Genus Pleistophora (Phylum Microspora): redescription of the type species, Pleistophora typicalis Gurley, 1893 and ultrastructural characterization of the genus

. Pleistophora typicalis Gurley, 1893, a parasite of the striated muscle of Myoxocephalus scorpius has been re-examined at light- and electron-microscope levels. Foci of infection were almost exclusively found in the ventral body wall where a mixture of schizonts, sporonts and masses of pansporoblasts lay in direct contact with unaltered host myofibrils and mitochondria. All stages were surrounded by a thick (0-5 μm) amorphous coat, external to the plasmalemma. In schizonts this was traversed by channels passing from the plasma-lemma to a layer of vesicles in contact with the muscle. This coat became modified as a pansporoblast envelope surrounding the mature spores: at first a layer of fine granules was laid down about mid-way across the previously amorphous coat, while the channels disappeared. At the time when the sporogonial plasmodium retracted away from the coat to produce the pansporoblast cavity, the layer of the coat within the granules disintegrated into spherical vesicles. The pansporoblast envelope around the mature spores was composed of two or three layers of different electron density, including one which was strongly electron dense. The pansporoblast envelope as defined here corresponds in function, but not necessarily in origin, to the sporophorous vesicle that encloses the spores in other pansporoblastic microsporidia. Schizonts had an extensive system of smooth endoplasmic reticulum, composed of expanded vesicles and divided by plasmotomy into smaller multinucleate segments. The endoplasmic reticulum of sporogonial stages was comprised of a network of fine channels, parallel arrays of flat cysternae and close-packed stacks of membranes. The sporogonial plasmodium divided stepwise through smaller multinucleate segments into uninucleate sporoblasts. Nuclei were isolated throughout development. Two types of sporonts were recognized: large sporonts gave rise to pansporoblasts containing up to 200 microspores. Macrospore pansporoblasts always contained eight macrospores. Microspores measured 4–4 × 2·3 μm (fresh) and were probably uninucleate. Macrospores measured 7·5 × 3·0 μm (fresh) and were probably binucleate. Another microsporidium, found in the muscles of Blennius pholis, which had been attributed to the same species by Thelohan (1895), was distinguished on the basis of spore size (microspores 3·9 × 2·3 μm and macrospores 7·7 × 3·8 μm fresh) and by the fact that electron dense components were laid down in the pansporoblast wall early in sporogony. The species was named Pleistophora littoralis n.sp.