Background. Achtheres percarum is an important copepod parasite (Crustacea: Copepoda) of European perch. Adult females permanently attach to the gill arches, roof of the mouth, tongue, and gill filaments. Attachment, at the latter site, may result in necrosis and epithelial hypertrophy, both compromising fish respiration during oxygen deficiencies. Adult males can move freely on gills of perch. To date there has been no published record of the complete set of developmental stages of this fish parasite. Provision of such may have practical implications for freshwater ichthyopathology, for example helping to monitor the dynamics of the parasite’s populations. It may also provide useful information regarding copepod phylogenetics. Materials and Methods. Early developmental stages (nauplius and copepodid) of A. percarum were acquired through incubation of eggs within egg sacs of females collected from European perch, Perca fluviatilis L., caught commercially in 1994 in Lake Dąbie, Szczecin, Poland. All subsequent larval stages were collected from gills of perch caught in the same lake, in 1990. All copepods were fixed and preserved in 75% ethanol. A modified "wooden slide" method was used to observe the collected developmental stages in a suspended drop of lactic acid, using a compound microscope. Specimens were stained in lignin pink and morphologic details of were drawn using a drawing tube. Results. The life cycle of A. percarum consists of 7 developmental stages, separated by moults (nauplius, copepodid, chalimus I, chalimus II, chalimus III, chalimus IV, and adult). The nauplius hatches from the egg and quickly moults into the copepodid. Both stages are free swimming and the copepodid is the infective stage, attaching to the host′s gill filaments, through the frontal filament. The subsequent chalimus stages (I through IV) "inherit" the copepodid′s frontal filament, modifying its proximal end, such that the structure of the proximal end of the frontal filament explicitly identifies the stage of a chalimus. Two adult males were found attaching, by means of claws of its maxillipeds, to the frontal filament, left over by previous stages. Conclusion. The number of developmental stages of A. percarum determined within the presently reported study is consistent with that hitherto found in the life cycles of other lernaeopodids.
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