Hyperparasitic Spore-Forming Eukaryotes (Microsporidia, Haplosporidia, and Myxozoa) Parasitizing Trematodes (Platyhelminthes)

© INVERTEBRATE ZOOLOGY, 2020. Trematodes serve as outstanding hosts for a variety of parasites. This paper restricts those parasites to three groups of non-related taxa parasitizing trematodes and forming invasive spores similar in external morphology, which is why in the past these three groups of parasitic eukaryotes were united into the taxon “Sporozoa” (Kudo, 1924). (1) Trematode-infecting microsporidia (type Microsporidia Balbiani, 1882: Rozellomycota: Holomycota) are represented by more than 30 species. Among those, the species with sequenced barcode (SSU rDNA) are phylogenetically associated with either microsporidia from invertebrates, as the species of the genus Unikaryon (7 species), or fish microsporidia, as the species of the genera Pleistophora and Ovipleistophora or alike forms (4 species). However, most of the species that we place in the incertae cedis group, are known only by light-microscopy descriptions. Those were attributed by the authors to the genera Nosema (14 species) and Microsporidium (8 species), but in fact their taxonomic affiliation, phylogenetic position, and origin remain unknown. (2) Trematode-infecting haplosporidia (type Haplosporida Caulleri Mesnil, 1899: Ascetosporea: SAR) are represented by only one genus Urosporidium with a broad range of hosts among marine helminths and free-living invertebrates. The literature describes 10 Urosporidium spp. infecting trematodes. (3) Myxosporidia or Myxozoa (subclass Myxosporea Butchli, 1881: class Myxozoa Grasse, 1970: Cnidaria: Holozoa) infect mainly fish. All three species known from trematodes, belong to the genus Fabespora, and, likely, switched to hyperparasitism from fish. These three spore-forming groups demonstrate a diversity of characteristics and group-specific physiological mechanisms in addition to common adaptations responding to similar environmental conditions. These forms developed from genetically dissimilar material and exemplify an amazing power of convergent evolution, a phenomenon A.A. Dobrovolskii and other Russian scholars of parasitology demonstrated for a variety of parasitic groups