Fish Louse, Fish Crab (Ergasilus sp.)

Volume 2, Issue 4
July 2000
Updated May 2002


Fig. 1 Ergasilus female removed from rainbow smelt gill.

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

Ergasilus has a direct life cycle using only the fish as a host. Ergasilus can spend prolonged periods swimming free, and mating takes place while the male and female are swimming. The male then dies. Egg incubation occurs while the egg clusters are attached to the female.

Female ergasilus

Fig. 2 Adult female Ergasilus on gill of rainbow smelt.

The offspring hatch and are broadcast into the water. The offspring undergo four molts before becoming adults.
There are several species of Ergasilids and none is too host specific. Ergasilids infect eels, gars, herrings, killifishes, paddlefishes, perches, pirate perch, smelts, sticklebacks, sunfishes, temperate basses and troutperch.

Male ergasilus
Fig. 3 Male Ergasilus.

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 modified antenna turned hooks. Its feeding apparatus further injures the host fish when it inserts the stylet into the epidermis and underlying host tissue causing hemorrhage. Ergasilus feed on the host’s blood and body fluids. The feeding apparatus also may release digestive enzymes which can cause systemic illness
similar to Argulus.

Special points of interest:

Ergasilus does not infect humans.

Ergasilus sp. can be found on the gills of many different species of freshwater fishes.

Cooking fish kills the parasite.

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.