Eye? Aye! Horseshoe Crab


This is an image of one of a horseshoe crab’s two compound eyes, distinguishable from its other seven eyes by the presence of multiple lenses.

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A horseshoe crab has two compound eyes, mainly used to detect another horshoe crab, and seven other eyes, on its carapace, used to detect differences in light levels. This animal is little changed from its appearance in the fossil record about 300 million years ago. It is not a “true crab.” In fact, it is not even a crustacean: it is a saltwater-dwelling arthropod more closely related to spiders and scorpions. Its blood cells incorporate copper, instead of iron, into its oxygen-carrying molecules; when exposed to oxygen, horseshoe crab blood turns BLUE instaed of red. The whole animal may look scary, but it is harmless to humans, with a diet that includes worms and carrion. And today, here is an image of an eye, magnified over 200 times, to start a conversation that begins with, “What is that?”

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