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

You Are What You Eat Marine organisms protect themselves from predators by eating toxic plants.

GEORGIA TECH SCHOOL OF BIOLOGY Professor Mark Hay studies the ecology of "yuck." Translated, that means Hay investigates the chemical defenses aquatic organisms use to fend off predators.

photo by Gary Meek In their field work, Hay and his research team selectively looked for small, less mobile herbivores – such as crabs, sea slugs, worms and shrimp-like animals called amphipods – that had to make their home on their food source, much like caterpillars do on land. Researchers found that many of these marine herbivores live on toxic plants to avoid being eaten by fish. (300-dpi JPEG version - 686k) Detail of photo above. (300-dpi JPEG version - 667k)

In his 20-plus-year career, Hay has studied "yuck" in oceans – particularly coral reefs – around the world. He has discovered some fascinating and elaborate ways that plants and animals protect themselves from being eaten by other organisms.

For example, some marine organisms feed exclusively on seaweeds that fish find noxious because doing so allows these specialized feeders to evade or deter their predators. In their field work, Hay and his research team selectively looked for small, less-mobile herbivores – such as crabs, sea slugs, worms and shrimp-like animals called amphipods – that had to make their home on their food source, much like caterpillars do on land. Researchers hypothesized that these marine herbivores live on toxic plants to avoid being eaten by fish.

"If you're a caterpillar and are living on what the buffalo like to eat when the buffalo migrate through, then you're a cow patty, not a happy caterpillar," Hay jokes. "An amphipod living on a seaweed favored by fish would suffer similar fate."

He adds: "If you're a little, non-mobile herbivore on the coral reef, you selectively eat toxic plants, but not because it may be good for you. In fact, it may slow your growth or your reproduction. So what? That's better than being dead. If you live on what the fish like on the coral reef, you're going to get eaten, even if you're hidden."

Some of these animals feed on toxic seaweeds, sequester the toxin in their bodies and then become chemically defended against their predators. Other small specialists that can't perform this metabolic trick achieve the same effect by "gluing" or "wrapping" portions of the toxic plant all over their bodies.

"They are like toxic tacos wrapped in this poisonous plant," Hay says. "In the lab, if we force these organisms to wrap themselves in the wrong plant, they get eaten by fish. If we allow them to wrap themselves in the plant they want and then feed them to fish, they go in the mouth and come right back out unharmed."

In some cases, these small animals are so specialized that they sense and use only one of three structurally similarly chemicals in the same plant to cue the gluing behavior and thus deter local fish, he says.

"What has driven the evolution of specialized feeding on these toxic plants is the need to avoid predators, not something that is great about the food," Hay says. Terrestrial ecologists are documenting similar patterns, he adds, citing the gypsy moth as an example. This moth eats plants high in tannins (a chemical used in tanning leather). Even though the tannins are difficult to digest, the moth eats them because the toxins protect it from attack by microbes.

Similar patterns of chemical defense in marine and terrestrial ecosystems are not that surprising to Hay considering the complex cascades of indirect interactions between these environments, he says. The professor cites an example from one of his current studies funded by the National Science Foundation. He and colleagues at the Skidaway Institute of Oceanography near Savannah, Ga., are examining Phaeocystis, one of the most abundant genera of phytoplankton in cold oceans.

courtesy U.S. Fish & Wildlife Service Larger birds, such as the albatross shown here, follow smaller birds feeding on zooplankton in the ocean to feed on fish that are attracted to these sites. These larger birds then fly back to their nests, often on desert islands, where they feed their young, defecate and sometimes get eaten by terrestrial predators. Stable isotope studies indicate that this marine-based input of energy and nutrients influences the growth of desert vegetation, such as cacti, and animal populations, such as coyotes and spiders.

"These organisms are the primary producers in several arctic and sub-arctic seas and so form the critcal base of the food web.... Sometimes they are palliative, and sometimes not," Hay says. "It's really variable. We are testing the hypothesis that they may be sensing the presence of consumers and producing more defensive chemistry in response. If so, this could determine whether their productivity cascades up the food web to support fisheries or is largely unused and sinks through the water column, supporting primarily bacteria and the detrital food web."

Specifically, some Phaeocystis species respond to consumers – primarily zooplankton – by releasing a chemical called dimethylsulfide (DMS), which reduces feeding by zooplankton. Some species of small seabirds that eat zooplankton apparently sense this release of DMS, which guides their flight across more than a hundred miles of feature-less ocean to find high-productivity areas where they can be the first to feed on zooplankton that concentrate there.

Larger birds, such as albatrosses, follow the smaller, more sensitive birds and feed on fish that are attracted to these sites. These birds then fly back to their nests, often on desert islands, where they feed their young, defecate and sometimes get eaten by terrestrial predators. Stable isotope studies indicate that this marine-based input of energy and nutrients influences the growth of desert vegetation, such as cacti, and animal populations, such as coyotes and spiders.

Because DMS rises into the atmosphere and helps clouds form, influences rain patterns and reflects the sun's energy back into space, it is even possible that DMS affects global climate patterns, Hay adds.

"Although all of these connections have not yet been proven, it appears that these complex biotic interactions and long-distance energy transfers between marine and terrestrial systems may be critically dependent on chemical signals produced by simple marine phytoplankton," Hay says.

"The point is that the mechanistic understanding of which chemicals are doing what to which animals can be scaled up," he adds. "If you do small science, it's interesting and useful. But if you ask, 'What does it mean?' you are looking for connections. You can understand something about the density of coyotes on islands by knowing the particular chemical that phytoplankton are releasing in response to zooplankton predation 300 miles away.... If we can understand the mechanisms, we can help fix ecosystems that need it. But for now, we're sort of like auto mechanics working on an engine that nobody fully understands."

– Jane M. Sanders

For more information, contact Mark Hay, School of Biology, Georgia Tech, Atlanta, GA 30332-0230. (Telephone: 404-894-8429) (E-mail: mark.hay@biology.gatech.edu)

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Last updated: April 12, 2003