Scientists say warming oceans have wiped out marine parasites, suggesting they may be particularly vulnerable to climate change.
More than a century of preserved fish specimens offer rare insights into long-term trends in parasite populations, according to a new study.
Research conducted at the University of Washington (UW) indicates that the number of fish parasites decreased from 1880 to 2019, or 140 years, when Puget Sound – their habitat and the second largest estuary in the continental United States – warmed significantly.
The researchers say that if a similar level of decline occurred in species that humans “care for”, it would trigger conservation action.
Scientists say while these tiny creatures evoke fear or revulsion in many, their demise is troubling news for ecosystems.
Lead author Chelsea Wood, UW Professor of Aquatic Sciences and Fisheries, said: “People generally think that climate change will cause parasites to grow, that we’re going to see an increase in parasite epidemics as the world warms.
“This may be true for some species of parasites, but parasites depend on their hosts, which makes them especially vulnerable in a changing world where the fate of their hosts is reshuffled.”
The researchers say the study is the world’s largest and longest collection of data on the abundance of wild animal parasites.
This suggests that the parasites may be particularly vulnerable to a changing climate.
While some parasites have a single host, many travel between host species.
The eggs are carried by one species, then when the larvae emerge, they infect another host, and the adult may reach maturity in a third host before egg-laying, the researchers said.
Dr Wood said: “Our results show that parasites with one or two host species remained fairly stable, but parasites with three or more hosts crashed.
“The degree of decline has been serious. This would trigger conservation efforts if it occurred in species that people care about, such as mammals or birds.
She added: “The parasite ecology is really in its infancy, but we know that these complex life-cycle parasites likely play an important role in pushing energy through food webs and supporting top-end predators.”
Scientists have used a new method of resurrecting information about past parasite populations.
Mammals and birds are preserved by taxidermy, which only traps the parasites on the skin, feathers or fur.
However, fish, reptile and amphibian specimens are preserved in a fluid that also protects 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 of the samples were from the UW Fish Collection at the Burke Museum of Natural History and Culture.
The scientists carefully sliced up the preserved fish specimens, then identified and counted the parasites found in them before returning the specimens to museums.
Scientists have discovered a wide variety of parasites, including arthropods or animals with an exoskeleton, including crustaceans, as well as what Dr. Wood describes as “incredibly wonderful tapeworms.”
In total, the team counted 17,259 parasites, from 85 types, from 699 fish specimens.
Dr Wood explained: “This study shows that there have been major parasite declines in Puget Sound.
“If it can happen unnoticed in an ecosystem as well-studied as this, where else can it happen?”
She added: “Our result highlights the fact that parasitic species may be in real danger.
“And that could mean bad things for us – not only less worms, but also less ecosystem services driven by the parasites we depend on.”
The findings were published in the Proceedings of the National Academy of Sciences.