Puget Sound’s Parasites Are Disappearing—but Don’t Be Glad to Say Goodbye

The decline, which was correlated with warming waters in a new study, is bad news for ecosystems

two brown horseshoe-crab-shaped parasites under magnification
Monogenean worms dissected from the gills of a preserved copper rockfish from the University of Washington Fish Collection at the Burke Museum Katie Leslie / University of Washington

The waters of Washington’s Puget Sound are not a paradise for parasites. According to a study published last week in Proceedings of the National Academy of Sciences, fish parasites that require three or more hosts declined dramatically in the estuary over the last century. Their loss was correlated with rising sea surface temperature, suggesting a connection with climate change.

“It’s really our first peek into what parasites have been up to over the past couple of decades,” study lead author Chelsea Wood, a parasite ecologist at the University of Washington, explains to the Seattle Times’ Isabella Breda. “It’s a warning. It suggests that there might be more loss of parasite biodiversity than we previously anticipated.”

Despite Hollywood’s common portrayal of parasites as villains, their decline isn’t anything to celebrate. This “substantial” loss in parasite diversity is “disturbing news,” Armand Kuris, a parasite ecologist at the University of California, Santa Barbara, who was not involved in the study, tells the New York Times’ Rachel Nuwer.

While parasites can harm their host species, a large-scale loss of these creatures could have major consequences for biodiversity and ecosystems. The much-maligned organisms do the important work of controlling populations of host species. They also can lead infected prey to act less cautiously, making it easier for predators to get a meal.

The severe decline recorded in Puget Sound’s parasites, from arthropods to tapeworms, “would trigger conservation action if it occurred in the types of species that people care about, like mammals or birds,” says Wood, who also co-authored a 2020 report outlining a conservation plan for parasites, in a statement.

preserved fish
A jar of fluid-preserved fish from the University of Washington Fish Collection at the Burke Museum. Katherine Maslenikov / UW Burke Museum

Using a method that might conjure up the scenes—and smells—of a junior high science lab, researchers dissected almost 700 preserved fish, most from the University of Washington Fish Collection, that had called Puget Sound home at some point over the last 140 years.

“It took a long time,” says Wood in the statement. “It’s certainly not for the faint of heart.”

In eight fish species, the scientists counted more than 17,000 parasites of 85 different types. They then looked at host species abundance, pollution levels and water surface temperature to try to explain their “parasite census” trends.

Through their painstaking work, the team found the population of parasites that rely on only one or two host species remained stable. But complex-life cycle parasites—which require three or more host species to complete the different stages of their lives, from egg to mature adult—experienced a significant decrease. These parasites, which represented more than half of the researchers’ sample, declined an average of 11 percent each decade between 1880 and 2019. Measured against the rise in surface water temperature, their population fell by 38 percent with every increase of 1 degree Celsius.

This suggests the parasite decline may be associated with climate change, though the researchers say additional factors, such as ocean pH or algal blooms, could have contributed as well.

According to the authors, the study is the world’s largest and longest data set of wildlife parasite abundance, making it an important contribution to the currently limited body of knowledge on how populations of parasites with non-human hosts are changing over time.

“There is no data set like this in the world,” Skylar Hopkins, a parasite ecologist at North Carolina State University who was not involved in the study, tells the New York Times. “Just imagine how long it took them to dissect all those stinky, ancient fish.”

Still, Wood says that more research is needed to find out if the trends in Puget Sound are part of a wider pattern. She has already begun to examine specimens from other locations, including the Gulf of Alaska and the Rio Grande, and she hopes other researchers will begin to do similar experiments.

“If this can happen unnoticed in an ecosystem as well-studied as this one, where else might it be happening?” she says in the statement.

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