Haplosporidian species (phylum Haplosporidia Caullery & Mesnil, 1899) are a small group of four genera of sometimes pathogenic protozoan parasites usually with uninucleated spores. They are widely distributed in marine and freshwater invertebrates, although their status in Africa, much of Eurasia and Central and South America, is largely unknown. They infect molluscs including commercially important bivalves and other molluscs, annelids, crustaceans, ascidians, trematodes, turbellarians, and probably many invertebrate groups. The morphology, development, and ultrastructure of the four haplosporidian genera (Haplosporidium, Minchinia, Urosporidium, and Bonamia) are described using light, scanning, and transmission electron microscopy. Several new haplosporidian species have recently been described based on their genetic sequences, spore morphology, and ornamentation. Ultrastructural organization of the spores and the origin of the spore wall ornamentation are discussed. This phylum contains 52 described species and several unnamed species reported in the four genera. Life cycle stages involve exosporulation of the endosporoplasm to form multinucleate plasmodia and sporoblasts giving rise to the spores that are described from some species. The phylogenetic analysis based on SSU rRNA strongly supports that the genera Minchinia, Urosporidium, and Bonamia are monophyletic, while the genus Haplosporidium is paraphyletic. The taxonomic positions and affinities between these genera within phylum Haplosporidia are discussed. The negative economic impact on the commercially important infected hosts is reported.
Carlos Azevedo, P. M. Hine, Living Reference Work Entry, Handbook of the Protists, pp 1-29, Date: 12 January 2017