It's a beautiful green water frond with delicate petals. Pleasant enough to look at with names like "Water Thyme" and "Star Vine."
But don't let its looks fool you. Hydrilla verticillata, which grows up to several feet thick and chokes the life out of lakes and ponds, has been dubbed the "killer" weed by those in the know.
"It's just a thug," says Leslie Mehrhoff, director of the Invasive Plant Atlas of New England.
Once a problem mostly in Southern states, hydrilla has become an aquatic scourge in a third of US states. Researchers blame the weed's spread not only on unwitting aquarium hobbyists and motor-boat propellers, but also the Internet.
Indeed, online sales of such noxious weeds - some of them illegal - have flourished so much in the past few years that the federal government is preparing a high-tech crackdown.
"We've seen a link between growing Internet sales, the mail system, and the spread of these plants," says Larry Fowler, a botanist with the US Department of Agriculture's Animal and Plant Health Inspection Service (APHIS). "We realize many people are simply unaware of our laws, but that still doesn't make it right. And there's still a segment that is quite aware of the law and is still selling."
The hydrilla - named after the hydra-headed monster of Greek mythology - is on the federal govern- ment's short list of "noxious weeds," making it illegal to buy or sell it nationwide.
Nevertheless, an online search leads within minutes to the website of a New York pet store offering not only hydrilla, but also another federally banned plant - Hygrophila polysperma, known as "Indian Water Star."
"We know our products," the website boasts. "We're proud to have the most knowledgeable staff in the industry."
Clearly, federal law is not the company's strong point.
Businesses selling banned plants can be fined up to $250,000 under federal law, while smuggling them into the US can bring criminal penalties.
Nearly three years ago, Mr. Fowler saw the danger of Internet sales of invasive species. Miscreants were selling with impunity invasive plants like hydrilla. Next-day courier services made it possible to ship them coast-to-coast or even from abroad with little scrutiny.
With the help of North Carolina State University researchers and the Internet search company Fast Search & Transfer, Fowler created a system that can identify and track Web pages and Internet operators selling outlawed plants.
The resulting high-tech enforcement tool, called the Agricultural Internet Monitoring System (AIMS), is to be unveiled in January. Already, its pilot test has identified 6,568 distinct pages on websites belonging to US suppliers who may be hawking banned plants (4,790 pages), mollusks (734 pages), and insects (1,044 pages). Those numbers could rise dramatically as researchers eliminate technical glitches.
As soon as January, US sellers of regulated plants will begin to get e-mail notices from APHIS warning them to produce a federal permit to sell such plants - or stop selling them.
Hydrilla has spread northward to 16 states, including Maine, Vermont, and Massachusetts, according to the US Agriculture Department. Hydrilla was found in Massachusetts in a pond on Cape Cod two years go. Scientists theorize it may have arrived with aquarium hobbyists who dumped fish tanks into local ponds - or even as tendrils stuck to propellers of motor boats.
But the latest reason given for the spread of such plants is technological.
"The Internet sales of plants and other organisms is quite a large unknown and there's little if any regulation," says Ted Grosholz, an ecology professor at the University of California at Davis. He leads a program that intends to educate the public and businesses as a way to reduce the introduction of nonindigenous species.
One of his concerns: the seaweed Caulerpa taxifolia, dubbed "killer algae" because of its propensity for spreading fast and pushing out other species. Popular with aquarium fanciers, caulerpa escaped in the mid-1980s into the Mediterranean, where it has become a huge problem. In 2000, it showed up in a lagoon in Carlsbad, Calif.
Efforts to eradicate it in California appear to be working, but the threat remains of caulerpa being shipped from overseas to California aquarium lovers. "We're working with both the pet and aquarium trade," Professor Grosholz says.
What's needed most is more consumer and pet-store education, the pet industry says.
"Internet sales can be a real problem," says Marshall Meyers, executive vice president of the Pet Industry Joint Advisory Council, an industry group overseeing the invasive-species issue. "You can buy giant salvinia over the Internet out of Europe. [But] this is an area that's confusing to the public. Not all nonnative species of fish and plants are invasive. Our industry relies on nonnative species. One thing we are trying to do is educate the public not to release these into the environment."
The council will soon begin mailing information to pet owners to acquaint them with the invasive-species problem. It also supports Agriculture Department efforts to police the Web, though some sellers may be doing so out of ignorance of the law, Mr. Meyers says.
At least initially, the AIMS Web-scanning program will focus on US sales of about 600 organisms, including plants and animals like the Giant African Snail - a voracious creature 6- to 8-inches long that can reproduce quickly and threaten crops. There are plans to expand the system to monitor international websites, Fowler says.
The AIMS program is working with Australia, Britain, and a few other nations to develop an information-sharing system to identify and shut down operations selling invasive plants. The goal is to use the new system to identify sales of "bad actors" abroad, well before they begin to arrive en masse, Fowler says.
People and plants aren't the only ones who move in and change ecosystems. Invasive species of animals are spreading globally. For example:
• Argentine ants, one of the world's most invasive species, have formed a "supercolony" 60 miles wide under Australia's second-biggest city, Melbourne. Also discovered in North America and Europe, the ants drive out local species.
• Cane frogs in New Zealand and rats, pigs, cats, and dogs in New Caledonia are a few of the targets of a $1 million New Zealand project to help protect the fragile ecosystems of Pacific Island nations.
• Sea squirts in America's Puget Sound created such alarm that divers this month used an experimental chlorine treatment to try to get rid of them.