HOW A HORSESHOE CRAB SEES

      Although the horseshoe crab has ten eyes, it still doesn't see very well. It can see movement and can make out the shapes of things, but it only sees black and white, blurry images. It doesn't depend on its eyes to find food or to even escape from enemies, although it really doesn't have very many enemies.
     A horseshoe crab has  two large very important eyes that are called compound eyes. There is one on either side of its shell. You can see one of those eyes in the picture above. The compound eyes are made up of one thousand photoreceptors. That sounds like a lot but it isn't. (People usually have one million photoreceptors each.) That is the reason why it sees blurry pictures.
     In addition to the compound eyes, a horseshoe crab has two small simple eyes together in the middle toward the front of the shell, five photoreceptors under the shell, and one eye made up of a group of photoreceptors along the top of its tail. But these eyes don't see images. They can just  tell light from dark. However, two of the small eyes on top of its shell can sense ultraviolet light. That is something human eyes can not do.


photo permission: Enchanted Learning
     Also, there is another thing a horseshoe crab can do that humans can't. A horseshoe crab has the ability to make its eyes one million times more sensitive at night and can see things, like another crab when it is time to mate, at night. Then when daytime comes, the eyes become less light sensitive. It is part of a daily cycle, but scientists do not know how exactly how they do this process.
     Researchers have been experimenting with horseshoe crab eyes for quite awhile. Because the photoreceptors in their eyes are the biggest of any animal in the world, they are easier to study than in other animals. A scientist named Dr. Hartline used the horseshoe crab's eyes to study the mechanics of human vision and won the Nobel Prize in 1967 for his research.

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