Eimeria that infect fish are diverse and are related to, but distinct from, those that infect terrestrial vertebrates

The Eimeria are ubiquitous parasites (Phylum: Apicomplexa; family: Coccidia) of the gut epithelium of vertebrates which complete their development in a single host species and whose sporocysts may be recognized by the presence of a Stieda body through which their sporozoites excyst. Their diversity and relationship to other kinds of coccidia have been successfully explored by molecular systematic studies based on the sequencing the 18S ribosomal DNA. To date, most attention has been paid to the diversity and evolutionary relationships of Eimeria spp. parasitizing terrestrial vertebrates, most especially those species infecting domesticated birds and mammals. Regrettably, no Eimeria have yet been considered from the Earth's first vertebrates: the fish. If Eimeria first evolved in fish, then extant piscine parasites should comprise a deeply branching assemblage at the base of well-constructed phylogenetic trees. Here, we sequenced portions of ribosomal DNA from several such isolates (from Eimeria anguillae, Eimeria daviesae, Eimeria percae, Eimeria variabilis, Eimeria rutili and Eimeria nemethi) and compared them to one another as well as to other available sequences from the parasites of fish and terrestrial vertebrates, in order to better understand their diversity and origins. By establishing that such piscine parasites comprise a deeply branching clade at the base of the Eimeriidae, these data substantiate the hypothesis that Eimeria may have originated in fish. Plainly, a great deal of coccidian diversity awaits future discovery and description.