THEY LOOK LIKE aliens from a science fiction movie. Their reputation -- a killer fish that breathes air -- is appealingly ghoulish. But the discovery of three northern snakeheads in the waters of the Potomac has implications far beyond the tabloid headlines they inspire. Jerry McKnight of the Maryland Department of Natural Resources puts it succinctly: "The northern snakehead is the nasty, slimy, toothy tip of the iceberg."

Indeed, the snakehead, originally imported from Asia, is only the most recent of several damaging "invasive species" to arrive in this region from other continents. Mute swans from Asia, although more attractive than the snakehead, are wreaking destruction on the underwater grasses of the Chesapeake Bay, ruining the habitat for crabs and native fish and pushing out native swans. The South American nutria, a water-dwelling rodent native to South America, is destroying Maryland marshes. Zebra mussels, imported to the Great Lakes in the ballast water of ships, have clogged up water systems, doing billions of dollars of damage, and are headed this way. Like all of them, the snakehead has no natural predators in this area and no organic role in the local ecosystem. The fish has the potential to destroy some or all of the Potomac's other fish and to take down fishing and tourist businesses as well. Worse, there is almost nothing state officials here or anywhere else can do about it, except ask anglers to kill them.

There are, however, steps that can be taken at the federal level, if not to destroy the snakehead, then at least to block other invasive species from doing even more damage. States can ban ownership of particular animals, but they do not have authority to ban their import from abroad. They can attempt to cull or control animals, but only within their own borders. Yet there is no reason why importers of exotic fish or animals should have fewer federal controls on their business than importers of other damaging or polluting substances have. A pair of bills making their way very slowly through Congress would mandate controls over the exotic fish trade and establish standards for ships carrying foreign ballast water as well. If it serves no other purpose, the latest snakehead scare should force politicians to focus harder on the damage that exotic plants and animals can do to native species, whether they get here accidentally or on purpose.