A scientist’s love for a brainless, spineless parasite
Janine Caira’s lifelong passion began on a beach in Baja California.
She was in her 20s, studying for her parasitology PhD at the University of Nebraska at Lincoln, when a friend who worked as a guard on the California-Mexico border invited her to join him — not for a traditional beach vacation, but for a rattlesnake roundup.
Caira recalls thinking, “Rattlesnakes . . . they’re apex predators — and that means they’re going to have awesome tapeworms.” So she hopped in her Subaru wagon and set out on a two-day drive southwest.
She missed the rattlesnake competition — her friend got the dates mixed up — so they went to Baja California instead. Caira suggested they walk down to a waterfront where fishermen were hauling the day’s catch onto shore.
“We had to eat,” she says. “And I wanted to dissect something.”
They bought two small sharks, and Caira cut both open.
One yielded a previously unknown variety of tapeworm — a tiny, squiggly creature with hooks on its head. Caira christened the discovery — her very first — for her border-guard friend, Evan: Calliobothrium evani. “It’s a real honour,” she still insists. Now a distinguished professor of ecology and evolutionary biology at the University of Connecticut, Caira is one of the world’s top specialists in the tapeworms of sharks and stingrays and has helped discover and name a whopping 170 tapeworm species. As for the second shark? Caira laughs. “We ate it.” In 2017, Caira released her magnum opus: Tapeworms from Vertebrate Bowels of the Earth. The 463page volume, which Caira edited with University of Kansas scientist Kirsten Jensen, is the result of an eight-year effort to survey the intestinal parasites of animals from around the world. The book inventories 4,810 species collected from two oceans and every continent except Antarctica. It also introduces 211 species that are entirely new to science. “We’re very proud,” Caira says. “Bowels of the Earth” is no metaphor. Caira and some three dozen fellow scientists gutted an estimated 14,884 fish, frogs, lizards, snakes, mammals and birds for this project. They traversed grasslands, waded through marshes and trekked across ice. At one point, Caira found herself abandoned with a seasick colleague on a tiny sandbar in the waters off Mozambique, hoping that their fisherman collaborators would pick them back up before the tide came in. (They did.)
The planet-wide survey was part of a larger push by the National Science Foundation to understand the diversity of Earth’s inhabitants — particularly its less cuddly and charismatic ones.
Noting that some 85 per cent of species on the planet have not been discovered, the agency sought scientists willing to scour the darkest and most disgusting corners of globe for every member of a given group. Among the targets: a genus of arachnids known as “goblin spiders,” a group of shrubs called “spurges” and, of course, the class Cestoda, better known as tapeworms.
With the funding, Caira and her colleagues didn’t only find new species. They sequenced genomes, restructured evolutionary trees and sought out previously unknown connections between parasites and their hosts. They built a global cestode database and created an illustrated glossary of tapeworm-related terms (the words “sucker” and “hooks” feature prominently).
“A lot of this is super-basic research,” Caira says. “But what’s great about it is tapeworms are now one of the most thoroughly known systems in the world. And now we can ask all these really cool questions because we have real data — on the identity of the tapeworms, on the identity of their hosts, on evolution, on parasitism.”
“We’ve set the stage for future generations,” she concludes, “and for us.”
Why would anyone devote her life to a brainless, spineless, gutless group of parasites?
If you have to ask that question, Caira says, you just need to get to know tapeworms a bit better.
These creatures vary in size and appearance. The smallest could fit on the head of a pin; the largest, found in the gut of a sperm whale, grows up to nine metres long. But their biology is basically the same. Lacking a digestive system of their own, they survive by absorbing nutrients inside another animal’s gut.
Contrary to popular belief, tapeworms don’t usually cause their hosts to waste away. In fact, many animals can spend their whole lives with a tapeworm in their gut. These creatures have probably coexisted with vertebrates for as long as we have been around; scientists cutting open a 270-million-year-old shark coprolite (a piece of fossilized poop) once found tapeworm eggs inside.
“It’s a super-long-term association,” Caira says. “And not only that, it’s a successful way of life.”
The life of a tapeworm unfolds over three stages. First, their larvae, which dwell in some animals’ muscle, are swallowed by another unsuspecting host. With hooks or suckers, they cling to the lining of the gut and get fat off nutrients predigested by the host as they develop. When it comes time to procreate, these hermaphroditic creatures make use of the full suite of male and female reproductive organs packed into their rear ends — they can self-fertilize or mate with another individual. Their eggs are swept out into the world via the host’s bowel movements, then swallowed by another host, when the cycle begins again. Many tapeworms require multiple hosts — one or two in which their larvae develop, another in which they reach maturity. The exact sequence of hosts is specific to each cestode species, so much so that if you handed Caira the spiral-staircase-shaped intestine of any shark, she could guess what animal she was looking at based solely on the tapeworms she found.
This makes understanding tapeworms useful for conservation — their presence can indicate the diversity and health of an ecosystem. It also means that tapeworm family trees might hold clues to our own evolution.
One of the many intriguing revelations from Caira’s survey: Deep-sea sharks have fewer tapeworms than their shallow-water brethren. “Maybe it’s harder for those life cycles to go on at depth,” Caira says.
And yet, they do go on — at the bottom of the ocean, in the guts of high-flying gulls, even in our own bellies, if we’re unlucky. Three tapeworm species make their livings off humans, according to the U.S. Centers for Disease Control and Prevention. If left untreated, they can cause abdominal pain, weight loss, even seizures.
Their omnipresence suggests that parasitism is a great evolutionary strategy — but the secrets of tapeworms’ success remain a mystery.
“I’ve described hundreds of them, and all that means is I can tell you what it looks like and where it’s found,” Caira says. “To me, it’s just this fountain of the unknown elements of the world. I mean, we’re talking about a whole group of animals whose habitat is the body of another animal. Just think about that. There has to be really big advantages to being a parasite in order for your life cycle to be so complicated . . . so how on Earth does that come about?”
“It just has to be a whole different way of living,” she says. “I think they have figured out answers to questions we don’t even know are questions yet.”
When I ask Caira about her favourite tapeworm, she directs me to Page 379 of her text, Figure E. There, I find a greyscale scanning electron microscope image of the scolex (head) of Yorkeria izardi, named for an Australian fisherman who
“Tapeworms are now one of the most thoroughly known systems in the world. And now we can ask all these really cool questions because we have real data.” JANINE CAIRA PARASITOLOGIST
provided specimens for the survey. “Isn’t that amazing?” she gushes. The Y-shaped structure in the image is in no way recognizable as a head. Two elephant-ear-like lobes extend to either side; they are topped by pointed appendages resembling antlers. “What are those? Antennae?”
“Those are hooks,” Caira corrects me. “That’s what it attaches to the surface of the gut with.”
“Oy.” I shudder. “I guess it’s kind of a face-only-a-mother-could-love thing?”
That makes Caira indignant. “Let me try another one.” In a matter of seconds, an email from her arrives in my inbox. Attached is a .JPG labelled “Caira_tapeworm_eyecandy.” I open it. “So?”
This one is colourized in shades of blue, purple and green. Something about it reminds me of a carnivorous plant — not beautiful, perhaps, but undeniably impressive.
“This is actually a species that we auctioned to raise money,” Caira informs me. A dentist purchased the naming rights to the species for $3,000. “I’m just saying.”
“She was like, ‘I want to name it after my sons,’ so I asked: ‘Oh why? Are they parasites?’ And she said, ‘Absolutely!’ ” Caira laughs.