How can fish see clearly underwater?

See clearly without diving goggles

The Moken people live from the sea. The semi-nomads traditionally travel with their boats between the islands of the Andaman coastal region west of Thailand, Myanmar and Malaysia and dive on the seabed for mussels, sea cucumbers and other marine life. Even the children bring them up from a depth of three or four meters. In contrast to European children, the Moken children can see objects underwater that are only 1.5 millimeters wide? Europeans already have difficulties with objects under 3 millimeters in diameter.

"The Moken children use the optics of the eye to the limit of what is humanly possible," explains Anna Gislén in the online service of "Nature". When diving, they make their pupils smaller and their eye lenses close-up, which means that the incoming light is refracted more strongly. In this way, blurred images are brought into an area of ​​higher visual acuity.

The human eye is inherently ill-suited to seeing underwater. Water has a higher density than air, so that light is refracted differently under water than on land and the view becomes blurred. In children under ten years of age, the ability to bend the lens of the eye more sharply for near vision and thus to compensate for the changed conditions under water is normally only very poorly developed? not so with the Moken children.

“It could be a learned answer that takes place subconsciously, or a genetic adaptation to better underwater vision that has arisen in the course of evolution,” suggests neurobiologist Howard Howland from Cornell University to “Nature”.

It remains unclear what skills the adult swimmers of the marine nomads have. “They're too shy, so we couldn't test them,” says Anna Gislén.

May 20, 2003

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