PHILADELPHIA - Biologists say six alien fish--known as northern snakeheads-- have been pulled from a pond in South Philadelphia. Slideshow: Snakehead Fish Photo Gallery

They say the pond connects to both the Delaware and Schuylkill rivers contained what has been called the "frankenfish" because of its habits and abilities.

That's a problem because the fish can wriggle short distances across land. And that means they could invade other waterways.

Experts say the fish are insatiable eaters that gobble up native fish, birds, frogs and small mammals.

The species is native to China.

Experts say some collectors keep them as pets, and then dump them into the local waterways when they outgrow the tank. A five-pound, two foot-long snakehead was recently pulled from the South Philadelphia pond.

Authorities say anyone who reels in a snakehead should kill it.