Myxozoans evolved as an endoparasitic radiation of cnidarians exploiting invertebrate and vertebrate (primarily fish) hosts in freshwater and marine environments. Currently, there are some 2,200 nominal species classified to 64 genera and 17 families. These groupings are mainly based on spore morphology. This chapter provides an updated spore-based taxonomic key that takes into account the recent recognition of the cnidarian origin of myxozoans as well as important revisions at generic, family and suborder levels over the last decade. A list of generic synonyms is also reviewed here. Myxozoan molecular phylogenies largely disagree with traditional spore-based classification systems, probably due to extreme plasticity in myxospore morphologies that has resulted in extensive convergence. Molecular phylogenies of myxozoans (based on all existing SSU sequences and those available for species with known actinospore-producing stages) resolve the malacosporeans, the freshwater myxosporeans, the marine myxosporeans and the sphaerosporid lineages. Within these clades species can be resolved according to definitive host type and, partially, according to host environment. Numerous exceptions are indicative of several recursions of species into freshwater or marine environments. Further resolution within clades identifies groups of species according to tissue tropism in their vertebrate hosts. We suggest future studies of myxozoan systematics to address persisting taxonomic and phylogenetic discrepancies and make recommendations for describing taxa in the absence of sequence data or when sequence and morphological data are incongruent.

Classification and Phylogenetics of Myxozoa

Myxozoan species and genera have been defined traditionally using morphological characteristics of spores and developmental stages, host preference and tissue specificity. Use of morphology is, however, limited in some taxa by a lack of reliable characters or ambiguities in their origin. For instance, morphological variation can characterise spores of closely-related species while similar spore morphologies in distantly-related species reflect convergence. Therefore, morphological traits are accompanied routinely by DNA sequences in the identification of myxozoan species. DNA sequencing methods have inherent limitations, too, which include co-amplification of host, inability to distinguish mixed infections and PCR and primer biases for some taxa over others. By combining several approaches, myxozoan researchers are revealing novel diversity and demonstrating that strains and morphologically cryptic species characterise many taxa. Extensive geographic sampling of economically significant myxozoans, including Myxobolus cerebralis, Kudoa thyrsites, Ceratonova shasta and Tetracapsuloides bryosalmonae, has demonstrated intraspecific genetic variation, strains linked with geographic localities, and species complexes. Application of new approaches to myxozoan classification is expected to facilitate reassessment of existing taxa, discovery of new taxa and further resolution of cryptic species.

Approaches for Characterising Myxozoan Species

Evolutionary origin of Ceratonova shasta and phylogeny of the marine myxosporean lineage

Parasitic Diseases in Aquaculture: Their Biology, Diagnosis and Control

Ultrastructure and phylogeny of Ceratomyxa auratae n. sp. (Myxosporea: Ceratomyxidae), a parasite infecting the gilthead seabream Sparus aurata (Teleostei: Sparidae).

Ceratonova gasterostea n. gen. n. sp. is described from the intestine of freshwater Gasterosteus aculeatus L. from the Klamath River, California. Myxospores are arcuate, 22.4 ± 2.6 μm thick, 5.2 ± 0.4 μm long, posterior angle 45° ± 24°, with 2 sub-spherical polar capsules, diameter 2.3 ± 0.2 μm, which lie adjacent to the suture. Its ribosomal small subunit sequence was most similar to an intestinal parasite of salmonid fishes, Ceratomyxa shasta (97%, 1,671/1,692 nucleotides), and distinct from all other Ceratomyxa species (<85%), which are typically coelozoic parasites in the gall bladder or urinary system of marine fishes. We propose erection of genus Ceratonova to contain both intestinal, freshwater species and reassign the salmonid parasite as Ceratonova shasta n. comb.

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Erection of Ceratonova n. gen. (Myxosporea: Ceratomyxidae) to Encompass Freshwater Species C. gasterostea n. sp. from Threespine Stickleback (Gasterosteus aculeatus) and C. shasta n. comb. from Salmonid Fishes

J. S. Foott

Papers

Erection of Ceratonova n. gen. (Myxosporea: Ceratomyxidae) to Encompass Freshwater Species C. gasterostea n. sp. from Threespine Stickleback (Gasterosteus aculeatus) and C. shasta n. comb. from Salmonid Fishes