The discovery represents the northern-most record of the parasite on the West Coast and is likely an indication of its ability to spread without human transport.
Orthione griffenis, a cough drop-sized crustacean native to Asia and Russia, has decimated mud shrimp populations in California and Washington over the past 30 years, causing the collapse of delicate mudflat ecosystems anchored by the shrimp.
By the 2000s, it had reached as far as Vancouver Island.
The discovery of O. griffenis at Calvert Island, described in a new study, represents a northward leap of more than 180 miles.
Scientists found the parasite during a 2017 bioblitz, organized by the Hakai Institute and the Smithsonian Institution's Marine Global Earth Observatory, in which they intensely surveyed and documented marine life.
"I was on the lookout for things that seemed out of place," said study lead author Matt Whalen, a Hakai postdoctoral researcher at the University of British Columbia who studies coastal biodiversity.
"But this particular parasite wasn't initially on my radar."
Most scientists believed the parasites' expansion was exclusively mediated by human transport -- O. griffenisis thought to have first arrived in North America by traveling in ships' ballast water.
Their appearance at Calvert Island, 150 miles from the nearest city of more than 5,000 people, shows "clearly, they can do it on their own," said study co-author Gustav Paulay, curator of invertebrate zoology at the Florida Museum of Natural History.
"This is such an astonishingly spectacular part of the planet," he said.
"During the bioblitz, one of the things we talked about was that there were no invasive species at all.
"And then we found this thing."
Whalen described the find as "a bit depressing."
"We tended to associate this parasite with places that have a lot of marine traffic and aquaculture, like California and Oregon," he said in a statement.
"Finding them on Calvert Island really suggests that there's very little preventing the spread because of the parasite's life cycle."
The parasite is a bizarre crustacean called a bopyrid isopod.
In the pre-adult part of its life, it hitches a ride on planktonic copepods an intermediate host that allows the isopods to travel to new and far-flung mudflats in search of shrimp blood.
As adults, the parasites attach to the gills of another crustacean host, in this case a mud shrimp, Upogebia pugettensis, and proceed to sap the life from it.
Infected mud shrimp are so hard done by that they lack the required energy to reproduce.