A Fellow, Her Fish, and Their Bloodsucking Parasites

Zoemma Warshafsky scoops an American eel from its tank. ©Chris Patrick/VASG
Zoemma Warshafsky scoops an American eel from its tank. ©Chris Patrick/VASG

By Chris Patrick, science writer

Five forest green American eels squirm through a cooler filled with ice cubes. But within minutes the jostling stops. The eels are still, temporarily anesthetized by the ice.

American eels are snake-like fish historically found in nearly every river and lake along the Eastern coast of Canada, the United States, and Mexico, as well as in the Caribbean. They aren’t at all like aggressive electric eels—which actually aren’t eels in the same way a koala bear isn’t a bear. And although they’re docile, American eels are slimy, making them difficult to handle. Hence the ice. In a semi-frozen state, the eels cease their incessant wiggling, allowing Zoemma Warshafsky, a master’s student at the Virginia Institute of Marine Science, to get to work.

With an admirable lack of reservation, she lifts an eel from the cooler. It’s about two feet long and hangs in her gloved hand like a thick, limp noodle. She lays it gently on a glass platform covered in plastic wrap, fiddling until the fish is straight and secure.

Then she sits down at a computer on the other side of a lead partition and clicks the mouse a few times. Soon, an x-ray image of the eel’s insides appears onscreen, captured by a box above the glass platform. On the x-ray’s black background, the eel’s long body appears entirely white, ghostlike except for a black oval situated midway between its head and its tail.

This oval is what Warshafsky, a Virginia Sea Grant graduate research fellow, is looking for. It’s the eel’s swimbladder, an air-filled, oblong balloon of an organ that helps fish maintain buoyancy to swim. But most eels have something in their swimbladders besides air: a non-native, bloodsucking, parasitic worm called Anguillicoloides crassus. And Warshafsky wants to know if it’s killing American eels, which were once abundant along the east coast.

“When European settlers got here you couldn’t step in water anywhere without seeing an eel,” Warshafsky says. They were an important food source for American Indians and then European settlers, who were thrilled to see a fish similar to the European eel living in waters back home, and adopted it as a staple of their diet.

Zoemma Warshafsky readies an American eel for x-raying. ©Chris Patrick/VASG
Zoemma Warshafsky readies an American eel for x-raying. ©Chris Patrick/VASG

Though not a staple of the modern American diet, eels are still fished today. From 1970 to the mid-1980s, eel fishermen in the United States harvested up to 3.6 million pounds of eel per year. That’s about 360,000 eels. In 1987, this harvest dropped to 1.6 million pounds. It has since dropped even lower, remaining between 700,000 and 1.5 million pounds each year. In 2012, the Atlantic States Marine Fisheries Commission announced that the American eel population was at historically low levels.

The U.S. Fish and Wildlife Service reviewed the status of the American eel twice, in 2007 and 2015. Both times, they determined Endangered Species Act protection is unwarranted. But the American eel’s situation is viewed more grimly by entities outside of the United States. The International Union for the Conservation of Nature (IUCN) assesses the conservation status of flora and fauna around the globe, assigning species labels of least to most dire, from “not evaluated” to “extinct.” Since 2014, the IUCN has listed American eels as “endangered.” But this classification carries no legal implications.

Biologists blame a combination of factors for the decline, including pollution, habitat loss, overfishing, and dams that block eel migration. And—though the decline began before the introduction of A. crassus—biologists suspect that this parasite is now part of the reason eel population is not recovering.

U.S. fishermen harvest eels from Florida to Maine, and around the Great Lakes. Fewer eels means fewer fishermen can support themselves on eels alone. The decline also might have irreversible effects on the ecosystem: Birds, bigger fish, and aquatic mammals depend on eels as source of food. When a once copious species like the American eel is severely depleted or removed from an ecosystem, the ripple effects are impossible to predict. Some species may lose a critical food source, or populations of others may go unchecked because eels aren’t around to gobble them up.

Zoemma Warshafsky readies an American eel for x-raying. ©Chris Patrick/VASG
Zoemma Warshafsky studies an x-ray of an American eel swim bladder. ©Chris Patrick/VASG

Which is exactly how A. crassus infects an eel in the first place. They gain access to their main hosts by using eels’ food like a Trojan Horse. American eels will eat whatever fits in their mouths, including smaller fish, amphibians, snails, and crustaceans. Little do they know that A. crassus often lurks without effect inside these creatures. When an eel eats its prey, it’s letting the Greeks into Troy. Once inside an eel’s gut, A. crassus penetrates the intestinal wall and migrates to the swimbladder, where it feeds on blood running through the organ walls.

Warshafsky has dissected her share of eels, and she says that A. crassus inside swimbladders are usually swollen with blood: “poke one and it explodes.” She’s found as many as 10 parasites in a swimbladder.

