Tiny worms accidentally imported from Asia are eating their way through eels in the Chesapeake Bay, impairing their ability to swim and potentially threatening the health of the eel population.

Scientists with the University of Maryland’s Center for Environmental Science discovered the parasite last year when Patuxent River watermen reported finding eels filled with “worms.”

Since then, the researchers have found the parasites in hundreds of eels throughout the middle and upper Bay and are calling on scientists all along the coast to keep an eye out for it.

Even before the parasite was found, eel reproduction along the coast appeared to be declining, and the discovery of a parasite that hampers the ability of the eel to swim — and in some cases may kill it — raises new concerns for the migratory species.

“It’s a question mark,” said David Secor, a scientist with CES’ Chesapeake Biological Laboratory. “As a stress, it could be leading to other infections, which could be either sublethal or lethal. We have commonly observed that when we see incidences of parasitism by the worm, we also see lesions on the fish.”

One recent survey of 200 eels from the

Potomac River found that about 90 percent were infected. “I’m concerned that it will have a significant impact,” said Eric May, a fish pathologist with the University of Maryland-Eastern Shore.

Secor and a colleague, Ann Barse, of Salisbury State University, say the worm may spread to estuaries all along the East Coast.

The parasite, Anguillicola crassus, is a blood-feeding worm which historically was found in the Japanese eel. But the parasite and that eel species apparently co-evolved, and the Japanese eel seems to have adapted to the worm.

That is not true for other eel species. In the 1980s, the worm was found infecting European eels, apparently after a shipment of Japanese eels was made to a German aquaculture facility.

Among European eels, the parasites have caused enlarged abdomens, swim bladder rupture and — in some cases — mass mortalities. Infected eels appear to have less of an appetite and occur near the surface more than uninfected eels.

In recent years, there were isolated reports of the parasites in Texas and South Carolina before they were discovered in the Bay last year.

The parasite reproduces in the eel’s swim bladder, a large organ critical to the eel’s ability to move in the water. The worms chew the walls of the swim bladder and lay their eggs. After they hatch, the young parasites leave the eel, sometimes by boring holes through the swim bladder.

Though not fatal, Secor and Barse say the damage done to the swim bladder may cause long-range problems for the eel. European studies have found the parasites significantly decrease swimming performance.

That, in turn, harms their ability to elude trawl nets, escape predators and feed. Perhaps even more importantly, it may affect their ability to make the lengthy migration to spawning grounds in the Sargasso Sea in the Atlantic Ocean between Bermuda and the Bahamas.

It’s possible, according to Secor and Barse said, that parasite infections could harm spawning success and therefore further reduce coastwide eel populations.

“No one has actually seen a spawning eel, so no one knows about their behaviors in the ocean, but presumably they are homing to the Sargasso Sea, and that takes specific swimming behaviors,” Secor said.

If the parasite interferes with the ability to make that migration, he said, “it would have a large effect.”

To complete its life cycle, the parasite must live in the eel and — at various life stages — in at least two intermediate hosts. Those intermediate hosts include a variety of fish, amphibians, aquatic snails and zooplankton.

Secor said that because so many hosts are involved, the parasite is probably easily transported from place to place in the ballast water of ships — the avenue he thinks most likely introduced the species to the Bay.

A ship routinely draws in huge amounts of water while at a port to help keep it stabilized at sea. It then releases the water when it reaches its destination. Numerous recent studies have shown that ballast water is responsible for inadvertently transporting aquatic species around the world, sometimes with devastating results.

Secor said the parasite may be affecting intermediate hosts as well, but that has not been studied.

“What we could have is an exotic parasite that is systemic in its effect, and it could be applying its effect to the entire ecosystem,” he said. “It’s an epitome of what can happen when you have an exotic species introduced into a system.”

Right now, though, concern focuses on the eel. Eels, as a whole, are a poorly studied species, and their exact status in the Bay and along the Atlantic coast is not well known, though there are indications that it may have declined in recent years.

Unlike anadromous species such as striped bass or shad, which spend most of their life in the ocean but return to rivers to spawn, eels are a catadromous species, which means they spawn in the ocean but spend most of their life in freshwater and estuarine habitats.

The American eel, Anguilla rostrata, is also known as the common, or freshwater eel. It can live up to 25 years.

It lives in the broadest diversity of habitats of any fish species in the world, occurring in freshwater rivers and lakes, estuaries, coastal areas and the open ocean from the southern tip of Greenland along the entire coast of North America and into the Gulf of Mexico and the northeastern portion of South America. In the Chesapeake, they can be found in all tributaries and streams.

Until recently, eels had little economic value and were used primarily as bait for striped bass, blue crabs and other species.

But in recent years, a market has grown for the export of small eels to Asia. The value of eels grew from 80 cents a pound in 1985 to $4.50 a pound in 1995, according to the National Marine Fisheries Service.

In 1996, the commercial catch of eels was worth $5 million, ranking it 18th among East Coast finfish fisheries. By comparison, the commercial catch of striped bass was ranked 16th, with a value of $5.5 million.

Only a few years ago, Secor said, eels were not even in the top 25 commercial species.

As a result, the eels have never been the subject of intense study — something that also makes it difficult to predict the exact impact the parasite will have.

“Eels have emerged,” Secor said. “But you can see the problem. Suddenly eels are an important fishery, and we know so little about their life history in estuaries which are the biggest producer of eels for export.”