64 Scientific American, May 2022
David Scharf/Science Source (
preceding pages
)
Rachel Nuwer is a freelance science journalist and author who
regularly contributes to Scientific American, the New York Times
and National Geographic, among other publications.
Wood’s answer arrived the next morning: Anisaki-
dae, she wrote—probably Anisakis simplex or Pseudo-
terranova decipiens —a common nematode that spends
its larval stage in fish or squid. Wood went on to con-
gratulate me: “What better way to start off the new year
than to find a real live worm in your cod fillet?”
Given that Wood had told me about the abdominal
pain, vomiting, diarrhea and bloody stool I would have
experienced had the live worm managed to find its way
into my esophagus, stomach wall or intestines, the con-
gratulations seemed odd. In her enthusiastic manner
she explained why such a discovery was positive: the
typical hosts of this parasite are whales, dolphins, seals
and sea lions—animals at the top of the food chain.
“The presence of the worms in the fish is actually a sign
that the ecosystem it came from is healthy and that
there is a healthy population of marine mammals
nearby,” Wood wrote. “Celebrate that squirmy harbin-
ger of good news!”
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. But many
parasitologists like Wood focus on multicellular meta -
zo ans: animals that encompass hundreds of thou-
sands of species, including up to 300,000 differ-
ent types of worms that parasitize vertebrates alone.
Metazoan parasites are as diverse as they are abun-
dant. They span 15 phyla, ranging from microscopic,
barely multicellular blobs to 130-foot-long tapeworms
snuggly coiled inside whale guts—species as phyloge-
netically different from one another as humans are
from insects and jellyfish. They live in every habitat on
every continent and in every orifice, organ and body
part of their hosts. And they are some of the world’s
most extreme specialists, with wildly intricate life
cycles sometimes requiring up to five different hosts to
allow them to get from egg to larva to adult. “It’s just
such a beautiful expression of the complexity of nature
and its interconnectedness,” Wood says.
Yet relatively few biologists—and hardly anyone
else—are more than faintly aware of parasites beyond
the tiny sliver of species such as tapeworms, pinworms
and hookworms that are irksome or harmful to
humans. As a result, nearly everything we know about
parasites today comes from studying how to kill them.
“The depth of our ignorance is really unforgivable,”
Wood says.
That’s beginning to change. “Disease ecology and
parasite ecology is the now fastest-growing subset of
the ecological sciences,” says Skylar Hopkins, a para-
site ecologist at North Carolina State University. With
a recent influx of early-career researchers, “we have this
critical mass of scientists and practitioners.” As the field
grows, more evidence is emerging that points to para-
sites playing an outsized role in nature. One new study
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 some-
thing 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.
Like a scene from a horror movie, I watched, entranced, as the serpentine creature swayed
its body, dismayed, it seemed, at finding itself ripped from the embrace of fish flesh. Before put-
ting it in the compost bin, I snapped a few photos. I knew exactly who to send them to for identi-
fication help: Chelsea Wood, a parasite ecologist at the University of Washington—and perhaps
the world’s only person who uses words like “beautiful” to describe bloodsucking worms.
