Above is a giant red hermit crab in a crowned conch shell. The conch is about 5" long. They are in about 6 inches of water. Note the eyes on the left which look like life saver candy.
Compared to humans, crabs have much smaller eyes and brains but they are still extremely competent in using vision to make snap decisions and spot trouble. This is more than most robots can. Research into how fiddler crabs use vision may one day help robots to act autonomously.
Their eyes are tailored to the specific needs in their flat crab world. For instance, crab eyes see the whole 360 degree panorama around them. They are able to perceive polarized light. The eyes are on stalks like periscopes which fold into protective grooves when the crab enters the burrow. Eyes act as an early warning system, provide compass information for navigation, enable the crab to judge distance, and see birds flying above and predators approaching from behind.