The walking legs, carapaces and chelae of ocypodid crabs, particularly the well-studied fiddler crabs, are often brilliantly coloured. However, despite this array of colours there is still some debate over whether ocypodid crabs are actually capable of colour vision and what role it might play.
The aim of this experiment was to test the colour vision capabilities of fiddler crabs in a biologically significant context. To determine whether Uca mjoebergi use differences in spectral composition to discriminate between colours, and thus possess colour vision, I tested their ability to consistently discriminate yellow from various shades of grey. Experiments were conducted in October 2005 on an intertidal mudflat in the East Point Reserve, Darwin.
When the crabs were reproductively active, during neap tide, wandering females were caught and placed under a container 15 cm from two identical plaster casts of a U. mjoebergi male's 21.8 mm claw. Females were given a choice between a yellow claw (Dulux Tiny Tin yellow enamel) and a claw painted white, 75% grey, 50% grey, 25% grey or black (made by mixing appropriate amounts of white and black Dulux Tiny Tin enamel paint). It was recorded which claw the female approached within 2 cm. Each combination was tested until 15 females successfully made a choice.
The same experiment was conducted with two tethered, size matched (within 1 mm claw length), and handedness-matched males in place of the plaster claws. The males were tethered with 1 cm of cotton super-glued to their carapace and tied to a nail stuck in the ground, allowing some restricted movement. I painted the males' claw either with the same yellow paint as above or with the light yellow paint, made by mixing one part yellow to 14 parts white paint. The experiment was repeated 25 times using different pairs of males and new females each time.