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.
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 a 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.
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.
