The Eye

Crabs have eyes that are very different to our own. They are on stalks for a start, and can swivel around wildly, as well as being withdrawn into their sockets.

Crabs have compound eyes, which means their eye is formed of hundreds thousands of individual light sensors with their own lens and cornea. These mini-eyes are hexagonal and called ommatidia.

Crabs don’t see a single image, instead they get a mosaic of repeated, blurry pictures.

In light conditions the ommatidia becomes pigmented to reduce the amount of light that enters. When it is darker, the pigment crystallises and lets light in.

Generally, the deeper the environment the crab lives in, the bigger the eyes, to let in what little light is available.

Pictured is the eye of a Fiddler Crab, these crabs live in a flat, intertidal environment, and have developed 360 degree eyesight.

They can identify a flying bird of 30cm size flying above at a height of 20m, and a person walking on the mudflat at 100m. Although their range is amazing for crabs, they can only see in 100x100 pixel resolution.

The eye stalks of crabs is also where you can find the X-organ, a nerve cell which controls the colour of the crab. If you remove the eyes of a crab, they become pale, but please don’t try this.

The X-organ is actually one of the most important organs in the crab’s body, and controls things like molting cycle, sexual maturity, and carbohydrate metabolism.