When a male and female are in a swimbladder, they can procreate. When the female releases her eggs, they exit the eel’s swimbladder into its intestine and enter the open water through its feces. There, they wait to be eaten by an intermediate host and make their way to another eel—it’s parasitology 101.

A. crassus arrived in United States waters in the early 1990s. About ten years before that, the parasite was introduced to European eels, a species almost identical to American eels. Like the American eel population, the European eel population was already declining, and biologists worry that the parasite might cause further shrinkage or prevent recovery. The details of A. crassus’ introduction to American and European eels are unknown, but most experts believe it arrived via the transfer of live eels from southeastern Asia, where the parasite shares its native range with Japanese eels. Because Japanese eels coevolved with A. crassus, it has natural defenses against it and is less vulnerable to the parasite’s effects.

American and European eels are less fortunate. Like most balloons, a swimbladder works best when filled with air. When it’s filled with worms, an eel is a shoddier swimmer, which may increase its risk of becoming lunch. The eel might also have a harder time migrating to spawning grounds. While research shows that infected American eels swim worse with their worm-filled swimbladders, scientists are unsure of a parasitized eel’s fate.

“We don’t have a good handle on the effect of this parasite on the health or survivorship of American eels,” says Troy Tuckey, a scientist at the Virginia Institute of Marine Science also studying American eels and a member of Warshafsky’s thesis committee. “Zoemma is taking the first crack at looking at how the parasite affects American eel survival.”

As a Virginia Sea Grant graduate research fellow, Warshafsky is x-raying the swimbladders of parasitized eels to find out if they can recover from infection or if they are doomed. The answer is important for organizations managing American eels, like the Atlantic States Marine Fisheries Commission.

The natural mortality rate is an important component in regulating fisheries. Knowing how many eels are dying naturally tells managers how many eels they can allow fisherman to harvest while allowing the population to recover.

A rendition of Anguillicoloides crassus, a nonnative, bloodsucking parasite that lives in American eel swimbladders. They are typically a half-inch to three-inches long. Courtesy of Sally Bell, Integration and Application Network, University of Maryland Center for Environmental Science (ian.umces.edu/imagelibrary/).
A rendition of Anguillicoloides crassus, a parasite that lives in American eel swimbladders. They are typically a half-inch to three-inches long. Courtesy of Sally Bell, Integration and Application Network, University of Maryland Center for Environmental Science (ian.umces.edu/imagelibrary/).

Warshafsky is x-raying parasitized eels harvested from four tributaries of the Chesapeake Bay: the York, James, Rappahannock, and Potomac Rivers.

“I’ve got an eel dealer,” she says with a smile. It’s nothing shady, but they do meet on a backcountry road in rural Gloucester, Virginia, where her campus is located. She x-rays her eels once a month to track the health of their swimbladders. She’s trying to address two main questions: Can eels recover from A. crassus? Or is infection fatal?

“No one knows the timeline of how the disease works,” Warshafsky says. “We’ll see if it gets worse or if it gets better.”

Warshafsky didn’t enter grad school intending to answer these questions about American eels. She just wanted to keep studying parasites like she did as an undergraduate.

“Someone suggested eels and I was like, Eels are weird. I don’t want to do eels. That’s horrible,” she says. These feelings of disgust morphed into a fierce devotion as she learned more about them. Now, she fondly considers the fish as “misunderstood emotionally and scientifically,” and wants to figure out if their parasite is killing them. Listening to her talk about eels and watching her handle them now, it’s hard to imagine she ever thought they were gross.

“No one understands these fish,” she says. “No one’s ever seen them spawn. No one’s ever caught an American eel in the open ocean. No one’s ever seen a spent [post-spawning] eel.”

An American eel’s life begins in the Sargasso Sea, a stretch of the North Atlantic Ocean containing Bermuda, where an eel hatches from an egg smaller than a poppy seed into a larva. At this point in its life, an eel is flat and transparent and shaped like a sharp, skinny leaf.

The leaf-like creature drifts northward along the Gulf Stream, away from its birthplace. When it’s about 200 days old and still floating with ocean currents, it transforms from a larva into its next life stage, the glass eel. A glass eel is more tube-like, but still transparent. Around this time in its life, some eels begin to move inland to streams and rivers, settling in coastal areas that will be their homes for the next 20 to 25 years.

For now, Warshafsky sits in a computer’s glow more than 1000 miles away from the Sargasso Sea. Her eyes fixate on the small black hotdog shape of an x-rayed swimbladder. But she can’t linger too long, lest the ice melt and her eels begin to wake up. Before it’s too late, she must try to make a misunderstood fish a little less mysterious.

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