An illustration representing trematode life stages—a worm, a fish, larvae, a snail
Gabriel Alcala

Life Is Tough for Teenage Parasites

For a deep-sea parasitic worm, the epic journey to adulthood starts in a fish’s intestines.

In my personal opinion, the greatest coming-of-age story on Earth does not take place in a Dickens novel or a Disney movie, but rather in a white fish at the bottom of the Pacific Ocean.

The fish in question, called a zoarcid, looks like a creature caught in an identity crisis, too long to be a fish but too short to be an eel, its lips permanently drooping in a sullen pout. Otherwise, the fish has no real reason to be grumpy. It lives in one of the cushiest parts of the deep sea, in the shimmering waters near a hydrothermal vent that warms the freezing seas to the temperature of a kiddie pool. It enjoys a spectacular view, at the bottom of a glittering black canyon 559 miles off the coast of Acapulco, Mexico. The site, called Nine North, is part of the East Pacific Rise, an underwater mountain range that stretches from the Gulf of California across the southeastern Pacific Ocean.

Here, for the zoarcid, life is good. It spends its days lolling and lazing. It weaves through tangles of tube worms and munches on snails, limpets, and worms, blissfully unaware of the saga taking place inside its body, and, to be very specific, its gastrointestinal tract. For when the zoarcid poops, the bildungsroman begins.

Perhaps I should clarify that our intrepid young protagonist is not the zoarcid, but the eggs contained in its feces, eggs belonging to a parasitic worm called the trematode. In order for many species of trematodes to reach adulthood, the young worm must journey through the bodies of three other creatures, penetrating the first two and being eaten by the third, where it finally matures and reproduces sexually. It is a cinematic adolescence, full of trials (just how does one penetrate a mollusk?), tribulations (sustained attacks by its hosts’ immune systems), and a long-anticipated first encounter with sex (losing one’s virginity in the intestinal tract of a fish).

But our understanding of the deep-sea trematode’s life cycle ends with the zoarcid. Finding an adult worm in a fish tells us nothing about the first two animals it used as hosts. We don’t know if the eggs move through a snail and then a polychaete worm before reaching the fish, or through a mussel and then a clam, or through a limpet and then a lobster. The possibilities are not endless, but they are dizzying, especially when coupled with the fact that an adult trematode worm looks nothing like its younger selves, which can resemble anything from tadpoles to free-swimming slices of hairy salamis. And the trematode’s journey to adulthood occurs on a scale virtually invisible to the human eye, in a place virtually inaccessible to the human body. So it’s unsurprising that no one has ever figured out the complete life cycle of any deep-sea trematode. As one researcher told me, “It would be like looking for a microscopic needle in an infinity-sized haystack.”

Against these impossible odds, a graduate student named Lauren Dykman thinks she’s nearly figured it out.

“This is a fishsicle,” Dykman announces over a Zoom call in September from her dissection station in a laboratory at the Woods Hole Oceanographic Institution in Massachusetts. She squeezes what looks like a tiny pink Otter Pop out of a Ziploc bag. It lands with a wet, hard thud. It’s a baby zoarcid, as long as a ballpoint pen and frozen since around last Christmas, when Dykman visited Nine North in the submersible Alvin. Dykman works in the lab of the biologist Lauren Mullineaux, who has spent decades studying the hydrothermal-vent community where this zoarcid lived, ate, and died.

Fishsicle, Dykman assures me, is not a technical term, but one she picked up while feeding frozen fish to seagulls at a wildlife-rehabilitation center. But it’s an apt name for her specimen, which rests as straight as a chopstick on top of the raised edges of a petri dish. Dykman, a graduate student in the MIT-WHOI Joint Program in Oceanography/Applied Ocean Science & Engineering, is alone in the lab due to the pandemic. This means that she has free rein of the place, but also that every 15 minutes she has to stand up and run around to remind the lab’s motion-sensitive lights that there is, in fact, someone there. Dykman has a near-supernatural ability to sit entirely still for minutes on end attending to the fast-thawing fish flesh under the scope. She moves just her fingers to tease apart the fish’s gallbladder with a tool she explains is “a needle hot-glued to a shish-kebab skewer.” Whenever the overhead lights flicker off, the scope’s clementine light gives Dykman’s face a mad-scientist glow, and the increasingly flaccid zoarcid the aura of a creature roused from the dead.

