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How to see shells on the sea floor
Southeast Asian water babies have supreme aquatic vision
16 May 2003

HELEN R. PILCHER


Moken children use their special powers to hunt for shellfish.
© A. Gislén

In the sea, most of us are half-blind - but the Moken are king. This Southeast Asian tribe of sea gypsies can see twice as clearly underwater as Europeans, researchers have found.

The semi-nomadic Moken, who have settled on Thailand's Surin Islands, use their superior visual skills to dive for food on the ocean floor. It's not known whether the ability is learned or genetic.

Most of us see blurred images when we dive without goggles or a mask. The eye is adapted to air, and struggles to focus light under water. But Moken children can pick out small shells, clams and sea cucumbers at depths of three to four metres.

Moken children can distinguish underwater objects less than 1.5 millimetres wide; Europeans struggle to make out anything less than 3 mm across1, biologist Anna Gislén of Lund University, Sweden, and her colleagues found.

"They use the optics of the eye to the limits of what is [humanly] possible," says Gislén. The team compared the sub-aqua vision of native Moken and holidaying European kids, aged 7-14 years.

Unlike visiting children, the Moken swimmers constrict their pupils while diving, the team found. They can also squeeze their eyes' lenses more, making them thicker and better able to bend incoming light. The two processes bring blurry images into sharper focus, explains Gislén.


Moken swimmers constrict their pupils while diving.
© A. Gislén

"This is the first report I have ever seen about underwater divers and vision," says neurobiologist Howard Howland of Cornell University in Ithaca, New York. "It could be a learned response, albeit totally unconscious," he suggests. Alternatively, evolution may have favoured those with genetic adaptations for better underwater vision.

The ability to squeeze the lens like this is normally weak in children under ten, and strong in adults, says Howland. The underwater abilities of Moken grown-ups remain unknown. "We couldn't test the adults," explains Gislén. "They're too shy."

References

  1. Gislen, A. et al. Superior underwater vision in a human population of sea gypsies. Current Biology, 13, 833 - 836, (2003).

 

Source: © Nature News Service / Macmillan Magazines Ltd 2003

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(For more information on Anna Gislen et al.'s report on "Superior Underwater Vision in a Human Population of Sea Gypsies", please see the Current Biology website)

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