Fish Louse (Argulus americanus)

Volume 2, Issue 5
July 2000
Updated May 2002

Argulus americanus is a member of a small group of parasitic crustaceans that prey upon freshwater and marine fishes. Argulus can be found on the skin, fins, and gills of fishes. They can cause significant morbidity and mortality when heavily infesting fish. They have also been implicated as vehicles for other fish diseases.

Argulus americanus has a direct life cycle using only the fish as a host. Argulus can spend prolonged periods swimming free and mating takes place while the male and female are swimming. Clusters of Argulus eggs are deposited on submerged objects and after hatching, juveniles must locate a suitable host within a couple of days or they will expire.


Figure 1. Live Argulus from skin of largemouth bass.

This is not a difficult task, however, since Argulus aren’t too fussy about their hosts. Argulus have been found parasitizing: bowfins, drums, gars, herrings, killifishes, perches, pikes, sticklebacks, sturgeons, sunfishes, salmonids, and temperate basses.

Argulus can be seen moving rapidly around on the surface of the fish, but will often swim away as soon as the fish is netted out of the water. Affected fish have patches of hemorrhagic and edematous affected skin, gills or fins. The parasite causes these injuries by attaching to the fish with its curved hooks and sucker. Its feeding apparatus further injures the host fish when it inserts the stylet into the epidermis and underlying host tissue causing hemorrhage. Argulus feed on the host’s blood and body fluids. The feeding apparatus also releases digestive enzymes which can cause systemic illness.

Special points of interest:

Argulus americanus does not infest humans.

Argulus can be found on many different species of freshwater fishes.

Cooking fish kills the parasite, but most will voluntarily leave the fish when it is removed from the water.

For more information see Hoffman 1999. Parasites of North American Freshwater Fishes.

Images in this document were made possible in part by a grant from the Maine Outdoor Heritage fund.