JOIN USAn international team has worked out how fiddler crabs -- who've all-round vision including overhead provided by 9000 separate eye facets, or ommatidia -- actually perceive the world and respond to it.
"Unlike our eyes, the crab's eyes do not move, so it uses different parts of its visual field for different tasks. Some require a sharp focus and some of which require just a general lookout to be maintained.
"Our work is aimed at understanding how they process the visual signals they receive and convert them into behaviour, a process common to all visual organisms, ourselves included," Dr Jan Hemmi, who led the team, said.
The team has demonstrated that the sharpest vision in the crab is in the horizontal plane immediately in front of it. Its eyes here are really adapted to fine detail. This it uses for identifying and communicating with potential mates.
The crab also sees quite well horizontally to either side with especially good spatial perception enabling it to see how far objects are from one another -- it uses this to keep watch for rival crabs and monitor how far it is from its own burrow, so it can run for cover, the scientists said.
The eye cells that make up its overhead and rear vision are much more thinly spread, sufficient just to provide warning of the approach of a predator like a gull, outlined against the bright sky.
To test the response of crabs to objects glimpsed above them, the team has also constructed a "crab treadmill" - a ball suspended on a column of air that tracks the direction the crab scuttles when it sees something scary.
According to the scientists, crabs also see their world in unusual colours and this ability may provide a way to recognise mates or rivals based on UV patterns on their shell.
"In the case of humans, all our acute vision is provided by the fovea, a small pit at the back of the eye where vision cells are thickly concentrated, enabling us to read, recognise faces and see detail.
"In the case of the crab, it has a concentration of vision cells a few degrees above and below the horizon and towards its front, where its most acute vision exists.
"So while the crab may seem a simple creature, its vision is exquisitely adapted to the featureless mudflats it inhabits, providing it with all the information it needs to navigate, feed, and dodge predators," Dr Hemmi said.
According to the scientists, the crab's eyes may hold important lessons for robot design.
"Most robots use TV cameras which are like the human eye and provide a flood of information which is hard to process quickly. The crab's eye, on the other hand, performs all the essential tasks needed in autonomy, but with far less information being processed.
"For certain types of robot, this type of machine vision may be far more practical and appropriate -- and the crabs can teach us much," Dr Hemmi of Australian National University said.