07 August 2009

Fish follow prey tracks left in the water

Much as an animal's footprint alerts a tracker to the direction it is headed, predatory fish can track the passage of their prey in water.
Flathead eating a goby!
Prey leave vortex rings that alert predatory fish to the trail of another swimmer. The orientation of the ring generated a specific pattern of excitation in the fish's sensory system that tells it where the prey is going.

Prey Tell: How Fish Track Their Quarry, Even When They Can't See It
A model incorporating fluid dynamics and neurology shows how fish detect the long-lived wakes generated by other swimmers
John Matson, Scientific American 7 Aug 09;
Fish are adept trackers of prey, able to detect the trace of another fish more than a minute after it has swum past.

A group of German researchers has now deconstructed that aquatic ability by modeling what fish wakes look like and how fish use cues from those wakes to follow prey. The research is set to be published in Physical Review Letters.

When their prey is out of sight and beyond earshot, fish rely on an intricate array of sensors below the skin. The subdermal sensors are known as canal neuromasts, because they reside in canals connected by pores to the water outside. When a pressure difference exists between two adjoining pores, water flows through the canal connecting them, and the neuromasts register the fluid movement.

Study co-author J. Leo van Hemmen, a theoretical biophysicist at the Technical University of Munich, says that the key to fishes's tracking ability is the fact that wakes generated by swimmers remain stable for long periods of time, owing to their rotational structure.

"Wakes are vortices, water that's rotating," van Hemmen says. Like a spinning top, he adds, the ring-shaped vortices "will spin for quite awhile if there's not too much friction." The amount of friction encountered in a fluid depends on that fluid's viscosity, a measure on which water ranks relatively low. "Fish are not swimming in syrup," van Hemmen says. "They're swimming in water." So the vortex rings persist for a long time.

Van Hemmen and his colleagues modeled how those vortex rings should translate to pressure differentials as detected by canal neuromasts along a fish's lateral line, a sensory "ribbon" that runs like a racing stripe lengthwise across both sides of a fish's body. Then the researchers monitored the neuronal output from a fish's canal neuromasts, verifying that their model correctly described the sensory process.

The researchers found that not only did vortex rings alert fish to the trail of another swimmer, but the orientation of the ring generated a specific pattern of excitation in the fish's sensory system. In other words, the wakes pointed the way to the swimmer, much as an animal's footprint alerts a tracker to the direction it is headed.

Sheryl Coombs, a sensory biologist at Bowling Green State University in Ohio, says it is not news to those who study the lateral line that fish can follow wakes, but it is significant that van Hemmen's group was able to quantitatively model how it is done. That advance, she says, owes its existence, at least in part, to ever more powerful computational tools. "Until we really had the engineering technology to visualize what these vortices look like, it was difficult to study them," Coombs says.

Although such modeling will likely not play a major role in her own work, Coombs says others will likely find uses for the new research. "I think it's going to be a very powerful model," she says. The U.S. Department of Defense, for instance, has funded research by Coombs and her colleagues that could someday lead to silent alternatives to sonar for underwater sensing.

Like sonar, a fish's wake tracking is a versatile sensory ability—it works at night and in other settings where visibility is impaired. "You can do it in complete darkness, or in the Mississippi River," van Hemmen says.

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