With real estate in high demand, foraging is risky for fiddler crabs, Uca vomeris. Ever vigilant for burrow thieves, the crabs stay within a meter of the entrance and rush back to defend the burrow when another crab gets too close. To do this effectively, they must be able to judge how close the intruding crab is to their burrow entrance. Simple as this may seem, for fiddler crabs this is not necessarily an easy task.

While depth perception is relatively easy for vertebrates that can use binocular disparity, fiddler crabs, with their closely set, low-resolution compound eyes, cannot use such cues. Arthropods, such as praying mantises, are known to use apparent movement to determine the distance between themselves and an object or prey. This strategy involves an egocentric frame of reference in which distance is measured with reference to the individual's location. The task facing fiddler crabs, however, is unusual in that it involves an allocentric framework, in which the relative distance between two objects, such as an intruding crab and a burrow entrance, is judged. The ability to use allocentric frameworks is assumed to be more sophisticated. A new study by Jan Hemmi and Jochen Zeil explains how this seemingly difficult task might be accomplished.

Using dummy crabs moving on a track, the authors found that the behavior of fiddler crabs indicates that the best predictor of a defense response is the burrow–dummy distance, regardless of the angle of view. In addition,crabs respond to approaching rather than departing dummies. Given the flat visual environment of the fiddler crabs' world, the distance of objects on the ground can be determined by the location of images on the retina. Hemmi and Zeil suggest that a given burrow–dummy distance may be signaled by a single “distance neuron”, which has a wide receptive field and is motion-sensitive. To adjust its response threshold according to its distance from home, a crab would need a collection of such distance neurons, each tuned to a particular burrow–dummy distance.

To make matters more complicated, a foraging crab often cannot see the entrance to its burrow. We know that a fiddler crab can find its way back home by keeping running tabs on its location relative to its burrow, a strategy called path integration. Hemmi and Zeil propose that a fiddler crab can also use path integration to judge how far it is from its burrow entrance, so it can start running earlier when it has further to go to protect its turf.

Specifically, fiddler crabs could use the information from path integration to determine which of the distance neurons to pay attention to. By comparing outputs across these neurons, a crab might also be able to gauge the relative direction of movement of the suspect crab. Thus, fiddler crabs may be able to accomplish a seemingly complicated task of judging relative distance using a surprisingly simple mechanism.

Hemmi, J. M. and Zeil, J. (
2003
). Robust judgement of inter-object distance by an arthropod.
Nature
421
,
160
-163.