Figure 1. Generalized life cycle of myxozoa. 1–7; Development in the fish host (myxosporean phase). 1, Actinosporean contacts surface of fish and discharges sporoplasm; 2, free unicellular sporoplasms in fish skin shortly after infection; 3, extrasporogonic multiplication; 4, trophozoites develop into sporogonic plasmodia, vegetative nuceli (v) and internal generative cells (g) formed by endogeny (internal budding); 5, sporogenesis: internal daughter cells form sporoblasts; s, sporogonic cell; 6, sporoblasts differentiate into the cells comprising the spore; c, capsulogenic cell; x, sporoplasm; v, valvulogenic cells; 7, fully-formed myxospore (Sphaerospora sp.), v, valve; pc, polar capsule; x, sporoplasm.

8–20: Actinosporean phase in the annelid worm host (actinosporean phase). 8, Myxospore is ingested by worm and released sporoplasm penetrates epithelium; 9, sporoplasms form multiple schizogonic stages; 10, plasmotomy (fusion) of two uninucleate cells to produce one binucleate cell; 11, division to form cell with four nuclei; 12, division of ‘11’ into four unicellular cells; 13, formation of early pansporocysts with two somatic cells (S) and two internal generative cells; 14, gametocytes form within pansporocysts; 15, meiosis of gametocytes results in transformation of diploid to haploid cells and polar bodies (pb); 16, fusion of gametocytes to form diploid ‘zygotes’. 17–20: Sporoblasts develops into actinospores, which are then released from the worm. Adapted from Moser and Kent (1994), El-Matbouli and Hoffmann (1998), El-Matbouli et al. (1995).

Figure 2. Wet mounts of Myxobolus arcticus, a myxozoon infecting the brain of salmonid fishes. (a) Myxospore from brain. Bar, 10 m. (b) Triactinomyxon actinospore from oligochaete worm host. Bar, 50 m; pc, polar capsule.

Figure 3. Phylogeny of the Myxozoa based on analysis of small subunit ribosomal DNA (SSU rDNA) sequences. Maximum parsimony analysis of the Myxozoa inferred from complete SSU rDNA sequences with a bootstrap resampling of the data set. The numbers at the forks represent the percentage of times the group occurred out of 100 trees. Branch lengths are proportional to the scale given in substitutions per sequence position.