A researcher holds open a preserved fish specimen that has been inspected for parasites. The study included eight fish species and 699 fish specimens, which yielded more than 17,000 parasites. Credit: Katherine Maslenikov/UW Burke Museum

More than a century of preserved fish specimens offer a rare glimpse into long-term trends in parasite populations. New research from the University of Washington shows that fish parasites plummeted from 1880 to 2019, a 140-year stretch when Puget Sound—their habitat and the second largest estuary in the mainland U.S.—warmed significantly.

The study, published the week of Jan. 9 in the Proceedings of the National Academy of Sciences, is the world's largest and longest dataset of wildlife parasite abundance. It suggests that parasites may be especially vulnerable to a changing climate.

"People generally think that will cause parasites to thrive, that we will see an increase in parasite outbreaks as the world warms," said lead author Chelsea Wood, a UW associate professor of aquatic and fishery sciences. "For some parasite species that may be true, but parasites depend on hosts, and that makes them particularly vulnerable in a changing world where the fate of hosts is being reshuffled."

While some parasites have a single species, many parasites travel between host species. Eggs are carried in one host species, the larvae emerge and infect another host and the adult may reach maturity in a third host before laying eggs.

For parasites that rely on three or more host species during their lifecycle—including more than half the parasite species identified in the study's Puget Sound —analysis of historic fish specimens showed an 11% average decline per decade in abundance. Of 10 that had disappeared completely by 1980, nine relied on three or more hosts.

This copper rockfish (Sebastes caurinus) was collected in 1964 in Puget Sound. The study included eight fish species and found a dramatic decline in the number of parasites over time. Credit: Natalie Mastick/University of Washington

"Our results show that parasites with one or two host species stayed pretty steady, but parasites with three or more hosts crashed," Wood said. "The degree of decline was severe. It would trigger conservation action if it occurred in the types of species that people care about, like mammals or birds."

And while parasites inspire fear or disgust—especially for people who associate them with illness in themselves, their kids or their pets—the result is worrying news for ecosystems, Wood said.

"Parasite ecology is really in its infancy, but what we do know is that these complex-lifecycle parasites probably play an important role in pushing energy through food webs and in supporting top apex predators," Wood said. She is one of the authors of a 2020 report laying out a conservation plan for parasites.

These monogenean worms (Microcotyle sebastis) were dissected from the gills of a preserved copper rockfish specimen from the UW Fish Collection at the Burke Museum. Credit: Katie Leslie/University of Washington

Wood's study is among the first to use a new method for resurrecting information on parasite populations of the past. Mammals and birds are preserved with taxidermy, which retains parasites only on skin, feathers or fur. But fish, reptile and amphibian specimens are preserved in fluid, which also preserves any parasites living inside the animal at the time of its death.

The study focused on eight species of fish that are common in the behind-the-scenes collections of natural history museums. Most came from the UW Fish Collection at the Burke Museum of Natural History and Culture. The authors carefully sliced into the preserved fish specimens and then identified and counted the parasites they discovered inside before returning the specimens to the museums.

"It took a long time. It's certainly not for the faint of heart," Wood said. "I'd love to stick these fish in a blender and use a genomic technique to detect their parasites' DNA, but the fish were first preserved with a fluid that shreds DNA. So what we did was just regular old shoe-leather parasitology."

Pacific herring (Clupea pallasii) specimens on a shelf in the UW Fish Collection at the Burke Museum. This behind-the-scenes collection provided most of the specimens used in the 140-year study of parasite abundance. Credit: Katherine Maslenikov/UW Burke Museum

Among the multi-celled parasites they found were arthropods, or animals with an exoskeleton, including crustaceans, as well as what Wood describes as "unbelievably gorgeous tapeworms:" the Trypanorhyncha, whose heads are armed with hook-covered tentacles. In total, the team counted 17,259 parasites, of 85 types, from 699 .

To explain the parasite declines, the authors considered three possible causes: how abundant the was in Puget Sound; pollution levels; and temperature at the ocean's surface. The variable that best explained the decline in was sea surface temperature, which rose by 1 degree Celsius (1.8 degrees Fahrenheit) in Puget Sound from 1950 to 2019.

A parasite that requires multiple hosts is like a delicate Rube Goldberg machine, Wood said. The complex series of steps they face to complete their lifecycle makes them vulnerable to disruption at any point along the way.

A jar of fluid-preserved fish specimens from the UW Fish Collection at the Burke Museum. These fish were collected in Hood Canal in 1991. Credit: Katherine Maslenikov/UW Burke Museum

"This study demonstrates that major parasite declines have happened in Puget Sound. If this can happen unnoticed in an ecosystem as well studied as this one, where else might it be happening?" Wood said. "I hope our work inspires other ecologists to think about their own focal ecosystems, identify the right museum specimens, and see whether these trends are unique to Puget Sound, or something that is occurring in other places as well.

"Our result draws attention to the fact that parasitic might be in real danger," Wood added. "And that could mean bad stuff for us—not just fewer worms, but less of the parasite-driven ecosystem services that we've come to depend on."

The UW Fish Collection is a state-supported facility that houses more than 300,000 adult fish specimens. The jar on the left contains herring (Clupea pallasii) collected in 1952. Credit: Katherine Maslenikov/UW Burke Museum

Co-authors are Rachel Welicky at Pennsylvania's Neumann University, who did this work as a UW postdoctoral researcher; Whitney Preisser at Georgia's Kennesaw State University, who did this work as a UW postdoctoral researcher; Katie Leslie, a UW Research Technologist; Natalie Mastick, a UW doctoral student; Katherine Maslenikov, manager of the UW Fish Collection at the Burke Museum of Natural History and Culture; Luke Tornabene and Timothy Essington, faculty members in aquatic and fishery sciences at the UW; Correigh Greene at NOAA's Northwest Fisheries Science Center; and John M. Kinsella at HelmWest Laboratory in Missoula, Montana.

More information: Wood, Chelsea L., A reconstruction of parasite burden reveals one century of climate-associated parasite decline, Proceedings of the National Academy of Sciences (2023). DOI: 10.1073/pnas.2211903120. doi.org/10.1073/pnas.2211903120

Journal information: Proceedings of the National Academy of Sciences