The phylum Myxozoa contains over 2100 species of parasites that principally infect fish, although some species infect amphibian and reptilian hosts (Kudo 1920; Lom and Dykova 2006; Yokoyama et al. 2012). Myxozoans are multicellular, comprised of 1 to 7 spore shell valves, 1-2 infective germ sporoplasms, and 2 to 7 polar capsules (Lom and Dyková 1992). The polar capsules contain an extrudible filament used for attachment to a host, similar to the stinging cells of Cnidaria. After attachment, 1 to 2 amoeboid sporoplasms infect the target tissue and begin development. See Lom and Dyková (1992) for details on development of the trophozooite, plasmodia, and mature spores. Several myxozoan parasites studied to date have a life cycle with two hosts: fish and aquatic oligochaete or polychaete worms (Lom and Dyková 2006). The myxospore stage infects the worm host when ingested, which produces the actinospore stage that infects the fish host. Yokoyama et al. (2012) listed 33 myxozoan species for which the corresponding actinospore stage has been identified. These species include Myxobolus cerebralis (Wolf and Markiw 1984), the first species deduced to have a complex two-host life cycle. Other species within Myxobolidae include M. cotti (El-Matbouli and Hoffman 1989), M. carassii (El-Matbouli and Hoffmann 1993), M. arcticus (Kent et al. 1993), M. drjagini (El-Mansy and Molnar 1997a), M. portucalensis (El-Mansy et al. 1998), M. hungaricus (El-Mansy and Molnar 1997b), M. psuedodispar (Székely et al. 2001), M. bramae (Eszterbauer et al. 2000), M. macrocapsularis (Székely et al. 2002), and M. parviformis (Kallert et al. 2005). Among Henneguya species, only H. exilis, H. ictaluri, H. mississippiensis and H. neusslini have had the actinospore stage discovered (aurantiactinomyxon or triactinomyxon; Yokoyama et al. 2012; Rosser et al. 2015). So, many species have not had their complete life cycle determined yet. Likely, future taxonomic revisions of Myxozoa will consider the morphology of the actinospore stage, which varies among the known life cycles (e.g., hexactinomyxon, raabeia, triactinomyxon; Lom and Dyková 2006). See Yokoyama et al. (2012) for a key to these actinospore types.
The organisms in this guide are in the family Myxobilidae within suborder Platysporina, order Bivalvulida, class Myxosporea, phylum Myxozoa. The family Myxobolidae Thélohan 1892 is characterized by spores flattened parallel to the straight sutural line and two polar capsules (which may be unequal in length). The genera of Myxobolidae that lack tails, i.e., Myxobolus, Spirosutoria, Neothelohanellus, Thelohanellus, Neothelohanellus, are not covered in this guide. This summary covers the genera that have appendages that look like ‘tails’, i.e., Henneguya, Hennegoides, Neohenneguya, Unicauda, Dicauda, Laterocaudata, Phlogospora, and Tetrauronema, and Trigonosporus. The tailed spores of Myxobilatus (family Parvicapsulidae, suborder Variisporina), which look similar to Henneguyaspores, but have striated shells and the suture line bisects the polar capsules, are also not included in this guide, other than in the family key. I have chosen to use the word ‘tail’ instead of ‘caudal appendage’ or ‘caudal projection’ to make the guide more user friendly and to use less space in the text boxes. For similar reasons I have chosen to use the word ‘cyst’ instead of ‘trophozooite’.
