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The West Indian Fuzzy Chiton. Credit: Hans Hillewaert

Tiny eyes in mollusc shell allow vision of nearby objects

The eye has taken on several different forms in the animal kingdom including the very unique eye found in a mollusc known as the West Indian fuzzy chiton.

This creature is covered in a hard shell made up of overlapping plates. Under a microscope, these plates are dotted with tiny holes, each a fraction of a millimeter in diameter. It turns out that each of these holes is actually a tiny eye.

These eyes have one very unique property. Ordinarily eye lenses are made out of proteins, such as crystallin in humans, but lenses in this mollusc are made out of aragonite, a form of calcium carbonate, which is also used in the shell that protects it.

An electron micrograph of the chitons shell showing the tiny holes where the eyes are contained. Credit: Wyss Institute at Harvard University.
An electron micrograph of the chitons shell showing the tiny holes where the eyes are contained. Credit: Wyss Institute at Harvard University.

Despite the lens being made out of aragonite, the eyes actually work fairly similar to the way ours do. Each calcium carbonate lens focuses light onto a retina, which is connected to photoreceptors that transmit the signal to a dense packet of nerves that acts as a primitive brain. The aragonite grains making up the lens are larger than the ones in the shell, so light has to pass through fewer grains on its way to the retina. This reduces the scattering of light ensuring a higher proportion reaches the retina.

Knowledge of these tiny eyes itself isn’t new but until now, scientists didn’t know how well chitons could see with them. A team of scientists lead by Ling Li of Harvard University took one of the lenses out of the shell and tested it. They found that the eyes, when working together, could provide a blurred image of its surroundings. While this may not seem helpful, it is good enough to give a chiton enough time to see a predator and react by clamping onto the seafloor and preventing attack to its soft underside.

The resolution provided by the tiny eyes that dot the chitons shell. Credit: Wyss Institute at Harvard University
The resolution provided by the tiny eyes that dot the chitons shell. Credit: Wyss Institute at Harvard University

These eyes provide a good field of vision for chitons and allow them to see objects up to six and half feet away. What is unknown is how chitons can process the information coming from hundreds of these eyes without a complex nervous system or brains.

Li thinks these eyes can have implications for designing structures with embedded sensors. These sensors could detect cracks or flexing in the structure. Although we are still far away from this technology, we can take something away from the structure of the chitons shell.

About Harry H

Harry H
Harry is currently studying biology and chemistry in University and hopes to go to grad school for evolutionary biology. He enjoys writing about sciences and sports and is a big fan of hockey and soccer. Some of his other interests are reading and rock climbing. Contact Harry: harry.h@youthindependent.com