Carp grow potbellies to foil predatory fish.

You're a little fish in a big pond filled with large, hungry fish. Everywhere you turn, a wide, gaping mouth tries to take a bite out of your tail end. What's a scared little fish to do?

Fatten up, suggests a new study of carp and carp-eating pike.

A nice, round potbelly may constitute a carp's best defense against predatory pike, report ecologists Christer Bronmark and Jeffrey G. Miner of the University of Lund in Sweden. The researchers have found that European carp blimp out over a period of several weeks in the presence of pike so they won't fit into the predators' mouths.

The discovery marks the first time scientists have demonstrated that vertebrates--animals with a backbone--can change their body dimensions over time in order to avoid predators. Ecologists had previously found the phenomenon only among botton-dwelling invertebrates such as barnacles and sea snails and among microscopic aquatic creatures called zooplankton.

Some vertebrates, including blowfish and several species of birds, try to scare off potential predators by gulping air to inflate their bodies or throats. However, this temporary protective response lasts only minutes, and the animals must then revert back to their normal shapes.

To demonstrate the more permanent change effected by European carp, Bronmark and Miner used weighted plastic curtains to bisect two small, natural ponds containing carp. They placed wild pike into one half of each pond and confirmed that the pike began preying upon the carp.

Three months later, the researchers determined the body dimensions of several carp taken from each half of both ponds. They report in the Nov. 20 SCIENCE that the carp exposed to pike predation developed deeper bodies--as measured from backbone to belly--than the carp that lived without pike.

To rule out the possibility that the carp fattened up simply because each individual had more food after the pike reduced their numbers, Bronmark and Miner conducted a second experiment in the laboratory.

The researchers maintained wild-caught European carp in three different aquarium environments: one with a small amount of food, one with a large amount of food, and one with a small amount of food and several pike that were hand-fed three times a week to keep them from eating the carp.

After two months, Bronmark and Miner found that the two groups of carp maintained without pike grew at roughly the same rate, regardless of the difference in their access to food. However, the carp exposed to pike grew much larger bellies, the researchers discovered--despite the fact that the added bulk increased their drag through the water, requiring them to swim harder.

Bronmark and Miner conclude that European carp tailor their body shape according to their environment. While they usually stay lean and mean, the better to compete for resources with their peers, the carp grow bellies to put off pike.

Aquatic ecologist John E. Havel of Southwest Missouri State University in Springfield suggests the carp might be responding to a chemical secreted by pike. However, no one has yet identified such a substance. "There's a lot of speculation," he says, "but it's still a puzzle."