Ergasilus

Ergasilus is a genus of small parasitic copepods in the family Ergasilidae. Members of this genus are ectoparasites that mainly inhabit the gills of fish, though some species may be found on the skin or buccal cavity. They occur in freshwater and marine environments worldwide and are of particular concern in aquaculture and aquarium fisheries.

Biology and life cycle: Like other copepods, Ergasilus species have a free-swimming larval stage and sessile or semi-sessile adults. Female copepods attach to the host’s gills and produce egg sacs, which release nauplius larvae that disperse in the water column. Males are typically smaller and mate with encountered females, completing the cycle on or near the host.

Hosts and impact: Ergasilus species exhibit varying host specificity, infecting a range of fish groups including salmonids, cyprinids, tilapia, and other freshwater and marine fish. Infestations can cause gill inflammation, mucus production, edema, and impaired gas exchange, leading to reduced growth, increased susceptibility to secondary infections, and, in severe cases, mortality. The parasites are especially problematic in crowded aquaculture settings and during periods of environmental stress.

Diagnosis and management: Diagnosis is based on microscopic examination of gill filaments to identify the parasites. Management emphasizes good husbandry, water quality, and biosecurity to prevent introduction. In aquaculture or veterinary contexts, approved antiparasitic treatments and bath therapies may be used under professional guidance, along with preventive measures such as quarantine of new stock and minimizing stress.