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The microsporidium Nosema pontogammari Ovcharenko and Kurandina, 1987 was detected in the muscles of Pontogammarus robustoides collected in the stream of South Baltic Coastal Zone (sample site N 54°16', E 19°25')
The parasite infects the cytoplasm of the muscle cells. The presence of remarkable intracytoplasmic vacuoles was noted inside of some parasite cells. Transmission electron microscopy showed that these intracytoplasmic vacuoles contained numerous microorganisms identified as rickettsia-like prokaryotic cells. Groups of pleiomorphic in shape bacilliform and rounded cells were enveloped in electron-dense cover 20-30 nm wide. The prokaryotic cells wall was constructed out of triple layered coat of different electron density. The cytoplasm of infected cells was degenerated with weakly developed endoplasmic reticulum and undeveloped organelles. The obtained data demonstrate pathogenic nature of host-parasite relationship between endosymbiotic rickettsia-like organism and microsporidia.
Key words: invasive amphipods, Microsporidia, prokaryotic endosymbionts