Simulated fish swim through virtual seas.

They glide gracefully through water, group into schools, and scatter when pursued by predators. They eat, mate, and teach themselves to swim smoothly. And yet they don't exist physically.

Such artificial fish exist only in a computer netherworld. The fish, displayed on a color monitor, swim together within a supercomputer's processors. Each emanates from an autonomous computer program nested within a larger program, which generates a simple underwater ecosystem.

"Our algorithms try to emulate not only the appearance, locomotion, and behavior of individual animals, but also the complex group behaviors evident in some aquatic ecosystems," says Demetri Terzopoulos, a computer scientist at the University of Toronto. "Artificial fish learn to control internal muscles to locomote hydrodynamically and exhibit a repertoire of realistic behaviors, relying on their perception of a dynamic habitat."

Speaking last week at an "Artificial Life" conference in Cambridge, Mass., Terzopoulos described how each fish uses muscles and fins to avoid obstacles, other fish, and plants. Each fish explores its environment and searches for food. Large predator fish hunt down small fish, which form schools, take evasive action, and scatter--as real fish do. Predators select, chase, and consume prey until satiated. Other fish forage for plankton and engage in mating rituals.

The "virtual" fish employ a perception system to detect their surroundings, a motor system to control movements, and a behavior system to coordinate relevant actions. An intention generator helps the fish to avoid collisions, evade predators, eat, mate, school, or wander. The researchers program each fish's affinity for darkness, coolness, and schooling, plus its level of hunger, fear, or desire to mate. Mimicking real fish, the virtual fish show courtship rituals; males dance, circle, ascend, and nuzzle female partners.

Undulating in a lifelike way, each fish's body is based on a computer model of 91 interacting springs in place of muscles. Hydrodynamic control systems sense virtual water and adjust simulated fins to send a fish where it wants to go. The fish, which do not know how to swim initially, learn with practice to move efficiently. In all, the computer-generated aquarium holds 10 fish, 15 food particles, and five plants.

"Some of these systems may at first look superficial, but they often provide biologists with insights into natural behaviors," says Christopher G. Langton, a computer scientist at the Santa Fe Institute in New Mexico.

Gene Levinson, a molecular biologist at the Genetics & IVF Institute in Fairfax, Va., agrees. "The fish presentation showed much more than just fancy computer graphics. The muscle system had a lifelike design that captured real biological complexity. The behaviors had a solid physiological basis, generating physical forces based on a body moving through a liquid."

"The simulated structures and behaviors strongly resembled what we see in nature," Levinson adds. "And that is scientifically interesting."