“I’m just going to go into its head right now,” Dykman says, unpeeling its cheek to reveal cranberry-colored gills. She turns the zoarcid’s head toward the camera so the fish looks like it’s staring directly at me, making eye contact just as Dykman cuts into its brain. She has dissected at least a dozen whole zoarcids and looked inside the preserved stomachs of many other hydrothermal-vent fish. But the process is never routine. Each fish might hold a new species of trematode or another parasite unknown to science—and its secrets can only be uncovered through the highly scientific process of squishing its flesh between two glass plates and shining a light through its stomach, gills, gonad, and liver under a microscope. To a parasite, a fish is a habitat just as complex as a kelp forest.

Chronicling the abundance and diversity of parasites at Nine North has become the focus of Dykman’s graduate research. According to the sparse scientific literature on vent parasites—epitomized by the paper “Deep-Sea Hydrothermal Vent Parasites: Why Do We Not Find More?”—Dykman’s project should have been a dead end. “I had read that parasites aren’t that common at vents,” she said. “But I kept finding them.”

Trematodes are far from the most bizarre parasite Dykman has found. Some, such as the copepod shaped like a gummy bear, might make you coo. Others might make you scream, such as rhizocephala, barnacles that inject themselves into crabs and branch like roots throughout the crab’s body, turning it into a castrated automaton. But in Dykman’s eyes, the grossest thing she’s found isn’t a parasite but a creature that appears impervious to them. The flesh of the giant tube worm, she says, squishes into glinting silver mush devoid of any recognizable organs and parasites. “I’ve never cut up an animal that has less going on inside it,” she says. “It’s the grossest thing I’ve ever seen.”

An hour into the dissection, Dykman is deep inside the fish’s gut. Suddenly, she sees something. “Ooh,” she says, cutting into the belly, “what are you?” Finally, she’s found trematodes. “Let me show you what these dudes look like,” she says, and presses her phone against the scope. After a few seconds of strobing halos, the camera focuses and I see two worms side by side, each a glassy vase of organs. “They’re like stained-glass windows,” Dykman says reverently. Near the end of the dissection, she takes a photo of one of the worms and walks me through its internal anatomy: eggs, a uterus, vas deferens, sperm ducts, and, by the tip of its tail, two perfectly circular testes that seem the Platonic ideal of what we call “balls.” “These worms are more human on the inside than tube worms are,” Dykman says, her voice disembodied over the photo of the trematode. I take a screenshot of the Zoom room, which I only later realize makes it look like I am video-chatting with a worm.

Even if you’ve never thought about deep-sea parasites before, you may have seen one. The world’s most famous—or at least most memified—photo of a deep-sea creature captures the blobfish Mr. Blobby with a white gob of spit dribbling from his pout. The spit isn’t actually drool, but a parasitic copepod.

Whether you like it or not, parasites are everywhere, lurking seen and unseen in almost every ecosystem on Earth. Nearly every free-living organism on the planet, including me and you, can host one or more species of parasite. Yet parasites are often overlooked, or studied only for the threats they pose to human health, says Armand Kuris, a parasite ecologist at the University of California at Santa Barbara. For half a century, Kuris has been studying how parasites affect the ecosystems they occupy and how much influence they can hold over their environment. As an example, Kuris describes an estuary near the town of Carpinteria, a short drive from his office. “There’s a few elephants’ worth of trematode tissue in that salt marsh,” Kuris says, and I imagine a Miyazaki-esque elephant formed by millions of writhing worms. “You could see them from the freeway if they weren’t all in the snails.”

