Then in June, officials from the Maryland Department of Natural Rescources e-mailed a photo of a strange fish that had been caught in the Crofton pond. Courtenay immediately identified it as a northern snakehead, confirming the fish was nesting 75 yards from the Little Patuxent River.

Officials later determined that a local man had dumped two adult snakeheads-one male and one female - into the pond two years ago after they outgrew his aquarium. Earlier this month, DNR officials captured about 100 young snakeheads. Courtenay fears thousands are lurking - merchants typically sell snakeheads when they are about two years old and at their sexual peak. Do the math, he says, and it adds up to a third generation.

But, some scientists say Maryland's fish story has been blown out of proportion.

"These stories tend to get exaggerated and take on a life of their own," said Paul Shafland, director of the Florida Fish and Wildlife Conservation Commission's nonnative fish research lab in South Florida. "You hear the fish have teeth, they breathe air. Pretty soon you hear they climb trees, they eat dogs, they tap-dance, whatever."

Shafland, who is also on the Maryland panel, said the question of the snakehead's danger is one of perspective, and that native fish are far more resilient than some think.

But the Gainesville team says the Crofton discovery bolsters their conclusions - that snakeheads are dangerous and federal law should prohibit anyone from importing them alive.

"People said, what's the probability of them ever reproducing? Well, now we know. How would two fish in 10 acres of water find each other? Well, they did," Williams said. "I can't predict today that snakeheads are going to enter every stream in the Mississippi and wreak havoc on all fish populations, but it certainly has the potential to happen."

The snakehead saga reminds Courtenay of an old fish story.

In 1967, Courtenay had just begun what would be a 30-year career teaching zoology at Florida Atlantic University when the Asian walking catfish arrived in South Florida. The working story is that it escaped from a fish farm, where a man raised it because he thought its albino color would be popular with collectors. Later, the fish farmer's brother told Courtenay it jumped out of its tanks as he drove from Miami International Airport.

Either way, the predatory fish navigated the canal system like a beltway, chomping on every fish it met. Officials soon banned them in the state, prompting farmers in Tampa to dump their collections. Tampa's fish moved south while Miami's headed north, and by the time the species met in Lake Okeechobee, Courtenay said, little could be done for the native fish.

The walking catfish hooked Courtenay on invasive species, which he's studied ever since. It also taught him an important lesson about prevention. He keeps a walking catfish paperweight on his desk as a reminder.

Courtenay has worked tirelessly on the snakehead problem - his wife, Pat, sneaked a laptop into the hospital last weekend after he was rushed to the emergency room for digestive problems. He read e-mails and news reports as he recovered.

And while he expects some belly-aching from snakehead enthusiasts for recommending a ban, he said the government should seize the opportunity to keep the willful fish from spreading.

As if to drive the point home, Courtenay reaches into the bull's eye snakehead's tank. Immediately, the asphalt-colored fish sprouts spots resembling tire tracks - a fight-or-flight reaction. Williams lowers a net, but the fish jumps out twice, pirouetting back into the water. It digs its teeth into the netting until Courtenay pulls it out.

"Here's your snakehead," Williams said. "Now, as for where it's going to go, and what it's going to do..."

"It will determine that," Courtenay interrupted. "Not us."