“I was shocked,” Daniel says. Other nonnative species have been known to hitchhike on plants—salamanders from the northwest, for instance, have been ushered eastward on Christmas trees—but “we never expected zebra mussels to travel through aquarium plants,” Daniel says. “It’s not a pathway we had ever considered.”

An employee at the Seattle store uploaded the sighting February 25. By March 2, Daniel had learned about the post, reviewed some photos, rallied colleagues at the state and federal level, and canvassed for the little green blobs in Gainesville, Florida, where he lives. At the first pet store he surveyed, Daniel found an adult zebra mussel clinging to a moss ball. That made him realize the Seattle sighting wasn’t “a weird fluke,” he says. “If they’re in Washington and Florida, I’m assuming they’re going to be distributed everywhere in between.”

Daniel showed the pet store manager how to kill the freshwater interloper by dunking it in super-salty water, and they removed the moss balls from the shelves. Still, by March 8, the USGS reported mussel sightings in moss balls in Alaska, California, Colorado, Florida, Georgia, Iowa, Massachusetts, Michigan, Montana, Nebraska, Nevada, New Mexico, North Dakota, Oklahoma, Oregon, Tennessee, Vermont, Virginia, Wisconsin, Washington, and Wyoming—with more states added since. Officials are investigating what Daniel refers to as the “chain of custody,” trying to determine whether the moss balls all came from a single producer, likely a wholesaler based in Ukraine, or from several sources. Meanwhile, with the US Fish and Wildlife Service and other partners, the USGS issued guidance for going ham on the moss balls to quash the new invasion—that’s where all the bleaching, boiling, freezing, and other pummeling comes in. Afterwards, the brutalized ball should end up in the trash, in a sealed bag.

Introductions from home aquaria aren’t the most common vector for species entering waterways such as the Great Lakes, but “they do happen,” says Christine Mayer, an aquatic ecologist at the University of Toledo. “Especially with fish—people don’t want to kill them. People don’t feel so bad about putting a plant in the compost, but they’re not sure they know how to humanely euthanize a fish.” Mayer says that ecologists sampling in the Great Lakes will sometimes come across a goldfish, either freed from a fishbowl or descended from others gone feral.

A preemptive strike is the best way to keep zebra mussel populations small, because established communities are incredibly difficult to eradicate. “Everyone who works with invasive species says prevention is better than cure,” Mayer says. “Keeping things out is cheaper, easier, and better than trying to kill them once they’re there.”

Introductions from large ships aren’t such a major issue anymore, because there are now procedures for how, where, and when ballast water can be dumped, says Eva Enders, a research scientist at Fisheries and Oceans Canada who has studied the mussels’ muscling through Lake Winnipeg, in Manitoba. Now, Enders says, “the risk that remains is inland—the transport by small leisure or recreational boats from lake to lake, river to river.” To aid in prevention, ecologists, naturalists, and others around the Great Lakes region have encouraged boaters to inspect their hulls and motors and thoroughly drain and dry water-holding compartments between outings.