Ornamental fish mortality reveals an old parasite introduction: A case study of Koi carp and fish louse

Koi carp are globally known for their colors and cultural significance. The introduction of these fish to new environments poses a threat to local biodiversity, in addition to releasing parasites, such as argulid ectoparasites. This study presents a record of Argulus japonicus infecting carp in an artificial lake in Southern Brazil using morphological and molecular methods, with a 100% prevalence (n = 3) and a mean intensity of 21.6 parasites per host, distributed over the body surface. The invasion history of hosts in the study locality indicates that the introduction of A. japonicus occurred decades before its first formal record in Brazil. •Argulus japonicus was found parasitizing carp in an artificial lake in Brazil.•The identification was confirmed by morphological and molecular methods.•This fish louse was likely introduced decades before its first record in Brazil.