May 2022, ScientificAmerican.com 69Martinelli is now studying the ecology of the para-
site in hopes of finding ways to help oyster growers treat
and contain it. She and Wood are also trying to untan-
gle the history of Polydora and other shell-boring poly-
chaetes. An accidental introduction seems like the obvi-
ous answer, but the story may be more complex. Mar-
tinelli is turning to oyster middens—essentially, piles of
shells left over from ancient oyster feasts—to unravel
Polydora’ s history in the Pacific Northwest. She con-
firmed that 1,000-year-old native Olympia oyster shells
recovered from Jamestown S’Klallam Tribe middens
bear signs of some type of burrowing worm. Martinelli
guesses that this is a different species—but it could also
be that Polydora has been lying in wait in very low num-
bers and only now has been unleashed by some as yet
unknown environmental trigger.
Martinelli plans to excavate more recent oyster mid-
dens to see if she can pinpoint the parasite’s introduc-
tion into local bivalve populations. “The tricky thing
about paleo work,” she says, “is we’ll never have the def-
inite answer. But we do have traces of the past that are
comparable to the present.”SAVE THE PARASITES
parasite increases still get most of the attention—which
is why Wood is attuned to parasite decreases and their
implications for humans and wildlife. Some are to be
celebrated, such as the effort to eradicate the Guinea
worm, a spaghettilike parasite that grows up to 2.5 feet
long inside an infected person’s digestive system before
migrating to and eventually breaking through their skin.
But for parasites that do not impact humans—the vast
majority of species—some of the losses are concerning.
One 2017 Science Advances paper estimated that up to
30 percent of parasitic worms may go extinct in the
coming decades because of climate change and other
pressures, and we’re only just beginning to learn how
such a staggering loss of biodiversity will reverberate.
Take, for instance, the phenomenon of parasitic pup-
pet mastery that occurs in many species. “Parasites
shunt energy from lower to higher trophic levels by
making prey reckless,” Wood says. Euhaplorchis cali-
forniensis, for one, is a trematode flatworm that, in its
larval stage, looks a bit like a sperm, with a big head
and long tail. The flatworm begins its life in a snail,
then moves into a California killifish, then to its final
destination in the gut of a predatory water bird, such
as a heron or egret. Killifish typically spend their days
hiding, however, which runs counter to the flatworm’s
agenda. So the parasite creates cysts on its host’s brain,
causing the hapless killifish to splash around on the
surface of the water and flash its shiny belly, baiting the
birds. Infected killifish, researchers have found, are 10
to 30 times more likely to be eaten by a bird than non-
infected ones. Collectively, trematodes make a signifi-
cant proportion of killifish populations more readily
available as meals for birds—effectively subsidizing
those predators’ diets. If certain parasite species are in
decline or even disappear, it’s possible that “it could be
way harder to conserve predators,” Wood explains.
Likewise, in Japan a 15-inch-long nematomorph
worm causes infected crickets to dive into streams,
where the adult worms burst out of their hosts to par-
take in a parasitic orgy. Meanwhile the doomed crickets
become food for endangered Japanese char, providing
up to 60 percent of the fish’s calories. Not only does the
nematomorph worm help feed an endangered species,
but by relieving pressure on other invertebrate species
the fish eat, it also changes the stream’s overall ecology.
As scientists learn more about parasites’ roles in
ecosystems, a small but growing cadre is beginning to
think seriously about the need for targeted parasite
conservation. In August 2020 parasite ecologist Colin
Carlson of Georgetown University, along with Wood,
Hopkins and nine others, published a 12-point plan for
conserving parasites over the next decade. For starters,
they wrote in the journal Biological Conservation, we
cannot care about or conserve what we do not know
exists. They challenged the scientific community to
shine a light on parasite diversity by describing more
than 50 percent of parasite species by 2030. “We basi-
cally have barely scratched the surface,” Hopkins says.
Once descriptions and data about each species’ ecol-
ogy and life cycle start rolling in, the authors suggest,
parasites in need of conservation could be identified,
then integrated into existing species-protection
schemes fairly simply. Parasite conservation can sim-
ply piggyback on existing efforts to save imperiled free-
living species. Threatened parasites can likewise be
added to various inventories for tallying and protect-
ing endangered plants and animals. Only one animal
parasite, the pygmy hog-sucking louse, is currently
included on the International Union for Conservation
of Nature’s Red List of Threatened Species, and none
are included on the U.S. Endangered Species List.
Hopkins, Wood and their peers know that parasites
have a serious image problem but are hopeful they can
be rebranded. They liken the state of parasite conser-
vation to where the field of predator conservation was
just a few decades ago. At the time, many researchers
and the public thought of bears, wolves and other meat
eaters as damaging to the environment and dangerous
to humans and livestock. Those assumptions proved
not only false but harmful. Scientists now know that
predators are keystone species—ones on which entire
ecosystems depend. Removing them can cause cas-
cades of negative impacts, from disease outbreaks and
disruption of nutrient cycling to shifts to entirely dif-
ferent habitat types. As researchers realized the impor-
tance of predators, the public warmed to them, too.
“My hope is that people are willing to peer into this
black box we’ve put parasites into,” Wood says. “Para-
sites aren’t this monolithic threat.”FROM OUR ARCHIVES
Surfing Parasites. Stephenie Livingston; November 2020.
scientificamerican.com/magazine/sa