Socated on the Seattle campus of the University of Washington, the Burke Museum is home to the largest repository of preserved fish in North America. More than 400,000 individuals representing 4,100 species populate the shelves of the museum’s ichthyology collection. Preserved in ethanol, these specimens are a window into the marine and freshwater ecosystems of the past. For the University of Washington (UW) ecologist Chelsea Woodhowever, the most interesting things in those thousands of jars are not the fish themselves, but the parasites they carried in and on their bodies.
Wood and his lab are studying these tiny creatures to answer a long-debated question: how has the abundance of the parasites changed over time? “Until recently, I didn’t think we were going to find answers to that question,” Wood says. Many ecologists have worked on the assumption that parasite loads in the past were lower than today, she explains. Indeed, parasite abundance is often seen as a sign of stress, as hosts may be less able to control their parasite loads when faced with stressors such as food shortages or pollution, conditions that have intensified in many areas in recent years.
But this hypothesis is untested, with very little data to support or refute it. While previous studies have been able to detect parasites in preserved fish specimens – some of them dating back years or centuries – these studies have yielded little information about parasite abundance. The problem, Wood explains, was that there was no way to check whether the measures taken to preserve these specimens affected the number of parasites detectable on them.
In 2020, Wood and his team found a way to collect this data. They took fresh fish from three species and preserved some of them in ethanol, the same method used by the Burke and other museums around the world. Several days later, the researchers compared the parasite counts of these experimentally preserved fish to the numbers of fresh specimens they immediately dissected and, for the first time, confirmed that the preservation process was not biasing the numbers. This validation study meant that any fish preserved and stored in this way — potentially millions of specimens in museums around the world — could be used to investigate the question of past parasite abundance, Wood says.
Walleye pollock specimens at the University of Washington’s Burke Museum
To get an accurate count of these tiny creatures, Wood’s lab used a variation of an established methodology that involves cutting fillets of preserved fish and flattening the muscle tissue between glass plates to detect the parasites. Wood’s team made an incision and opened the body cavity, then shined a powerful light through the side of the fish. The parasites appeared as shadows against the bright background of the preserved muscle, allowing the researchers to pull them out and identify them under a magnifying glass or microscope.
While previous studies have been able to detect parasites in preserved fish specimens – some of them dating back years or centuries – these studies have yielded little information about parasite abundance.
Over the past few years, the lab has continued to refine the technique and apply it to a greater number and variety of samples. In a recent article published in the Journal of Animal Ecology, Wood explores how such a methodology could help researchers answer ecological questions not only about fish but other aquatic taxa as well. The article introduces historical parasite ecology as a new sub-discipline dealing with the abundance of parasites in the past as well as the biotic and abiotic factors that affect their populations, both yesterday and today. As the method is applied more widely, it challenges, or at least complicates, the assumption that early seas were less dense in parasites. On the contrary, “our research suggests that many metazoan parasites are declining in abundance,” says Wood.
Joshua Brian, a postdoctoral research associate at King’s College London who was not involved in the research, says the team has produced “a brilliant paper”. Brian studies host-parasite interactions in freshwater mussels, focusing on the susceptibility to extinction of these small, unloved parasitic species. “These kinds of methods, to look in the past at where the parasites were and what their abundance was, and tie that to the host variability, to the environmental variability, it’s so important to start building that picture of the how parasites change,” adds Brian.
A rockfish specimen collected in the 1970s, at the University of Alaska Museum of the North
The Wood Lab is currently working on its largest analysis of fish parasites to date, covering dozens of parasite species and spanning 130 years of Puget Sound’s history in the Pacific Northwest. Wood says she expects the upcoming study to provide much more insight into ecological changes for the parasites and their hosts over the past century.
Apart from preserved fish, mammalian specimens preserved in museums also prove useful for the historical study of parasites and diseases. In a recent paper, University of Richmond researchers have extracted bacterial DNA from mammalian skins to learn more about the spread of Lyme disease in the eastern United States. By testing specimens of mice — and the ticks they carried — from the Virginia Museum of Natural History’s mammalogy collection, the paper’s authors were able to trace the spread of the bacteria that causes Lyme disease (Borrelia burgdorferi) to the south in recent decades.
“Specimens are collected and archived for a reason, and prove to be incredibly valuable historical records for further study,” says the co-author Nancy Moncrief, curator of mammalogy at the Virginia Museum. She collected some of the mouse specimens used in the study herself, for research that had nothing to do with Lyme disease. “We can’t predict what technology will be here ten years from now, but we’ve archived the samples and documented them properly, so we can answer those questions.”
Wood notes that this is an advantage of his lab’s less invasive methodology for dissecting fish: it minimizes damage to irreplaceable original specimens. Everything used in laboratory studies returns directly to museum collections, down to the last tiny parasite (labeled and stored separately from the source specimen), leaving them available for future researchers to explore their own questions.
The only concrete limits to what can be done with museum specimens are how the specimens make them a collection and the survival of the collections themselves. “There’s not as much archived material,” says Moncrief, pointing to a decline in new museum collections and a worrying trend of small collections, especially at struggling universities, closing their doors. “You can’t go back in time,” she adds, but museum specimens offer a rare glimpse into the past. As long as the collections survive, their applications should only continue to grow.