Phylum Haplosporidia Caullery & Mesnil, 1899: ultrastructural morphology of spores of microparasites of aquatic fauna

The Kingdom “Protista” or “Protoctista” encompasses a diverse group of microparasites, which includes the phylum Haplosporidia Caullery & Mesnil, 1899, where around 50 species, most of them reported as pathogenic, are described. This phylum divided into four genera: Minchinia, Haplosporidium, Urosporidium and Bonamia. Most species of haplosporidia infect bivalves and gastropods, and may infect other taxonomic groups. The taxonomic nomenclature was very controversial at the time the name of the phylum was created, with a nomenclatural variation, with the unanimously accepted designation of Haplosporidia prevailing, defended by Caullery & Mesnil in 1899. The first haplosporidian species described were the genera Minchinia Labbé, 1896 and Haplosporidium Caullery & Mesnil, 1899. However, the taxonomic characteristics described for each of the genera were not based on the same morphological aspects of the same organelles. Most haplosporidian species in this taxonomic group were initially described based on comparative morphology by LM observations; subsequently, ultrastructural morphology and, more recently, molecular technology of SSU rDNA sequences have allowed us to clarify the taxonomic position of some species. In few species the life cycle has been reported, although there are isolated states without continuity of development and cellular differentiation.