Hemocyanin transports oxygen in the hemolymphof many arthropod species. Within the crustaceans,this copper-containing protein was thought to be restrictedto Malacostraca, while other crustacean classes wereassumed to employ hemoglobin or lack any respiratory protein.Only recently it has become evident that hemocyaninsalso occur in Remipedia and Ostracoda. Here we reportfor the first time the identification and characterisation ofhemocyanin in the fish louse Argulus, which belongs to theclass of Branchiura. This finding indicates that hemocyaninwas the principal oxygen carrier in the stem lineage of thepancrustaceans, but has been lost independently multipletimes in crustacean taxa. We obtained the full-length cDNAsequences of two hemocyanin subunits of Argulus foliaceusby a combination of RT-PCR, RACE and Illumina sequencingof the transcriptome. In addition, one full-length andone partial cDNA sequence were derived from the transcriptomedata of Argulus siamensis. Western blot analysisconfirmed the presence of at least two hemocyanin subunitsin A. foliaceus, which are expressed at the mRNA level ata 1:3.5 ratio. The addition to the branchiuran hemocyaninsubunits to a multiple sequence alignment of arthropod,hemocyanins improved the phylogenetic resolution within the pancrustacean hemocyanins. Malacostracan, ostracodand branchiuran hemocyanins are distinct from the hexapodand remipede hemocyanins, reinforcing the hypothesisof a close relationship of Remipedia and Hexapoda. Notably,the ostracod hemocyanins are paraphyletic with respectto the branchiuran hemocyanins, indicating ancient divergenceand differential loss of distinct subunit types.