Parasitic Nematode
Common names
Eel swimbladder parasite, swimbladder nematode
Scientific name
Anguillicoloides crassus
Recent discovery in Cape Breton
Anguillicoloides crassus is a parasite that lives in the swimbladder of eels and has recently been discovered in American eels (Anguilla rostrata) sampled in Cape Breton Island (click here for news story). The discovery was made by graduate student Lydia Rockwell, who is being co-supervised at Saint Mary’s University by Project UFO member, Dr. Martha Jones. This was the first documented finding of this highly invasive parasite in Canadian waters.
Description
In the adult stage inside eel swimbladders, Anguillicoloides crassus is dark grey-black, and it is large enough to see with the naked eye. The nematode has a body covered with a soft wrinkled cuticle with a small circular mouth. The nematode feeds on the blood of its host. The overall body length of males is up to ~4cm, and females can reach up to ~5cm body length. Here in Cape Breton, we have observed up to 15 nematodes per individual American eel, and in European eels, up to 30 nematodes per eel have been reported.
Native range
A. crassus originates in Asia (Japan, China), where it infects the Japanese eel (Anguilla japonica) without causing them serious harm—but the European eel (Anguilla anguilla) and the American eel (Anguilla rostrata) are less resistant to its effects. International trade in eels led to the accidental introduction of the parasite into Europe in the 1980s; the first documented case in North America was in 1995, at a southern-Texas aquaculture facility. It is thought to have been sent infected elvers by a supplier. The parasite spreads rapidly within an eel population and can threaten the sustainability of the population.
Life cycle
The adult female nematode releases thousands of eggs in the eel's swimbladder, which pass through the eel’s excretory system and settle of on the substrate as larvae. The larvae are ingested by a crustacean or fish (intermediate host). The may pass through a number of intermediate hosts before it infects an eel. The parasite finds its way to the swimbladder where it can be destructive. Eels with the parasite can exhibit hemorrhagic lesions, fibrosis and collapsed swim bladders. Other symptoms include susceptibility to disease, decrease in growth rate, and death. As well, the infected swimbladder may decrease the eel’s ability to reach spawning grounds, resulting in decreased reproductive rates.
Photograph of an American eel (Anguilla rostrata) on a measuring board.
Invasive swimbladder nematodes (Anguillicoloides crassus) found in one eel. Note that one of the females (left bottom corner) has exploded. Ripe female nematodes will explode in the swimbladder to release their larvae.
This is a dissected eel showing a full stomach (forefront) and the translucent, air-filled swimbladder just behind. There is a dark blotch near the posterior end of the swimbladder that may be scarring or hemorrhaging due to the nematodes found living inside.
Current research
Cape Breton University, Saint Mary's University, the Inland Fisheries Division of NS Department of Fisheries and Aquaculture, and Fisheries and Oceans Canada are assessing the current distribution of A. crassus in the Maritimes. We are also interested in determining the rate of spread and the infection levels in local populations.
What you can do to help
Monitoring this invasive species is essential for the protection of our native eel populations. If you catch eels for consumption (in accordance with provincial fishing regulations), and notice worms in the swimbladder while cleaning the eels, please contact us at: info@ProjectUFO.ca. It would be useful for us to know the location and date the eels were sampled.
Additional websites about eels and the swimbladder nematode
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Click on the image for a poster about the eel swimbladder parasite
(it will open in a new window)
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