The northern snakehead, an invasive species of fish native to Asia, has successfully broken out of the Potomac River system and established itself in the Rappahannock River, according to state officials. The fish drew headlines when it turned up in a Maryland pond in 2002, in part because of its prodigious teeth and ability to survive several days out of water by breathing air. Spotted less than a decade ago in the Potomac, the snakehead has not just survived, but spread. Some of the snakeheads likely got into the Rappahannock after being dropped into a Fredericksburg-area pond that’s close to the river, said John Odenkirk, a state fisheries biologist who has been dealing with the snakeheads in the Potomac.