Kidnapped plankton shares its defenses.

Kidnapped plankton shares its defenses

In the frigid waters of Antarctica's McMurdo Sound, some plankton turn to kidnapping as a survival tactic, according to new observations.

Scientists have discovered that tiny, shrimp-like animals called amphipods capture snail-like pteropods and carry them around piggyback-style for days at a stretch. On its own, the shrimpy Hyperiella dilatata -- about the size of a rice grain -- lacks defenses against predators. But the captive pteropod, Cline limacina, provides a chemical that protects them both, report biologists James B. McClintock of the University of Alabama at Birmingham and John Janssen of Loyola University in Chicago in the Aug. 2 NATURE.

"It's the only case I know in which one organism captures another for a chemical defense," says marine ecologist Diane K. Stoecker of the Woods Hole (Mass.) Oceanographic Institution.

Janssen and McClintock observed that plankton-eating fish -- which normally feast on amphipods and avoid pteropods -- nearly always spit the duo out, often with violent head shakes. But while the abduction routine appears to help the amphipod avoid becoming a meal, the unwieldy "backpack" also slows the amphipod, making foraging more difficult. And although the pteropod can no longer forage for its normal fare (other pteropods of even smaller size), abduction seems neither to harm nor help it in the long run.

The odd coupling falls under the umbrella category of symbiosis but doesn't fit neatly into the standard sub-categories of parasitism, mutualism or commensalism, McClintock says. "This is a good example of how sophisticated some of these interrelationships can become," he adds.