Killer cell.

It's a lot like a sci-fi movie monster. It thrives in polluted salty water. It can disguise itself as a harmless plant. And it eats anything that comes near it.

It's called Pfiesteria piscicida (pronounced FEAST-eer-ya pes-key-SEE-da), and it is killing fish along the Eastern seaboard at an alarming rate.

Pfiesteria is a dinoflagellate, a type of microscopic organism that whipping its long, thin tail. It may be small, but it's very deadly. As few as 250 cells per milliliter of water can kill a healthy fish within minutes, says JoAnn M. Burkholder, a botanist at North Carolina State University. The tiny beast injects two poisons into its prey: one stuns the victim and the other dissolves its flesh, creating large, ugly sores.

After it has eaten, Pfiesteria changes its shape. It can mimic common green algae, actually producing its own food, just like a plant. But it also displays other forms. This makes Pfiesteria hard to detect, even by scientists searching for it. In fact, Burkholder is one of the few botanists to have actually seen the killer cell under a powerful electron microscope.

Scientists are worried that Pfiesteria may represent a new kind of threat to the environment. There seems to be a direct link between Pfiesteria and pollution. In particular, the little monsters like waters rich in nitrogen (a chemical used in farm fertilizer) and laced with human sewage.

The pesky cell may be causing trouble in the Chesapeake Bay, which has had pollution problems in the past. Biologists with the Department of Natural Resources in Maryland are investigating why fish there are dying. But scientists caution that toxic chemicals in polluted water can produce lesions on fish that are similar to those left by the cell.

No matter what the cause of the fish-kill, however, the solution is the same: clean up polluted waters, making them less inviting to Pfiesteria and more friendly to healthy fish.

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Pfiesteria has killed every species of fish and shellfish it has encountered, in nature and the lab.

Pfiesteria changes shape constantly. It has at least 19 stages in its life cycle, compared to 4 for most insects.

People exposed to toxins excreted by Pfiesteria feel dizzy and suffer memory loss. Some say they feel `drugged.'