Horseshoe Crabs have 10 Eyes – 2 of them are compound
The 2 large Compound Eyes
are located on the top of the shell. These eyes are made up of a thousand light sensors that see in shades of gray. The crab combines all these separate sensors together as an image that they see. These eyes are probably used for finding a mate.
The compound eyes are larger and have a optical nerve that is easy to identify making the crabs ideal for studying how an eye works. The research into their eyes has helped the study and understanding of how the human eye works.
Horseshoe crabs also have another 6 eyes scattered around the body. One on it’s tail and the other 5 on the bottom side. These eyes are merely light sensors and help with navigation. The eyes have the ability to detect uv light and are sensitive enough that the horseshoe crab sees as well at night as it does in the day.