Of course, a Santa Barbara beach is more or less heaven on Earth for any living thing, human or parasitic worm. Life in the abyssal deep sea is generally less halcyon, dominated by vast slopes and plains where your chances of encountering anything living are slim. But hydrothermal vents are like the Santa Barbaras of the abyss, peaceful respites where life can be abundant and flourishing. In other words, it’s a great place to be a parasitic worm.

No matter where they’re found on Earth, trematodes require at least two hosts to complete their life cycle, which makes life both exhausting and bewildering to contemplate. Certain species that pose a threat to human health, such as the trematodes that cause schistosomiasis, have narrowed down their life cycle to just two hosts. After I ask Dykman over and over about particular stages that confuse me, she draws me a diagram by hand. I make it my desktop background so I can reference it whenever needed, which, while writing this story, is about every 15 minutes.

“The life cycle starts,” Kuris says with a sigh, the sigh of a man who has had to tell this story many times, “when the fish craps out the eggs, or spits out the eggs, or pisses out the eggs …” Kuris trails off, and I assure him that I get the picture. Once the eggs are liberated from the intestines of the fish, they hatch into ciliated larvae called miracidia, which look like the aforementioned hairy slices of salami. These larvae swim to the single species that serves as their unsuspecting host: a mollusk such as a snail or limpet. When they find one, they burrow inside the mollusk’s flesh, grow into a tube-like sack, and begin cloning themselves by the hundreds, forcing their host to shed these clones like sperm-shaped dandruff, called cercariae. Time is precious for the cercariae, which generally have 24 to 48 hours to live, so they wriggle through the water until they find another creature to penetrate. During this second stage, they’re less particular about the species of their host but often still restricted to a certain taxon. So one species of trematode might target a fish, another a snail, another a lobster, and so on. The larva digs into its host’s flesh, encloses itself in a cyst, and lies in wait for whatever it has entered to be gobbled or swallowed by another—or perhaps even the original!—fish. And on that fateful day when the snail’s life ends, the rest of the trematode’s life begins. Once inside the fish, the cysts mature into worms and become adults in the biblical sense, flinging themselves into the orgy of trematode sex that begets the eggs that will soon be spit, crapped, or pissed out to begin the cycle anew.

Different stages of trematode life
Gabriel Alcala

Dykman deciphered her first trematode life cycle in Santa Barbara in the spring of 2016, while she was an undergrad in Kuris’s parasitology lab. Dykman was helping a graduate student, Dana Morton, map out the parasite food web of giant kelp forests, when she found tessellations of trematodes and eggs in most of the kelp bass she dissected. One of the trematode’s second hosts was already known—the candy-cane-striped red rock shrimp—but its first-stage hosts remained a mystery. Dykman’s first idea to solve the cycle was a bust. “I dissected thousands of snails and found nothing,” she says.

So she started again, this time beginning her search with the fish. She scooped the trematode eggs out of the fish’s uterus and into a cup of seawater. Fifteen days later, the eggs began to wiggle. Thirty-three days later, they hatched into microscopic fringed larvae. Dykman plopped the baby parasites into water with two of the most abundant snails in the kelp forest: a pink round snail and a white pointy snail. She watched as two of the wee trematodes made a beeline for the pink snail and disappeared into its flesh. Seventy days later, Dykman dissected the pink snail and found the worms alive and well in the snail’s gonad.

Dykman didn’t have the time or resources to turn her discovery into a paper, but her identification of the snail that this particular trematode penetrated immediately after hatching was the last missing clue of the worm’s cycle. “She pretty much figured out the life cycle,” Kuris says.

Though the field of deep-sea parasitology is small, its defenders are passionate.

The first time I Zoom with Chuck Blend, a parasitologist from Corpus Christi, Texas, he presses his business card against the camera; it reads, in red lettering in all caps: whatever your terms, we want your worms. Blend, who has a white beard as dense as a microbial mat, flips the card over to reveal an illustration of Podocotyle nimoyi, a species of deep-sea trematode he recently named after the actor who played Spock in the original Star Trek. Blend, a resident scholar at the Corpus Christi Museum of Science and History, has identified and named more than 50 species of parasitic worms, many hailing from the deepest parts of the Gulf of Mexico. No one loves deep-sea parasitic worms like Blend. “The other day I was looking at a deep-sea fish at the museum, and a worm just popped out of the intestine,” Blend tells me, leaning in toward the camera, his eyes glinting. “I could just feel that giddiness, the butterflies in my stomach, and I almost fell out of my chair!”

