Anguillicola crassus | Eel Swimbladder Nematode



Conservation status

Not Assessed

Native status


First reported in the wild



Invasive species - risk of High Impact

Irish status


Introduction pathways - 1

Transport Contaminant

Introduction pathways subclass - 1

Parasites on animals

Invasive score


NAPRA Ireland risk assessed


Species Biology


Parasitic nematode (worm) found in the swim bladders of the European eel (Anguilla anguilla). Only found by examining the body cavity of an eel for 3cm length worms (Minchin, 2009).


While there appears to be no changes to the condition of eels infected with the parasite (Sjöberg et al., 2009), it has been shown to cause mortality in wild and farmed eels subjected to additional stress (Kirk, 2003). Infected eels show an increase in spleen size likely due to the stress placed on the eels blood supply from parasitism (Lefebvre et al., 2004). This stress makes eels more likely to be captured than uninfected eels (Sjöberg et al., 2009) due to decreased swimming performance (Palstra et al., 2007). This decreased swimming performance is thought to make eels unable to reach their spawning ground (Palstra et al., 2007), leading to population level declines.


Inland surface waters


Each nematode may contain 500,000 eggs which are laid in the swim bladder of the eel before passing into the gut and expelled in the faeces (Minchin, 2009). The intermediate host for newly hatched larvae include frog and newt tadpoles and the nymphs of aquatic insects (Moravec & Skorikova, 1998), which are consumed by transfer hosts or directly by eels, before burrowing through the stomach wall of the eel to attach to the swim bladder (Minchin, 2009).

Pathway and vector description

Introduced into Europe from Asia in the 1980s (Kirk, 2003) by 1987 it had reached Britain (Kennedy & Fitch, 1990), mainly transported throughout Europe in vehicles transporting live eels but may also have been facilitated by natural dispersal of eels and birds regurgitating infected fish (Kirk, 2003). The commercial eel trade is thought to be the most likely source of introduction into Ireland (Evans & Matthews, 1999; Minchin, 2007).

Mechanism of impact


Broad environment


Habitat description

Ectoparasite of the swim bladders of the European eel (Anguilla anguilla), adult eels may contain more than 70 nematodes each (Minchin, 2009).

Species group


Native region

Temperate Asia


World distribution(GBIF)

Irish distribution

Established - Widespread. As it was first recorded in 1998 (Evans & Matthews, 1999) and is thought to reach a nearly 100% infection rate within 1 year (Kennedy & Fitch, 1990) it is likely virtually every waterway in Ireland is infected.

Native distribution

Thought to be originally native to Japan, though due to the long history of eel cultivation there the parasite may have been introduced from another area in the Indo-Pacific, where it is a parasite of the Japanese eel Anguilla japonica (Lefebvre et al., 2012). Now found throughout the native range of its European host, it has also been introduced to North America and Africa (Kirk, 2003).

Temporal change

Date of first record category


Fifty year date category


Records submitted to Data Centre in 2023

The following map is interactive. If you would prefer to view it full screen then click here.

Further information

Delivering Alien Invasive Species In Europe (DAISIE) project list this as one of the 100 Worst Invaders in Europe.



Evans, D. W., & Matthews, M. A. (1999). Anguillicola crassus (Nematoda, Dracunculoidea); first documented record of this swimbladder parasite of eels in Ireland. Journal of Fish Biology, 55(3), 665-668.

Kennedy, C. R., & Fitch, D. J. (1990). Colonization, larval survival and epidemiology of the nematode Anguillicola crassus, parasitic in the eel, Anguilla anguilla, in Britain.Journal of Fish Biology, 36(2), 117-131.

Kirk, R. S. (2003). The impact of Anguillicola crassus on European eels. Fisheries Management and Ecology, 10(6), 385-394.

Lefebvre, F., Mounaix, B., Poizat, G., & Crivelli, A. J. (2004). Impacts of the swimbladder nematode Anguillicola crassus on Anguilla anguilla: variations in liver and spleen masses. Journal of Fish Biology, 64(2), 435-447.

Lefebvre, F., Wielgoss, S., Nagasawa, K., & Moravec, F. (2012). On the origin of Anguillicoloides crassus, the invasive nematode of anguillid eels. Aquat Invasions, 7(4), 443-453.

Minchin, D. (2007) A checklist of alien and cryptogenic aquatic species in Ireland. Aquatic Invasions, 2(4), 341-366.

Minchin, D. (2009) Anguillicola crassus Kuwahara et al., eel swim-bladder nematode (Anguillicolidae, Nematoda) In: DAISIE Handbook of Alien Species in Europe. Spring.

Palstra, A. P., Heppener, D. F. M., Van Ginneken, V. J. T., Székely, C., & Van den Thillart, G. E. E. J. M. (2007). Swimming performance of silver eels is severely impaired by the swim-bladder parasite Anguillicola crassus. Journal of Experimental Marine Biology and Ecology, 352(1), 244-256.

Sjöberg, N. B., Petersson, E., Wickström, H., & Hansson, S. (2009). Effects of the swimbladder parasite Anguillicola crassus on the migration of European silver eels Anguilla anguilla in the Baltic Sea. Journal of Fish Biology, 74(9), 2158-2170.

Sjöberg, N. B., Petersson, E., Wickström, H., & Hansson, S. (2009). Effects of the swimbladder parasite Anguillicola crassus on the migration of European silver eels Anguilla anguilla in the Baltic Sea. Journal of Fish Biology, 74(9), 2158-2170.

CABI - Datasheet

 DAISIE - Factsheet