SYSTEMATIC BIOLOGY					KINGDOM ANIMALIA
HOME	SYLLABUS	WEEKLY ASSIGNMENTS	J. SYSTEMATIC BIOLOGY	TAXA OF LIFE
PHYLUM MYXOZOA

INTRODUCTION TO THE MYXOZOA

Myxozoa (mix-o-ZO-a) is a combination of two Greek roots meaning slime animals [myxo (μύξα) slime; and zoa (ζώο) animal].  The reference is to the former association of this group with the slime molds.

Members of the Myxozoa are intracellular parasites of vertebrates, mainly fishes but do occur in amphibians and certain reptiles.  They are taken into a host animal by ingestion of the spore, from which amoeboid forms emerge and penetrate the gut.  From there, the amoeba travels in the circulatory system until it arrives in the target organ.  At that point, the amoeba penetrates a cell and grows as a plasmodium.  They produce distinctive spores that are multicellular and develop a nematocyst-like structure (see Figures A-D).

SYNOPTIC DESCRIPTION OF THE MYXOZOA (BOTH ANIMAL AND PROTOZOAN FORMATS)

SYSTEMATICS OF THE MYXOZOA

The identification of the Myxozoa as a group of animals is a spectacular example of the coalescence of evidence from many avenues: natural history, cell biology, development, ultrastructure, molecular biology, etc.  Classical taxonomy had placed the Myxozoans in a group called the Sporozoa, a class within the Protozoa (e.g. Kudo 1966).  Grell (1973) seemed at a loss to deal with the myxozoans (he called them Myxosporidia) and placed them together with the Microsporidia in an uncertain class of Protozoa called "Cnidosporidia".  With the advent of the 5-kingdom system, the classical sporozoan groups rose to phylum-level status within the new kingdom.  In that system, Margulis and Schwartz (1988), Sleigh et al (1984), and Weiser (1985a) raised the Cnidosporidia to phylum-level status.  Later, evidence from cell biology, development, and ultrastructure showed that the cnidosporidian groups were very different and did not form a natural grouping.  The Myxozoa were elevated to phylum-level status within the kingdom Protista (=Protochtista) [(Lom 1990; Weiser 1985a, and Margulis and Schwartz 1998)], and the Microsporidia were removed from them into a separate phylum (Weiser 1985b).  Then, evidence began to mount that Microsporidia were not protists at all but belonged to a group of parasitic fungi (Fast and Keeling 2001; Keeling 2002; Sokolova et al. 2003; and Ragan et al. 2003).  

The association of Myxozoans with the animal kingdom was first proposed by Weill (1938) but began to enter the mainstream of systematic biology with Lom (1990) who supported the Weill hypothesis that the remarkable structural similarities between the polar capsules of the Myxozoa and the nematocysts of the cnidarians suggested that the Myxozoa evolved from that animal group. In particular, Weill (1938) proposed that the Myxozoa evolved from the Narcomedusae whose larvae resemble the Myxozoa in form and in lifestyle (that is, some of the Narcomedusae are also parasitic). Lom (1990) further claimed that the Myxozoa have the highest degree of cellular differentiation among any of the protist groups. This characteristic was also consistent with non-protist origins.  Smothers et al (1994) confirmed the structural evidence with molecular evidence that the Myxozoa are metazoans.   Siddall et al. (1995) presented convincing evidence that the Myxozoa were coelenterate animals.  Thus, I have placed them into the Animal Kingdom and allied them with the Cnidaria.

This taxonomy follows that of Lom (1990) and is roughly based on Sleigh et al. (1984), Grell (1973) and Weiser (1985a). In particular, I have followed the systems of Lom (1990) and Weiser (1985a).

HIERARCHICAL CLASSIFICATION OF THE MYXOZOA.  

This page is maintained by Jack R. Holt.  Last revised: 03/14/2008