Scientists have discovered the first known example in nature in which one species of animal abducts another and carries it around as a weapon to ward off would-be predators.

The abductor is a tiny, bright orange shrimplike crustacean called Hyperiella dilatata that lives in McMurdo Sound off the coast of Antarctica, where it is a favorite food of fish. On a research trip to the continent last year, biologists James B. McClintock of the University of Alabama at Birmingham and John Janssen of Chicago's Loyola University noticed that in some areas, almost 75 percent of the quarter-inch-long crustaceans were toting around a smaller animal, a sea slug called Clione limacina. They grasped it between leglike appendages.

In experiments published in last week's issue of Nature, the pair proved that the crustaceans use the slugs as a chemical deterrent. Fish refused to eat the slugs, and when they were presented with one of the crustaceans clasping a slug, they tasted the morsel and immediately spat it out. In two instances, a fish managed to pull the slug loose in its mouth, spit it out and eat the crustacean. In some cases, McClintock said, a fish would swim up to one of the crustaceans holding a slug, eye it, and then swim away.

McClintock said the fast-moving crustacean catches slugs by swimming up to one and grabbing it. Then it hoists the slug onto its back like a backpack and holds it fast. The slug cannot feed in that position, and appears to gain nothing from the arrangement.