Parasites play an outsize role in balancing ecosystems, and some species may be in danger
I was preparing dinner, portioning a piece of cod, when a small, pink blemish appeared in the pristine white muscle of fish. Removing the splotch with a knife tip, I realized something was very wrong. What had looked like a bulbous vein began unfurling into a thin squiggle the length of my pinky finger—and it was moving.
Parasites are organisms that live in an intimate, lasting and costly relationship with their hosts, and scientists estimate that fully 40 to 50 percent of all animal species fall into this group. Just about every free-living species on the planet has at least one parasite specially evolved to exploit it. The broadest definition of “parasite” includes pathogens such as bacteria, viruses, fungi and protozoans.
Like predators, parasites can exert an effect on populations of other organisms in their habitat, which shapes everything from nutrient cycling to the types of plants that grow there to the abundance of top predators. In other words, parasites “play a major role in the natural world that was previously just overlooked,” says Armand Kuris, a parasite ecologist at the University of California, Santa Barbara.
Wood’s mindset shifted slowly, then completely. In parasites, she began to discover an unseen world operating in parallel to the one of free-living species. Her undergraduate courses had hardly mentioned those animals. “It’s possible to get a degree in biology and never learn anything about parasites,” she says, citing a 2011 study that found that 72 percent of 77 conservation biology textbooks either did not mention parasites at all or only portrayed them as threats to the hosts they occupy.
Wood refers to this as the “sky is falling” narrative. In a 2015 paper published in the Proceedings of the National Academy of Sciences USA, for example, researchers wrote that because “host diversity inhibits parasite abundance ... anthropogenic declines in biodiversity could increase human and wildlife diseases.” This is based on an assumption, however, that parasites are always bad.
Historical Ecology Katie Leslie is sorting through ribbons of intestines belonging to a rockfish that’s been dead for 41 years. So far Leslie, a research technologist in Wood’s lab, has found only the remnants of the animal’s last meal. Rockfish are notoriously wormy, but this specimen is proving to be exceptionally parasite-free, untilUnder the microscope is the first parasite of the day, a thorny-headed worm. Leslie goes on to tally seven more parasites, including flatworms and nematodes.
According to Lafferty, who was not involved in the research, those results “demonstrate a new value for the millions of pickled fish in jars on museum shelves across the world.” The findings themselves are notable, he continues, because they add an important data point about how parasites respond differently to environmental change. English sole parasites have been surprisingly stable over time, but for those whose populations did shift, not all went up.
In addition to the possible impacts on health for humans and wildlife, spikes in parasite populations can harm certain industries. The Puget Sound, for example, is famous for producing Pacific oysters with pearly, unblemished shells. But in 2017 a colleague dropped a shell on Wood’s desk marked with squiggly canals and dark, ugly spots—signs of a shell-boring oyster pest called Polydora.
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