Winter 2003

COVER STORY – "SIGNALS in the SEA" The Broad Side You Are What You Eat Where's That Smell? Hot on the Trail Avoiding Costly Mistakes Key Ingredient A Gut Feeling

Hot on the Trail Microcrustaceans overcome the odds to find the right mate.

IF SINGLE PEOPLE think the odds are bad for finding the right mate, they may be inspired when they learn what transparent microcrustaceans have overcome in searching for a partner in the vast, viscous depths of the world's oceans.

courtesy of Jeannette Yen Often, a male copepod goes the wrong way on a female's pheromone trail, then turns around and swims more erratically as he searches for the narrow trail. He uses the sensors on his long antennae to compare from side to side. This is his strategy for staying on the trail. As he does this, he messes up the trail and leaves behind what looks like a spiral helix. (300-dpi JPEG version - 281k)

The chemically dependent mating patterns of these organisms – called zooplankton – are the focus of research by Georgia Tech Biology Professor Jeannette Yen. Zooplankton are short-lived organisms that range in size from several microns to a few centimeters. They live in oceans and almost all other aquatic habitats, traveling with the flow of water.

Though zooplankton are abundant, the probability is low for them to randomly encounter and mate with another individual of their own species. First, millimeter-size copepods – one type of zooplankton that Yen studies – are widely dispersed in a huge ocean. Their environment is dark, where vision is often limited, and the animals lack image-forming eyes. In addition, because of their size, the water is like a viscous medium to copepods, and they must use well-coordinated movements to easily navigate through it.

Facing these odds, copepods have developed efficient strategies for finding each other. They can sense light, and some have learned over time to migrate vertically to congregate with other copepods near the ocean's surface at night when the chances of predation by fish are much lower.

photo by Gary Meek To study the chemical signals involved in copepod mating, Professor Jeannette Yen and her students use several techniques to visualize and track the transparent or semi-transparent animals in the lab. A setup called Schlieren optical paths, a complex maze of mirrors and lasers, enhances the difference in the refractive light index to pinpoint the animals' 3D coordinates through time. (300-dpi JPEG version - 918k)

Also, copepods and other zooplankton communicate with potential mates by chemical signals. To attract males, the females emit chemical compounds called pheromones that are slowly dispersed by molecular diffusion, not by flow, in their viscous environment.

"It's sort of like people not feeling the Earth spinning because they're in their own pocket of the world," Yen analogizes. "Well, copepods are also in their own world. Things don't move so much relative to each other."

In the challenges of this environment, female copepods emit pheromones that become encased in a narrow, tunnel-like structure created by their hydrodynamic wake. The tunnel increases the animal's apparent size by 100 times, enhancing the probability of an encounter with a potential mate.

"The males have up to 10 seconds to find the female after she emits the pheromone," Yen observes, based on her laboratory experiments. "So the male is swimming around randomly. When he intercepts the female's trail, he apparently senses the chemical. He accelerates his swimming speed and catches up with his mate. About half of the time, the male goes the wrong way on the trail, then turns around and swims more erratically as he searches for the narrow trail. He uses the sensors on his long antennae to compare from side to side. This is his strategy for staying on the trail. As he does this, he messes up the trail and leaves behind what looks like a spiral helix."

To study these mating patterns, researchers use several techniques to visualize and track the transparent or semitransparent animals in the lab. Yen's research group uses Schlieren optical paths, a complex maze of mirrors and lasers that enhance the difference in the refractive light index to pinpoint the animals' 3D coordinates through time.

courtesy of Jeannette Yen Female copepods emit chemical compounds called pheromones that become encased in a narrow, tunnel-like structure created by their hydrodynamic wake. The tunnel increases the animal's apparent size by 100 times, enhancing the probability of an encounter with a potential mate. (300-dpi JPEG version - 270k)

For this study, Yen and her research team create what they call a "copepod tea" to track the male's response to the female's pheromone. The tea, which is released as a thin stream into a tank containing males, consists of water conditioned with the pheromone and a high-molecular-weight compound so they can see the trail and its tracker.

To study the interactions between female and male copepods, Yen also uses computer-based image analysis and her own fluid-mechanics-based technique for mapping the water flow at various points in the experimental setup. The result is a 3D representation of the animals' mating behavior in their fluid environment, she explains.

As experiments continue, Yen and her team plan to investigate several questions that have arisen from their research to date. One, are the female copepod pheromones and/or the hydrodynamic wakes species-specific? Two, does the male's erratic swimming pattern along the trail diffuse the pheromone more rapidly?

"For such widely dispersed animals and infrequent encounters with trails, males might follow any female's trail and rely on intimate contact for their final mating decision," Yen hypothesizes. "However, a preference for the odor trail of only their own species enables them to recognize mates at a distance. This remote detection behavior serves to maintain reproductive isolation and the integrity of the species."

photo by Gary Meek Schlieren optical paths, a complex maze of mirrors and lasers, enhances the difference in the refractive light index to pinpoint the animals' 3D coordinates through time. (300-dpi JPEG version - 870k)

Much of Yen's research depends upon interdisciplinary collaboration with her colleagues in Georgia Tech's aquatic chemical signaling research program. With biochemist Julia Kubanek, an assistant professor in the School of Biology, Yen and her research team are identifying the pheromone molecules that females emit to attract males.

"We want to track the 'aha'," Kubanek explains. "We hope to understand their mating behavior in a molecule-to-molecule, moment-by-moment way."

Then Yen's research team can ask larger biological questions, such as whether multiple species possess the same molecule, and if so, how they remain distinct species, Kubanek says. Also, researchers may be able to track the evolution of zooplankton from their pheromone structure or discover how males sense pheromones. Other research questions will center on how the female releases the pheromone, at what life stage she makes it and how much she makes.

"If we can understand the behavior and physiology of these organisms, we can see what effects these characteristics have on the structure of the population (i.e., who mates and how often)," Kubanek adds. "Then we can understand the community dynamics (i.e., effects on all species in the community)."

– Jane M. Sanders

For more information, contact Jeannette Yen, School of Biology, Georgia Tech, Atlanta, GA 30332-0230. (Telephone: 404-385-1596) (E-mail: jeannette.yen@biology.gatech.edu)

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Last updated: April 12, 2003