Blend sources many of his worms from well-preserved specimens that would otherwise lie around and collect dust—what he calls “pickled fish.” One advantage of pickled fish is that Blend can peek inside rare specimens that might not come up in a trawl net. In 2014, Blend spent a month slicing up worms in the biologist Ted Pietsch’s lab at the University of Washington in Seattle. “Ted’s the deep-sea anglerfish god,” Blend tells me, “and his lab was Shangri-La.” One of Blend’s many methods of extracting eensy worms from a long-dead fish is to snip the tips of the creature’s pyloric caeca, the tiny tubes that connect the stomach to the intestine and tend to teem with parasitic worms. “They’re just like pigs in a blanket,” he says. “Cut the tips off, and the worms go boop! boop! boop! Just like that,” Blend says, demonstrating with his own fingers.

Blend has collected thousands of larval parasitic worms from deep-sea fish, and has even speculated on potential life cycles, but he’s never been able to prove anything experimentally. In his eyes, it’s like looking for something the size of an ant in the ocean, which spans 71 percent of the globe. “Good luck!” Blend laughs.

The easiest way to sleuth out the life cycle of a parasite is to present it with potential hosts, just as Dykman did with the worms in the kelp bass. But tests like this only work if you can keep enough hosts and parasites alive and healthy in your lab. A snail plucked from a sun-dappled blade of giant kelp might not notice the difference between life near the ocean’s surface and life in a cup of seawater, and won’t mind the journey. But a deep-sea fish pulled up through thousands of feet of seawater—an enormous gradient of pressure—will certainly notice, and die. For most fish, this hours-long journey can trigger a physiological reaction that even makes a parasitologist squeamish. “The stomach is everted through the mouth,” Rodney Bray, a retired parasitologist and scientific associate at the Natural History Museum in London, tells me over Zoom in a refined British accent. He sighs, perhaps thinking, as I am, of how much data are lost when a fish’s stomach comes out of its mouth and its rectum comes out of its butthole.

Bray is one of the few people who have studied parasitic worms in deep-sea fish. His research has not focused on hydrothermal-vent ecosystems but rather on abyssal plains, where the seafloor is 20,000 feet below the sea. The only way to retrieve lots of fish from these depths is to lower a net for 12 hours, fish for three, and then retrieve the net for another 12 hours. Bray remembers that the grinding sound of the winch was so grating he could barely sleep—which was perhaps convenient, as everyone had to wake up when the net returned, no matter the hour. “You never know what fish you’re going to get,” Bray says, adding that there were always horrible, heaping mounds of dead sea cucumbers. Sometimes the only living thing in the net would be the parasitic worms, which had been cushioned by the dead fish during their journey up.

Unlike the open expanse of the abyss, Nine North sits in a valley running along the crest of the ridge of an enormous mountain range. The hydrothermal-vent fields lie inside the valley, with towering spires of rock that would snag any trawl—not to mention the hydrothermal fluid gushing out of the spires, which can reach 750 degrees Fahrenheit and melt any net. The only way to get there, as a human, is aboard a manned submersible, like Alvin. The descent into the canyon takes hours, and sunlight quickly fades into twinkling bioluminescence until the sub lands and its lights snap on, revealing a sparkling, obsidian world of pillow lava and black smokers ringed with tube worms. Once safely in the rift valley, the pilots can maneuver the sub’s bulky manipulator arm like a claw machine to snag stationary critters such as tube worms and mussels. To catch anything moving, the pilots have to pull out the slurp gun, which is exactly what it sounds like: a tube that can vacuum up anything that draws near, and the only match for something as slippery as a zoarcid. Alvin chief pilot Bruce Strickrott’s secret to slurping zoarcids is to park Alvin right next to the biggest cluster of tube worms and sit and wait. “So they have nowhere to go but the hose,” he says. For Dykman’s return to Nine North this March, the engineers at the Woods Hole Oceanographic Institution are developing a high-capacity suction sampler powered by the engine thruster of the unmanned submersible Jason, or, as Dykman deems it, “a big slurp.” She wants to catch some Thermichthys hollisi, a larger species of vent fish that hosts even more trematodes than the zoarcid.

