In other words, Weissburg wants to understand the chemical signals that aquatic animals, such as blue crabs and snails, use to track food and mates from some distance, generally a few meters. To understand the story from multiple perspectives, Weissburg, an animal behavior biologist, collaborates with Georgia Tech colleague
Don Webster, an assistant professor of civil and environmental engineering.
Webster, an expert in fluid mechanics, uses a 24-meter, 1,400-gallon flume – essentially an artificial river – to conduct experiments on the turbulent transport of chemical odors through water. Specifically, he investigates the physical mixing mechanisms of the turbulent flow, and quantifies the distribution of the odor filaments. Meanwhile, Weissburg collects information on how animals respond to the chemical signal through behavioral experiments in a smaller flume. They integrate their data to get "the big picture."
"Even the best biologists can't do much more than wave their hands about the physics part of this process," Weissburg says. "And even the most open-minded engineer may not know enough biology to understand what the chemical signal means. We can do more together."
The research questions Weissburg and Webster address are complex, but they have made some important discoveries in the past several years. For example, they have observed that when blue crabs get a whiff of odor from potential prey, they will move upstream into the current to pursue the cue. As they navigate, they use their legs as an extended sensing array to steer through the flow toward the odor.
"The crabs detect a burst of odor and hit the accelerator," Webster explains. "Then they perceive a contrast between their left and right legs and use that information to steer in the correct direction."
The crabs also appear to perform a cost-benefit analysis in deciding how to position their bodies, Weissburg says. If they decide to pursue a food odor, they position themselves at a large angle relative to the flow so they can better receive chemical signals necessary to easily navigate upstream. This takes a lot of energy because of increased drag force, but they decide it's worth the effort for the payoff, he adds.
The researchers have also examined the chemical signal effects of environmental conditions, such as roughness on the ocean floor and its associated increase in turbulence. In rough areas, such as an oyster reef, chemical odors are diluted, and animals such as blue crabs have difficulty sensing them. Meanwhile, tidal channels are sandy, smooth environments with high-concentration patches of odors that are easily detected by blue crabs.
"This is where management issues come into play," Weissburg says.
courtesy of NOAA
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Starfish sense chemical signals more slowly but perform well in highly mixed odor plumes.
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"Turbulent environments provide a refuge for animals that blue crabs find using their sense of smell. Clams are a good example. If you farm clams, then you want their environment to be reasonably turbulent.... So there are a lot of real-world applications for this research."
Another application for these research findings is in technology development programs – such as a U.S. Defense Department robotics initiative – based on biological design, he adds. That program funded some of Weissburg and Webster's initial studies four years ago. The hope is that researchers can use their understanding of chemical signaling to design algorithms for devices such as odor-tracking robots.
"For example, if you build a robot, one initial question is how many sensors you put on it and where," Webster explains. "If we have a firmer understanding of how animals collect and use chemical information, it can help in design projects. It may tell us we need a minimum 10-centimeter spacing between sensors or that we need sensors that respond rapidly to abrupt concentration changes. Do we want to act like a blue crab that can respond to signals quickly, but doesn't perform well in turbulent environments? Or do we want to respond like a starfish that senses more slowly but performs well in highly mixed plumes?"
The understanding researchers are seeking depends, in part, upon continued data collection in flume experiments. In those studies, Webster injects tiny patches of food odor mixed with a fluorescent dye, which moves downstream and becomes mixed in the flume.
courtesy of Don Webster and Marc Weissburg
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Blue crabs appear to perform a cost-benefit analysis in deciding how to position their bodies in response to food odors injected with dye during flume experiments. The crabs position themselves at a large angle relative to the flow so they can better receive chemical signals necessary to easily navigate upstream. This takes a lot of energy because of increased drag force, but they decide it's worth the effort for the payoff, researchers say.
(300-dpi JPEG version - 136k)
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Webster uses sophisticated measurement equipment to determine the magnitude of the odor concentration, the size of odor patches and changes in the odor concentration as they move through the flume. Researchers need to measure these properties to determine what chemical signals are available to the animals. Through Weissburg's behavioral experiments, the animals can suggest how they are using the signals.
"So the measurements Don takes in the flume are essential to understanding the animal's behavior," Weissburg explains. ".... We take Don's analysis and compare it to the animal's behavior. Then if we have a hypothesis about what the animal is doing, we can predict what the animal should do under specific conditions."
The data Weissburg and Webster have collected so far has formed what they call a "virtual odor environment" in which they can simulate how well various aquatic species track their prey under various conditions.
"What's important is the relationship between the size and scale of the animal and the size and scale of the odor plume," Weissburg says. "In fluid mechanics, dynamic similarity is used to build scale models. We are starting to use this approach in analyzing animal behavior. That suggests that when animal dynamics are simulated, and the animals have the same scale relationship to the odor plume, they will act the same."
As their research collaboration continues, Weissburg and Webster plan to observe animal behavior in the flume while simultaneously visualizing the odor plume with lasers. "There's a correlation there, and we've probably been missing some important subtleties that make the process work," Weissburg adds.
– Jane M. Sanders
For more information, contact Marc Weissburg, School of Biology, Georgia Tech, Atlanta, GA 30332-0230. (Telephone: 404-894-8433) (E-mail:
marc.weissburg@biology.gatech.edu); or Don Webster, School of Civil and Environmental Engineering, Georgia Tech, Atlanta, GA 30332-0355. (Telephone: 404-894-6704)
(E-mail: dwebster@ce.gatech.edu)
