Mud's eye view: to understand the world of the fiddler crab, ecologists peer through a lens that renders a landscape as a doughnut-shaped panorama

Natural History, April, 2004 by Douglas Fox

Analyzing data from hundreds of such maneuvers, Hemmi has shown how a fiddler seems to measure the distance between its burrow and a dummy. As Hemmi manipulated the dummy, a crab would consistently rush back to its burrow whenever the "invader" came within about ten to twelve inches of the burrow. The same result held no matter what the angles between the dummy, the burrow, and the resident crab--and no matter how far the resident crab was from its burrow. Apparently, the crab that owns the burrow treats a circular area around its burrow, with a radius of roughly ten to twelve inches, as inviolate; if an invading crab crosses into that circle, the home crab will retreat to its burrow, even if it is closer to the burrow than the invading crab is.

A crab measures its own distance from objects such as other crabs according to how high they appear in its visual field. (People use the same trick.) In a world that is almost perfectly flat, far-off objects look high, close to the horizon, whereas nearby objects appear relatively low in the visual field. The crab knows how far away its home is because it has been counting steps and keeping track of its direction, and is constantly refiguring the distance to its burrow. That information, plus the height and direction of any potential arthropod invader, would be enough to triangulate the distance between the potential invader and the burrow.

Of course no fiddler crab has the brainpower to understand triangulation and trigonometry. So how does a crab combine visual and odometric, or travel-distance, information in its brain to determine whether an intruder has entered the forbidden zone around its burrow? How, in other words, does it solve what is essentially a trigonometric problem without knowing how to do the trig?

Zeil and Hemmi think the crab has a shortcut hardwired into its brain. As a crab moves about the mudflat, it keeps the same side of its body pointed toward its burrow. That way, even when the burrow is invisible, it remains in the same region of the animal's visual field. The cluster of ommatidia that point toward the crab's burrow and the surrounding forbidden zone could then be wired up so that whenever those ommatidia detect movement, the crab races back to defend its burrow. There is one complicating factor: As the crab moves about, and as its distance from the borrow grows or diminishes, the forbidden zone becomes smaller or larger in the crab's visual field. Evolution could have solved that problem by wiring the crab's eye to its odometer: then, as the crab walked toward or away from its burrow, the cluster of intruder-detecting ommatidia in its eye would automatically grow or shrink.

To reinforce his point, Hemmi performed the initial run of an experiment on the crabs while I watched. Pulling a fishing line, he raised the dummy half an inch higher, and then passed it through the crab subdivision again. We watched a male that had consistently rushed to defend its burrow from the dummy on previous runs. But even when the raised dummy came within just three inches of the crab's burrow, the crab didn't respond.