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

As I stood on a coastal mudflat in northeastern Australia, the morning sun glistened off the wave-rippled surface of the muck. To the west, I was the only vertical feature for a quarter mile; to the east, the flatness reached clear to the horizon. All around me, hundreds of spidery fiddler crabs, of the genus Uca, milled about on the mud; as they ate their soggy porridge--quite literally, the mud--their faint, wet squelching noises registered just above the threshold of my hearing. The army of arthropods slurped bits of organic material out of the muck, then ejected balls of it like so many wads of chewing tobacco.

Welcome to Crabworld. To my eyes, the place looked mind-numbingly monotonous. But the crabs weren't seeing what I was. Crabworld, it turns out, isn't where you are; it's what you see. That little secret was my first lesson in a burgeoning discipline called visual ecology. Under the expert tutelage of Jochen Zeil and Jan Hemmi, two visual biologists at the Australian National University (ANU) in Canberra, I learned that the guiding principle of the discipline is to minimize--to eliminate, if possible--the biased guesses about animal behavior that human vision introduces. Instead, say the visual ecologists, begin with the ways the animals' own visual systems render the world.

That might sound like a straightforward call to study the anatomy and physiology of animals' eyes in the laboratory. But to understand how an animal behaves, Zeil says, you also need to study what the animal perceives in the environment to which it has adapted, and how in analyzes that information. Of course, some animals are easier to study than others. The fiddler crab, with its simple nervous system, its pancake-flat world, and its limited home range, makes a great place to start.

At least to a human being, Crabworld is bizarrely shaped. A fiddler crab's eyes are mounted on stalks that point straight up, and they command a panoramic, 360-degree view. The mudflat comprises the entire outer edge of the visual field, and the arching sky dominates the middle [see image at left].

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Unlike human vision, the crab's vision is sharpest around the edges. That's a reasonable emphasis. After all, the outer edge is where other members of the species are scuttling about: both rival animals looking to steal one's precious burrow, and females in the market for a mate. But in the great round center of the crab's visual field there is nothing but sky--and the occasional bird swooping in for cold crabmeat cocktail. A crab in such precarious circumstances doesn't need to see its would-be predator in detail; all it has to do is sense the movement overhead, then scurry for safety in its burrow.

Ever since my first conversation with Zeil several years ago, I had wanted to experience first hand the convoluted, crab's-eye view of the world. And so one morning I wandered onto the mudflat, found a crab colony, and lay down in the mud. Viewed down low, Crabworld was trickier to deal with than it had seemed at first. For one thing, the mud is imprinted with wavy bumps, or corrugations, the marks left by the water at high tide. The bumps pose a navigation problem to the crab: as soon as the animal gets more than six inches from its burrow, they obscure its view of home. Yet the animals still venture as far as a yard away from the safety of their precious mud-holes.

Furthermore, down at crab height, the entire mudflat is compressed into a narrow horizontal band. In every direction the band bustles with a dozen scuttling crabs--some twelve inches away, others twelve feet. For me it was hard to tell the difference. But for a fiddler crab, the distance to each of the other crabs, relative to itself and to its burring; is a fundamental piece of information.

Of course, my efforts at seeing crabstyle were deeply flawed; I was relying on human vision to explore another creature's world. With few exceptions, no two species share the same visual system. Vision can vary in pigment sensitivity: The eyes of some species of mantis shrimps can respond to sixteen distinct wavelengths of light, giving them an unmatched ability to distinguish colors, whereas human vision is skewed toward red and green.

Vision can also vary in the maximum rate at which retinal cells can fire in response to light (the "flicker fusion point," at which a blinking light no longer seems to blink). Most people can't detect more than about sixty blinks a second. A fluorescent light, for instance, actually blinks on and off sixty times a second, but most people perceive the light as continuous. For dogs, though, the maximum detectable blink rate is closer to eighty blinks a second, and so fluorescent bulbs might prove a lot more irksome to dogs than they do to some people: a dog may actually see the flickering.

Other features of human vision further complicate a visit to Crabworld. "Our vision doesn't sample the world like a video camera," Zeil points out. The highest resolving power for people is focused on a minute bit of retina, and so we have to move our eyes to scrutinize things.