THE Moken people of South-East Asia, who live by fishing and diving, can see twice as well underwater as Europeans. This extraordinary ability might have evolved during the thousands of years the Moken have led a semi-aquatic life, or it could be a skill learned during childhood.
The difference between the refractive index of air and that of the eye enables us to focus images. Things appear blurry underwater because water has a similar refractive index to the eye, so the eye cannot bend light enough to focus it on the retina, effectively making us very long-sighted. So Anna Gislen at the University of Lund in Sweden was intrigued when told that the Moken can spot small objects underwater without goggles.
The team tested the vision of Moken children in Thailand against children of European holidaymakers. Above water the Mokens鈥� vision was no better. But when they asked the children to look at barcode-like patterns of black and white lines underwater, and indicate whether the lines were vertical or horizontal, the Moken children correctly identified the orientation of lines more than twice as fine 鈥� over 6 lines per degree (Current Biology, vol 13, p 833).
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Infrared pictures taken underwater revealed that the Moken children contract their pupils, which sharpens their focus just like a pinhole camera. To fully account for their better vision, says team member Marie Dacke, they must also make the lens as convex as possible, which is what people do when looking at a very close object. It was not possible to measure this underwater. The team is now training children in Sweden to see if they can improve their underwater vision.
The ability of the Moken to contract their pupils underwater is at first sight surprising, says underwater vision expert Ron Douglas of City University, London, as the natural reflex is for the pupils to dilate in dim light. But the size of the pupil can change even when the light level is constant, he points out, narrowing when we focus on close objects, or responding to changes in patterns, colours or even someone attractive of the opposite sex. Seals use a similar trick, narrowing their eyes to a slit to compensate for their short-sightedness above water.