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Reconstructing a fiddler crab's view of the world

Dr Jan Hemmi and his colleagues from the School of Biology Visual Sciences group have spent the past seven years exploring how the visual system of fiddler crabs (Uca species) has been shaped by the perspective properties of the flat world they inhabit. Fiddler crab eyes are raised high above their substratum enabling them to differentiate between potential predators (seen above the horizon) and other crabs (seen below the horizon).

Fiddler crabs possess an equatorial ‘acute zone' retinal sampling array that makes a very significant contribution to the decision making of this simple autonomous animal. Their remarkable visual armoury includes double panoramic vision (360° each eye, with monocular overlap), and a panoramic visual field which enables them to simultaneously monitor everything that is going on around them without any need for eye movements - at any one time they can see other crabs up to about two metres away; a 30cm bird flying overhead to about 20 metres; and a person walking on the mudflat up to about 100 metres. However, although fiddler crab visual range is excellent, their visual resolution is poor, being approximately equivalent to a camera with a fish eye lens and a 100x100 pixel array.

The principles now understood through the study of fiddler crabs - particularly their ability to enhance / magnify vertical components of their visual world through the increased resolving power in the acute zone - are potentially applicable to navigation of pilotless aircraft and the operation of ground-based robots.

To further understand the visual system of fiddler crabs, Jochen Smolka and Jan Hemmi completed a technically advanced study which mapped the resolution and sensitivity of a crab's entire eye with respect to behavioural tasks. This study was achieved by holding a crab at the centre of a rotating stage and observing eye behaviour using a microscope which can pivot 360° around the eye. In a complementary study, other members of the team showed that the properties of visual pigment and therefore the crabs' colour vision, also varied across the visual field according to different visual tasks. Jan Hemmi's team is currently exploring how fiddler crabs see colours and polarised light and how this impacts on their ability to navigate across their flat world, detect predators and interact with other crabs.


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