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Eel swimbladder nematode (anguillicola crassus)
Photo credit: dirtydudley,

Eel Swimbladder Nematode

(Anguillicola crassus)

Report this Species!

If you believe you have found this species anywhere in Pennsylvania, please report your findings to state and federal authorities immediately!

Species at a Glance

This species uses the Japanese eel (Anguilla japonicas), European eel (A. anguilla) and the American eel (A. rostrata) as hosts. Eggs hatch inside the eels, and as larvae emerge they are released into the water from the host intestine. Larvae survive in a free living stage until they are ingested by crustacean intermediate hosts, which are then consumed by the eels.


The eel swimbladder nematode is a small, 3 cm (1.2 in) long parasite. 


Native to Southeast Asia, this species is currently found in the United States in New York, Delaware, Maryland, North Carolina, South Carolina, and Florida.


Note: Distribution data for this species may have changed since the publication of the Mid-Atlantic Field Guide to Aquatic Invasive Species (2016), the source of information for this description.

Environmental Impacts

With American eel populations already declining, the introduction of this nematode may have a large negative impact on survival of native eel species. Symptoms in infected eels include reduced swimming ability, rendering the eel vulnerable to predators, reduced growth rate, and mortality.


Information for this species profile comes from the Mid-Atlantic Field Guide to Aquatic Invasive Species (2016).

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