No matter the method, parasite collection requires a strong stomach. A few years ago off the coast of Costa Rica, Strickrott caught a huge red crab that Greg Rouse, a biologist at the Scripps Institution of Oceanography, dissected on board to reveal an enormous, ocher-colored lump on its butt: the egg sac of the parasitic rhizocephalan barnacle. “I’ll never forget the look on our deck officer Ronnie’s face,” Strickrott told me, laughing. “Horror, pure horror. He’ll never be the same.”

Dykman first realized that she might be able to figure out the life cycle of a vent trematode when she found a trematode larva in a sandwich. The larva had hitchhiked on one of the “colonization blocks” her lab places at Nine North—cubes of layered plastic that Dykman’s lab refers to as “sandwiches.” When Dykman was first learning to identify the species found on the sandwiches, she looked through her scope and spotted a tiny oval of a creature with two suckers and a tail—remarkably like a trematode cercaria, the sperm-like stage that lives for a day or two. By some serendipitous twist of fate, the larval worm happened to be in the waters near a sandwich when both were collected and brought up to the ship.

Dykman suspected that the larva had come from the most common species of snail found on the sandwich, the milky-white Bathymargarites symplector. But when she dissected the snails, she found no trace of parthenitae—the sack of clones that would have shed the larva she found. Still, Dykman was determined to keep looking. She’d now found every larval stage of a deep-sea trematode. By this time, she’d found ten different morphotypes of grown worms, meaning distinct appearances that could indicate unique species, as well as the unhatched eggs inside their bodies. She’d found five morphotypes of the worm’s cyst-like stage in a handful of species of polychaete worms, one shrimp, and the Bathymargarites snail. And now, improbably, she’d found the sperm-like cercariae. She’d begun sequencing tissue samples from all of these morphotypes, hoping that the DNA from an adult worm in the fish might match the DNA from a cyst in a polychaete or shrimp. She just needed one more piece to complete the picture of a deep-sea-trematode life cycle: the clonal sack in the worm’s first host.

While fish require hours of careful dissection, tiny snails are easy to squish. To save time, Dykman often pops 20 of them out of their shells and separates them like balls of cookie dough on a single glass plate. She’s dissected roughly 600 snails this way, each splatting to reveal a big fat parasitic nothing. The odds of this might seem unlikely; Dykman dissected 12 zoarcids and found trematodes in all of them. But larval trematodes don’t need to infect many of their first hosts to be wildly successful. “You could have just one out of 100,000 of those snails infected,” Kuris says. Once a single larva winds up inside its designated host, it refurbishes the mollusk into a clonal factory, which castrates the animal and forces it to pump out massive numbers of clones—meaning that one out of 100,000 is fairly good odds for the trematode, but poor odds for Dykman.

Three months after the Zoom zoarcid dissection, Dykman called me with news. A few weeks before, she was dissecting a batch of snails and limpets from Nine North late into the night, and finding the usual: cookie sheets of nothing. And then, suddenly, salvation in a limpet, Eulepetopsis vitrea. The species looks almost like a ghost of a limpet, with a shell as translucent as fogged glass. But the limpet’s peachy insides were riddled with clonal sacks of parasitic worms, bobbing in the flesh like the noodles in alphabet soup. Dykman had found the last missing piece of the life cycle, the parthenitae, in a species she’d encountered much less frequently than her prime suspect, Bathymargarites. It was a microscopic needle in an infinity-sized haystack, and she’d found it. Maybe there’s a reason another name for a trematode is a fluke.

