Invasive, shrimp-sucking parasite continues northward Pacific expansion
The cough drop-sized parasite Griffen’s isopod, native to Asia and Russia, has decimated mud shrimp populations along the West Coast. The parasite on the right is a female with the much smaller male attached. (Florida Museum photo by Amanda Bemis and Gustav Paulay)
By Josh Silberg and Natalie van Hoose
Researchers have identified an invasive, blood-sucking parasite on mud shrimp in the waters of British Columbia’s Calvert Island. The discovery represents the northernmost record of the parasite on the West Coast and is likely an indication of its ability to spread without human transport.
Griffen’s isopod, 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 Griffen’s isopod at Calvert Island, described in a new study, represents a northward leap of more than 300 kilometres.
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.
The parasite attaches to the gills of its host – here, a blue mud shrimp. (Florida Museum photo by Amanda Bemis and Gustav Paulay)
“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. Griffen’s isopod is thought to have first arrived in North America by traveling in ships’ ballast water. The species’ appearance at Calvert Island, 241 kilometres 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.”
Click to expand » The parasite was found at Calvert Island in a 2017 survey of marine life. The record represents a northward leap of more than 180 miles from its last known northern boundary. (Image by Whalen et al. in BioInvasions Records)
Matt 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 — an oddball relative of pill bugs in your backyard wood pile. 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 blue mud shrimp, and proceed to sap the life from it. Infected mud shrimp are so hard done by that they lack the required energy to reproduce.
“They’re essentially castrated,” Gustav said.
Mud shrimp may not be much to look at — much like crayfish with stumpier claws — but these homely crustaceans play an outsized role as environmental engineers in the mudflats of the Pacific Coast. They cycle nutrients when they filter food, pumping oxygenated water into an expansive network of tunnel dwellings, which provide housing for a suite of creatures, including gobies, worms, clams and other shrimp species. The shrimp’s presence affects how the entire mudflat ecosystem functions — or doesn’t.
When a parasite coevolves in the same place as its host, they both often reach a sort of agreement, Gustav said. After all, the parasite needs a host to survive, and killing it off at once would not make a great long-term strategy. But when a parasite is introduced from elsewhere, that balance may never arrive.
Florida Museum curator Gustav Paulay, left, shows a sea urchin to Hakai Institute’s Josh Silberg during a 2019 bioblitz in Los Angeles. (Photo courtesy of Katrina Pyne/Hakai Institute)
“The infection rates on Calvert Island were higher than I would’ve anticipated,” Matt said. “About one in four hosts were parasitized. That’s a pretty good chunk of the population.”
For now, scientists are tracking the northward spread of the parasite. The parasite’s prevalence on Calvert Island shows that it may only be a matter of time before it reaches the North Coast of British Columbia and moves onward to Alaska, the upper edge of the mud shrimp’s range.
For Gustav, the discovery of Griffen’s isopod also underscores how marine bioblitzes can function as early warning systems for invasions.
Florida Museum’s Amanda Bemis photographs a specimen during a marine bioblitz. (Photo courtesy of Katrina Pyne/Hakai Institute)
“Every bioblitz we do, we find invasive species,” he said. “If you catch them early enough, you have a chance to do something about it.”
The researchers published their findings in BioInvasions Records.
Funding for the research was provided by the Smithsonian’s Tennenbaum Marine Observatories Network and the Hakai Institute.
Sources: Matt Whalen, [email protected];
Gustav Paulay, [email protected]
This blog originally appeared on Hakai Institute’s website and an adapted version is reposted with permission from Florida Museum’s blog.
