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. © Katherine Maslenikov/ UW Burke Museum.

More than a century of preserved fish specimens provide 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 US – warmed significantly.

The study is the world’s largest and longest dataset of wildlife parasite abundance. It suggests that parasites may be especially vulnerable to a changing climate.

Lead author Chelsea Wood, a UW associate professor of aquatic and fishery sciences, said: “People generally think that climate change will cause parasites to thrive, that we will see an increase in parasite outbreaks as the world warms.

“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.”

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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. © Katie Leslie/ University of Washington.

historic fish specimens

While some parasites have a single host 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 fish – analysis of historic fish specimens showed an 11% average decline per decade in abundance. Of ten parasite species that had disappeared completely by 1980, nine relied on three or more hosts.

Wood said: “Our results show that parasites with one or two host species stayed pretty steady, but parasites with three or more hosts crashed.

“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 children, or their pets – the result is worrying news for ecosystems.

Wood explained: “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.”

parasite declines

Among the multi-celled parasites found in the study, 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 fish specimens.

To explain the parasite declines, the authors considered three possible causes: how abundant the host species was in Puget Sound; pollution levels; and temperature at the ocean’s surface.

The variable that best explained the decline in parasites was sea surface temperature, which rose by 1°C (1.8°F) 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.

She stated: “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?

“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 species might be in real danger, 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 study is in the Proceedings of the National Academy of Sciences.