by Whit Gibbons

January 20, 2008

This past week I read two scientific reports about planetary temperature changes. One report concerned a space probe we launched years ago that is now circling the planet Mercury and sending back data. On Mercury, surface temperatures can change from more than 800 degrees above freezing in daytime to more than 200 below at night. The other report was an update on Antarctica. Because of global climate change, the icy continent is melting away before our eyes. An inhabitant of Mercury would of course smile at the thought that we consider an increase of a few degrees to be a planetary problem.

Nonetheless, Earth's gradually rising temperatures are indeed a serious problem. Being human, our first thoughts might be how much air-conditioning costs will increase to cover a few extra months of cooling each year and whether beachfront property will soon be only a few miles from Atlanta. Many people also wonder what will happen to polar bears at the North Pole and penguins in Antarctica. Rises in sea level and responses of large vertebrate animals are clear indicators of global climate change. What other less-obvious polar organisms might be affected as temperatures continue to spiral upward?

Almost two decades ago I wrote about the discovery of a chemical association in Antarctica that involved fish and two species of tiny marine organisms. The scientists who made the remarkable observations were J. B. McClintock (University of Alabama in Birmingham) and J. Janssen (Loyola University in Chicago). They studied an amphipod, an invertebrate related to sow bugs, or roly-polies. Amphipods are defenseless against small, predatory fish in the region. In the clear waters of the Antarctic sea, the amphipod is easy prey. Dissections of Antarctic fish have shown that amphipods are common prey for a variety of fish species.

In contrast, another small invertebrate known as a pteropod, which is chemically noxious to the fish, is not a prey item. Pteropods are in essence tiny, bright orange snails without shells that move through the water by flapping winglike appendages. But pteropods are not eaten by fish, presumably because they are distasteful. Fish that grab a pteropod will shake their heads violently and spit the animal out. Fish avoid brightly colored, bad-tasting pteropods but readily eat amphipods. As a tasty animal with no defense whatsoever, what's an amphipod to do? Amphipods apparently capture pteropods as a chemical weapon against fish predation.

After drilling holes in the sea ice, the investigators observed many amphipods carrying pteropods on their backs. Amphipods placed in aquaria were observed to pursue small pteropods and grasp them with their pincers. The amphipod actually holds the smaller animal captive as it goes about its business. The scientists ran laboratory experiments that revealed that amphipods carrying a pteropod were immune to fish attack. Those without pteropods were readily consumed.

Skunks, scorpions, and venomous snakes also use chemical defenses against predators, but each produces its own chemical arsenal. The amphipod-pteropod relationship is a rare example of a prey species actively exploiting another species for chemical protection. Amphipods pay a price for carrying around a kidnapped bundle of protection due to added weight while swimming. But not having to worry about every fish that swims by apparently makes it worth it.

Meanwhile, what do pteropods get out of this relationship? Maybe nothing, or maybe we do not yet know. No hostage pteropods were observed to die during periods of up to a week, but they presumably do not maintain a normal diet. Being dragged around on another animal's back for a week or more would not seem to be in a pteropod's best interest. Ironically, the trait that spares them from fish predation is the very trait that results in their being abducted by another animal.

No one can predict exactly what will happen to these and other creatures large and small as temperatures continue to rise at the North and South Poles. We can be sure, however, that scientists will continue to explore the complex relationship among species-in the polar regions and elsewhere around the globe. And we should all appreciate the greater understanding of our world that such researchers provide.

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