Light and electron microscopic examination of so-called piroplasms of fishes from Atlantic Canada and systematic revision of the Haemohormiidae (Incertae sedis).

Haemohormidium terranovae infections in American plaice and infections with a similar parasite in oceanpout were examined by light and electron microscopy. All plaice were infected at the time of capture and remained infected for over 2 yr. Prevalence in oceanpout varied seasonally between 0% and 80%. Uninucleate, binucleate, and tetranucleate forms were found in both species. In addition, octonucleate stages were observed in some erythrocytes of infected plaice. The presence of DNA in parasite nuclei was confirmed. There was no evidence of any ultrastructural feature characteristic of the phylum Apicomplexa. It is proposed that the genus Haemohormidium Henry, 1910 be considered incertae sedis and the senior synonym to Haematractidium Henry, 1910.