Marine parasites are vulnerable to warming oceans

Parasites may fill us with revulsion and disgust, but they are integral components of ecosystems and play important ecological roles. They regulate host populations, and influence food webs, competition and biodiversity, even shaping community and ecosystem structure. In fact, there are way more parasitic species than there are predators, indicating how widespread this life-history strategy is in nature. 

However, while we are deeply concerned about the impacts of global warming on the species we value (think birds and mammals), we don’t often imagine how the world’s parasites will be affected. In a new study, researchers from the University of Washington and colleagues have made use of fish specimens preserved in museum collections over the past 140 years to assess long-term trends in parasite populations. 

“People generally think that climate change 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.”

The current study focused on eight species of fish caught in Puget Sound, the second largest estuary on mainland U.S. These species are common in the collections of natural history museums and most of the specimens came from the UW Fish Collection at the Burke Museum of Natural History and Culture. While mammals and birds are preserved with taxidermy, which retains parasites only on skin, feathers or fur, fish are preserved in fluid, which enters their tissues and thus also preserves any parasites living inside them at the time of their death. 

The researchers had to dissect and open up the preserved fish specimens and identify any parasites they found inside the body. They counted the parasites before returning the specimens to the preserving fluid. In total, the scientists found 17,259 parasites, of 85 types, in the 699 fish specimens.

“It took a long time. It’s certainly not for the faint of heart,” said Professor Wood. “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.”

The results of the study show that, for some parasite species, numbers decreased dramatically between 1880 and 2019, in fish from Puget Sound. These parasites included multicellular species of arthropods (animals with an exoskeleton), such as crustaceans, as well as what Wood describes as “unbelievably gorgeous tapeworms:” the Trypanorhyncha, whose heads are armed with hook-covered tentacles. 

Those parasite species that suffered the largest population declines were the ones that are dependent on three or more host species during their lifecycle. Some parasites have a single host species, but most require more than one and will travel between host species. For example, the eggs of some parasites 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 its own 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 average decline in abundance of 11 percent per decade. Of 10 parasite species that had disappeared completely by 1980, nine relied on three or more hosts. 

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

“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,” added Wood, who is one of the authors of a 2020 report laying out a conservation plan for parasites.

To understand the possible causes of these declines, the researchers investigated three factors over the 140 year time span; the abundance of the fish species in Puget Sound, pollution levels in the estuary, and water temperature at the ocean surface. The variable that best explained the decline in parasites was sea surface temperature, which rose by 1 degree Celsius (1.8 degrees Fahrenheit) in Puget Sound between 1950 and 2019.

For the parasite species that require multiple hosts to complete their lifecycle, warming temperatures just need to affect one of the hosts and this will prevent the parasite from reproducing successfully. These parasites are thus vulnerable to disruption at any point along the way. 

“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 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 research is published in the journal Proceedings of the National Academy of Sciences. 

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By Alison Bosman, Earth.com Staff Writer

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