Ultrastructural studies on the development of Enteromyxum scophthalmi (Myxozoa), an enteric parasite of turbot (Scophthalmus maximus L.).

The ultrastructure of the developmental stages of Enteromyxum scophthalmi is described. Scarce intracellular, early uninucleated stages appeared within intestinal epithelial cells whereas proliferative stages were abundant both intraepithelially and in the intestinal lumen. In the proliferative stages, food reserves were abundant in the cytoplasm of P cells and consisted mostly of carbohydrates in the intraepithelial stages and lipid inclusions in the luminal stages. Sporogenesis could occur in enveloped cells or by direct division or clustering of generative cells. The abundance, shape and size of mitochondria as well as the number and shape of their cristae were very variable in the different developmental stages. The cristae were usually tubular and sometimes plate-like, discoidal or lamellar. True flat cristae were not observed. We found elements of closed (cryptomitosis) and open mitosis as well as structures reminiscent of microtubule organising centres, hitherto not described in myxosporeans. The significance of these findings is discussed in relation to the taxonomic and phylogenetic position of the Myxozoa.