By Safiya Aldris
The Mergui Archipelago, a group of around 800 islands in the Andaman sea, is home to the Moken. They are a semi-nomadic Austronesian people living a hunter-gatherer lifestyle at sea, though this way of life is increasingly under threat (Survival, n.d.). The children of these small Moken tribes spend much of their day diving in the sea, collecting food from the sea floor or catching fish, and as a result have become uniquely adapted to seeing incredibly well underwater.
The reason for human’s inability to see underwater is due to the structure of the eye. When light travels from air into another medium, such as water, it is refracted. The refractive index of a substance is how much slower light travels in it compared to in a vacuum; air slows light down less than water does, which is why light bends more when it enters water. Human’s exploit this effect by having rounded corneas and the aqueous humour in front of their eyes; the cornea and liquid bend incoming light onto the retina (Holcombe, 2011). Because the difference in medium between the water and the aqueous humour is too similar, the light is not bent completely on to the retina. It was hypothesised that in the Moken children, their eye structure may be different, allowing the light to bend more effectively, allowing clarity in sight underwater.
If the Moken had evolved a flatter cornea, the light would refract successfully onto the retina underwater, thereby improving their resolution when diving; this is observed in amphibians. They could also be severely myopic, improving underwater vision, but resulting in blurred vision on land. While these strategies exist in the animal kingdom, they were not observed in the Moken children.
Visual acuity was tested in Moken and European children of the same age range who were on holiday in the same area. The Moken children had significantly better underwater acuity, able to see more than twice as well as the European children. However, on land, there was no difference between the corneal curvature or refractive state between the two groups of children. This means that their ability to see underwater is not due to the structure of their eyes (Gislén et al., 2003).
The most crucial difference seen between the two groups of children concerned their pupil size when underwater. On land, pupil size barely affects resolution, and both groups had the same pupil sizes in differing light intensities. Underwater, where it is incredibly blurred, a smaller pupil size will improve resolution through decreasing the diameter of the ‘blur circle’ on the retina (Thibos and Thibos, 2011). There are two opposing functions as the Moken children dive; as they go underwater, light levels decrease, which would trigger their pupils to dilate. At the same time, their eyes are constricting, and their lens’ accommodate to see their surroundings. In the European children, there seemed to be no accommodation. The Moken children push their eyes to the known limit of human performance when accommodating (Horstman, 2003).
It has been argued that genetics also play a part in their ability to see better, with natural selection favouring those who are able to intrinsically accommodate better underwater. To test the plausibility of this, an experiment was conducted to see if it was possible to teach these accommodative and constrictive abilities to European children who did not live off the water (Gislén, Warrant, Dacke and Kröger, 2006). The European children took part in diving training tasks over a month and achieved the same level of underwater acuity as the Moken children, despite their eyes still being irritated by the water. They retained this ability 8 months after the training sessions, and this achieved performance can be explained by the combined effects of the strengthened accommodation, and pupil constriction.
Moreover, Moken adults do not show the same underwater vision; as we age, our lenses become less flexible and less able to accommodate, explaining this loss in clarity. Both this and the training of the European children acts as evidence against genetics having a role in the Moken children’s adaptations.
Due to the political climate the Moken are trying to survive in, as well as the changing world, they are now spending much less time in the water; this means further studies of the Moken people and their underwater vision is not possible. While the children studied retained the ability into their adolescence, it is doubtful that any newly born children will develop it (Thompson, 2016). The tests done may mean it is possible to train children to accommodate underwater, giving them the ability to see as clearly as the Moken.
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