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Fish feel the flow

New model explains how these swimmers use their lateral lines to read wakes

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3:16pm, July 31, 2009
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Special sensory cells in fish respond reliably to swirly wakes, allowing fish to hunt down prey or join a friendly school by reading the watery traces, a paper to appear in Physical Review Letters suggests.

As fish, and other objects, move through water, they leave behind long-lasting vortices, or wakes, says study coauthor Jacob Engelmann of Bonn University in Germany, like the residual swirls left by a canoe paddle in a lake. “You can tell where the fish was, even minutes after the fish is gone,” he says. Researchers knew that fish, which rely heavily on senses other than vision, detected such footprints, but the details of how were unclear.

Fish feel pressure changes in the water around them with a system called the lateral line, which sits just below the scales. A typical fish has a stripe of sensory units called neuromasts down each side and various rows of the units on its head. Each sensor is made up of hair cells, similar to the ones in human ears, covered by a jelly sack. As pressure in the environment changes, the jelly sacks bend the small hairs below, which then relay a message to the brain.

Putting together many such signals may allow the fish to figure out which direction a wake came from, the new study shows. “The lateral line is good for feeling what’s going on in your direct neighborhood,” says coauthor J. Leo van Hemmen, a theoretical biophysicist at the Technical University of Munich in Germany.

In the new study, the researchers mathematically describe the pressure changes that a swirling wake causes. These pressure differences alone can produce neuromast signals that could collectively tell a fish which direction the wake came from, the model shows.

To test their predictions, the researchers eavesdropped on the activity of neuromasts in tethered goldfish as wakes rolled down the fish’s bodies. Electrodes recorded signals as they moved from neuromasts to the fish’s brains. Wakes coming from different directions produced distinct patterns of signals, the team found. And the findings fit well with the model’s predictions. “The theory tells you what the fish feels,” van Hemmen says.

The model could be used to predict lateral line activity in different kinds of fish for wakes of different directions and strengths, the researchers say.

“What’s new and unique is that they’ve been able to predict experimental results based on a model,” says Sheryl Coombs, a sensory biologist at Bowling Green State University in Ohio. “They have a model that actually works.”

Earlier studies of the lateral line system used vibrating spheres, which are not always the best approximation of what a fish encounters in the wild, instead of swirling wakes. “There is lots of mileage to be gained from studying fish in the context of real fish behaviors—what fish do in the wild,” Coombs says.

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