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In the gill filaments of common carp Cypnnus carpio f~ngerlings heavily infected with Sphaerospora molnan, intracellular parasites of unknown taxonomic position were detected with TEM. Cells, up to 3.5 pm in size, displayed surface projections which were often micro\fillous in shape One or more nuclei often in diplokaryon arrangement were seen in sectioned cells. Except for numerous dense bodies, no cytoplasmic organelles were observed. The possible relation to amphizoic amoebae, haplosporidia, rnicrosporidia and myxosporean stages is discussed; these parasites could not be identified with any of the them. While studying the ultrastructure of the myxosporean Sphaerospora molnari Lom, Dykova, Pavlaskova and Grupcheva, 1983 in the gills of common carp Cyprinus carpio fingerlings we came across parasitic cells which looked completely different from stages of the sporogonic cycle of S. molnarj. We hesitated to publish these rather enigmatic findings. However, as we have not encountered these stages since, we have decided to present our observations in order to draw attention of future investigators to these organisms. Material and methods. Small pieces of carp fingerling gill filaments infected with Sphaerospora molnari were sampled and processed for transmission electron microscopy using cacodylate buffered osmium tetroxide as a fixative and Epon-Araldite for embedding. Double stained ultrathin sections were observed in Jeol JEM lOOB electron microscope. Results and discussion. All the parasitic cells were found intracellularly in epithelia1 cells of the gill filaments and secondary lamellae (Figs. 1, 2 & 5) close to developmental stages of Sphaerospora molnari. The cells, up to about 3.5 pm in their largest dimension, are rounded or ellipsoidal. Their surface may be raised into stout projections (Fig. 3) or these projections may rather resemble fine villosities (Figs. 2 & 6) often seen in circular transverse sections about 2.7 pm in diameter around the circumference of the cell (Fig. 4 ) . The stages seem to be embedded directly in the cytoplasm of the host cell without eliciting formation of any parasitophorous vacuole. At higher magnification (Fig. 7 ) the surface appears to consist of closely superimposed unit membranes. The cytoplasm is rather dense, with numerous ribosomes and dense bodies (about 150 nm in diameter). The dense bodies are rounded, either homogenous or some of them (Fig. 8) have between their dense wall and dark core a more lucent layer. Except for the ribosomes and dense bodies, no cytoplasmic organelles could be discerned. The nuclei appeared rather indistinct against the background of rather dark cytoplasm. They were endowed with a distinct doublenuclear envelope. Distinct nucleoli could not be safely discerned. In some cells, a pair of nuclei closely apposed, as if forming a diplokaryon, could be seen (Figs. 3 & 4). In other stages, a single nucleus was seen (1 of the 2 nuclei of the diplokaryon?) while in still other stages, 3 nuclei, 1 diplokaryon and a cross section of a single nucleus could be found. No division stages were encountered and no traces of transformation in other developmental sequence (e.g. sporogony). The observed intracellular parasites are not easy to identify. From among the protozoan parasites, 3 groups come into question for possible comparison. Amoebae occur in cyprinid fish as intracellular parasites (Voelker et al. 1977, Dykova & Lom unpubl.). They have been described to cause generalized infections, a s well. However, if located intracellularly, they occur within parasitophorous vacuoles, they do have digestive vacuoles, and they do not have triplelayered cell walls. Genera of amphizoic amoebae most likely to be found in fishes (e.g. Acanthamoeba, Hartmanella, Naeglena and Vahlkampfia) do possess

Intracellular parasites of unknown taxonomic position in gills of common carp Cyprinus carpio