Abstract
Parasites belonging to the phylum Microsporida comprise more than 1300 species, which are obligate parasites of marine fish. These parasites cause tissue atrophy to the infected fish. In the present study, five (16.66%) out of 30 examined specimens of Pagrus major (Sparidae) captured off Jizan, the Red Sea coast of Saudi Arabia, showed natural microsporidian infection. The parasite formed tumor masses or plasmodia in the muscle tissue and the intestinal epithelium of the peritoneum. The microsporidium was characterised by light as well as transmission electron microscopy (TEM). Light microscopy studies showed xenomas encircled by a fibrous layer enclosing the various developmental stages. Mature spores were 1.7 +/- 0.6 (1.5-2.7) mu m long and 1.5 +/- 0.3 (1.2-1.8) mu m wide, with an apparent posterior vacuole. Examination of ultrathin sections by TEM revealed the presence of uninucleated spores with anterior lamellar polaroplast and lateral 5-8 polar filament coils with electron dense centre. Generative cells were the early recognised stages underneath the surface coat, followed by uninucleated, binucleated, multinucleated meronts, sporoblasts and finally immature and mature spores. Xenomas as well as developmental stages of the parasite compared with microsporidian species recorded previously form different geographical areas, showed that the microsporidium belonged to Pleistophora pagri described from Pagrus pagrus.