After putting the limpet through a bona fide photo shoot, she extracted its DNA and shipped the sample to the Sequegen lab, which will sequence the DNA for her. Hopefully. “I’m getting a little nervous,” Dykman confided to me. Sending precious samples into an increasingly harried mail system always invites some degree of risk, and Dykman was waiting for confirmation that the lab has received her trove of parthenitae DNA. If the sample were lost, it would mean that she had forfeited a month of work. Seconds later, Dykman got a confirmation email from Sequegen, and we both breathed a sigh of relief.

A month later, the Sequegen results came back: inconclusive. The parthenitae DNA closely matched the DNA of some known species of vent trematodes but also closely matched a shallow-water trematode from Sweden, indicating that the parthenitae might have come from a new species not yet recorded in the Sequegen database. It’s possible that the parthenitae might match one of the adult vent trematodes Dykman has already collected but has yet to sequence, which she plans to do in the coming months. But Dykman isn’t dispirited, and is hopeful that the match is already with her in Massachusetts, somewhere in the minus-85-degree freezer cradling her collection of worms.

When I think about the sheer abundance of trematodes at Nine North, I feel nauseous—not the gross kind of nausea, but the existential kind that oozes over me when I think about the Big Bang or any concept involving the word space-time. And I haven’t even mentioned the wildest thing about the trematodes at Nine North. It’s not just that the worms maintain their ludicrously complicated life cycle in a rare oasis of life in one of the most hostile regions on Earth. Nine North sits on a fast-spreading mid-oceanic ridge, where the planet’s tectonic plates pull apart from each other to create three to six inches of new ocean floor each year—and, every 10 to 20 years, a cataclysmic volcanic eruption. In other words, the communities at Nine North manage to survive in a world that obliterates itself on a regular basis.

When Woods Hole Oceanographic Institution scientists aboard Alvin first witnessed Nine North erupt, in 1991, it was as if a lush meadow had been replaced with a parking lot. Gone were the bouquets of tube worms, whirling octopuses, and meandering fish, all replaced by gleaming curtains of volcanic glass. Charred and tattered tube worms littered the seafloor, and the mussels that clung to their stalks had been steamed on the half shell, a banquet for brave scavengers. The burbling, diffuse flows of the vents themselves had turned to cloudy water, emerging from cracks and holes. It was as if life had to reinvent itself, thousands of feet away from the sun.

And yet, each time, it does. Within days of an eruption, white bacterial mats effloresce like dustings of snow, blizzardings dancing in the currents. Soon enough, scavenging crustaceans flock to graze on the mat, and fish and octopuses come to feed on the scavengers. Small tube worms sprout within the year, giant tube worms a year later, and then the mussels come and bedazzle the tube worms. Forget the eruptions, forget the inevitable extinguishing of the vents—life down here is raucous, insistent. Nine North last erupted in 2006, meaning apocalypse is almost overdue. But the creatures of Nine North only know the present, and in it, they flourish.

For better or worse, I feel like I know what it is like to be a zoarcid. The instability of Nine North has infiltrated my mind amid the tremors of the pandemic. When I lost my job and my health care, and with them my sense of stability and hope for the future, I felt like the earth was pulling apart beneath my feet. I felt unmoored, adrift in hostile seas and searching for a way to survive the coming eruption.

Dykman says she often tries to imagine what it’s like to be a parasite. “I put myself in one of their bodies,” she told me over the Zoom call, her shish-kebab skewer deep in the zoarcid’s gut. “It could be cozy and warm, because you’re not doing much, just hanging out and eating the thing you’re living in.” Sure, she clarifies, you’re technically under constant attack, but you’ve evolved to handle it. I wonder if I should stop thinking like a zoarcid and start thinking like a trematode.

And so I think of the trematodes, which are born in shit and freed in saltwater, and manage to find refuge in a limpet, and then again in a polychaete worm, and then in a fish. I marvel at the lives they manage to lead in a quaking, eruptive world, and at the titanic amount of luck on which their existence depends. And I think that if these worms can make it, so can I.