How Does the Horseshoe See?
A quick glance at the horseshoe will show the crab's two compound lateral eyes-- unusual because no other living Chelicerate possesses compound eyes. These are used primarily for finding mates, but the horseshoe has many more light-sensing organs.
There are 5 additional eyes on the top of its shell (two median eyes, one endoparietal eye and two rudimentary lateral eyes). The median eyes have cells sensitive to visible light and others to the ultraviolet range. The rudimentary lateral eyes are photoreceptors that become functional just before the embryo hatches. A clock in the anterior part of the brain sends out signals that control the sensitivity of the lateral and median eyes.
The tail also has a series of light sensors along the top and side that keeps its brain synchronized with cycles of light and dark. Additional signals from the small median eyes enhance the degree of adaptation to darkness, according to the amount of ultraviolet light those eyes receive from the sky at night.
On the underside of the crab, there are two ventral eyes, located near the mouth, which may help orient the animal when swimming.