gypsy children saw far better underwater, according to the study published in the May 13th issue of Current Biology.

Southeast Asia is the home to a number of populations known collectively as sea gypsies who are renowned for their swimming and diving skills. Unlike most people, who can make out only blurry images when diving without goggles, sea gypsies appear to be able to spot small items such as clams, shells and sea cucumbers without the help of visual aids.

To discover how the sea gypsies could spot such small objects on the sea floor, Swedish researchers set up an experiment in which they compared the underwater vision of children from a tribe of semi-nomadic sea gypsies called the Moken with that of European children vacationing in Southeast Asia.

The researchers tested the children's vision underwater by asking them to look at gratings -- black and white stripes -- of various widths, the study's lead author, Anna Gislen of Lund University, explained in an interview with Reuters Health.

"The children were asked to dive down and then come up and tell us which way these gratings were oriented," she said. "If they managed to correctly identify one pattern underwater, we then used a finer grating -- thinner stripes -- until the child made mistakes, which meant he or she could no longer see the pattern."

The researchers determined that the Moken children could see twice as well underwater as their European counterparts.

The better vision is most likely due to practice, according Gislen.

"I think it's mostly practice, since they swim and dive so much," she said. "But, of course, there may still be genetic components that make it easier for them to learn these things than for European children. We can only speculate."

The researchers examined all the children's eyes on shore and determined that there was no physical difference between the two groups. When they observed the children underwater, the researchers discovered that the Moken children constricted their pupils when diving.

Gislen compares the constriction of the pupil to the shrinking of the aperture in a camera.

"If you take a picture of, for example, a child in front of a painting and choose a large aperture, you will only get good resolution of the child, and not the painting. If you choose a small aperture you will get good resolution on the painting as well."

"In optical terms," she explained, "we say that the focal depth is improved when the aperture is smaller."

A person's pupils constrict automatically when he or she moves into bright light. When diving, the European children's pupils expanded, which was probably a response to dimmer light underwater, Gislen said.

But a person can constrict their pupils -- whether the light is bright or dim -- if they are bringing something into focus, Gislen said.

"When you look at something at close range, you automatically accommodate to see clearly -- accommodation is when the lens in your eyes is squeezed into a more compact shape to refract the light more," she explained.

"What is interesting with the Moken children," Gislen said, "is that they accommodate all through their dive, which (should) be very strenuous for them. Yet they seem to do it effortlessly."