Hepatic microsporidia were investigated from 5 species of fish, namely Taurulus bubalis (Euphr.), Crenilabrus melops, (L.), Gaidropsarus mediterraneus (L.), Ciliata mustela (L.). and Scophthalmus maximus (L.) and intramuscular microsporidia from 2 species, namely Taurulus bubalis and Cottus gobio L. Detailed ultrastructural studies carried out on hepatic microsporidia from T. bubalis showed it to be a new species belonging to the genus Microgemma (Ralphs) and it is named here as Microgemma dunkerli sp. n. Although hepatic microsporidia from the other species of fish are included in Microgemma, insufficient data was available to determine whether these were synonomous with M. dunkerli or a separate species. Intramuscular microsporidia were identified as Pleistohora sp. and Pleistophora vermiformis (Leger) from T. bubalis and C. gobio respectively. Unsuccessful attempts were made to experimentally transmit spores by per os and intraperitoneal injection using a variety of experimental hosts. Viability studies indicated that spores survive for at least one month under controlled conditions. A detailed study of host/parasite relations was carried out at Portwrinkle and comparisons made with populations from Roscoff, Wembury, Widemouth and Aberystwyth. Prevalence and intensity of hepatic microsporidiosis in T. bubalis decreased markedly northwards probably due to temperature. Overall infection levels were at their highest in late summer and autumn, 'O' group fish being the most severely affected. The main ecological events of the life cycle of Microgemma dunkerli are presented in the form of a flow diagram. It is hoped that this work will form the basis for an epidemiological model.

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