Say goodbye to a phylum:
Myxozoa are Cnidarians
Clockwise from the top-left are Chloromyxum sp., Henneguya sp., Myxobolus
sp., and Myxidium sp.
There's also a couple of
images of pathology caused by these
parasites in the eye, gills, and kidney tubules of
To date the myxozoans have always been classified with protistan parasites. Early
classifications (e.g., Butschli, 1881; Dogiel, 1965) placed the myxozoans with the
microsporidians, and with parasites now comprising the phylum
1970, together in the class Sporozoa. As the complexity of life
histories was better
understood, the class Sporozoa subsequently referred only to the apicomplexans while the
microsporidians and myxozoans remained together in the Cnidospora Doflein, 1901. More
recently, following recognition of profound differences in the ultrastructural composition of
these parasites, Microspora Sprague, 1977 was raised to the rank of phylum and has been
found, by molecular phylogenetic techniques, to be representative of 1 of the earliest
branching eukaryote lineages (Vossbrinck et al., 1987; Sogin et al., 1989). This has left
Myxozoa to stand alone as a phylum without obvious phylogenetic affinities to other
protists. Systematists have failed to be decisive about myxozoan phylogenetic affinities,
either finding the suggestion of a cnidarian connection to be preposterous, or considering
the recent suggestion (Smothers et al., 1994) of a relationship with nematodes to be an
obvious failure of molecular phylogenetics.
Here, we present both morphological and molecular phylogenetic evidence that the phylum
must be abandoned because myxozoans have their origins in a clade of parasitic cnidarians.
The findings presented herein are published in Siddall, M. E., Martin, D.S., Bridge, D., Cone, D.M., Desser, S.S. 1995. The demise of a phylum of
protists: Myxozoa and other parasitic Cnidaria. Journal of Parasitology 81: 961-967. None of the figures or data
presented here can be reproduced without the consent of the authors or of
the Journal of Parasitology.
Mark E. Siddall