Myxobilatus mictospora (Kudo, 1920) (Myxozoa: Myxosporea) in the largemouth bass (Micropterus salmoides Lacepede): Plasmodium morphology and fine structure [Alabama]

Plasmodia of Myxobilatus mictospora (Kudo) in the urinary bladder of the largemouth bass (Micropterus salmoides Lacepede) were studied with light microscopy and scanning and transmission electronmicroscopy. Three distinct forms of plasmodia were observed. During the winter months, when water temperatures were 25 C, sheetlike plasmodia gave rise to long, fingerlike plasmodia which then produced free-floating spherical or oval forms by a budding process. Spore production (sporogenesis) was confined mainly to fingerlike and free-floating plasmodia; sheetlike forms contained very few late pansporoblast stages and mature spores. The winter, sheetlike plasmodia had a surface topography of ridges or folds from which numerous, branched microvilli extended. Surface topography of summer, fingerlike plasmodia differed distinctly from that of the winter, sheetlike form in that the former had numerous ridges or folds devoid of microvilli. Plasmodia were attached to host tissue by pseudopodiumlike structures inserted into parasite-induced channels of the host cell membranes. The pseudopodiumlike structures formed extensive vacuolar networks and contained what appeared to be host cytoplasmic components. This report represents the first scanning electronmicroscopic study of plasmodium surface topography of a coelozoic myxosporidan, and also is the first report of the fine structure of any member of the genus Myxobilatus. Myxobilatus mictospora (Kudo) was described originally by Kudo in 1920 as belonging to the genus Henneguya. He described the parasite from the urinary bladders of Lepomis cyanellus Rafinesque, L. humilis (Girard), and Micropterus salmoides Lacepede in Illinois. In 1944, Davis redescribed this parasite and placed it in the new genus Myxobilatus. Of the 23 species of Myxobilatus described, M. mictospora is the only one found parasitizing fishes of the genus Micropterus. In the brief descriptive accounts of Kudo (1920) and Davis (1944), plasmodia of M. mictospora were reported as being polymorphous. In the present paper, three distinct forms of plasmodia were described from the urinary bladders of largemouth bass. Plasmodial morphology, plasmodial surface topography, and attachment of plasmodia to host cells were investigated with light microscopy, and scanning (SEM) and transmission (TEM) electronmicroscopy. This is the first report of the ultrastructure of a member of genus Myxobilatus, and it is the first account of surface topography of a coelozoic myxosporidan plasmodia as revealed by SEM. Received 4 November 1980; revised 26 January 1981; accepted 15 June 1981. MATERIALS AND METHODS Micropterus salmoides were collected periodically from November 1978 to May 1980 by angling or seining. Fish were taken from several ponds located north of Auburn, Lee County, Alabama that are managed by the Department of Fisheries and Allied Aquacultures, Auburn University. All specimens were transported to the laboratory alive and necropsied within 24 hr of capture. All but two of 69 fish were found to be infected with Myxobilatus mictospora. Parasites were identified on the basis of measurements and morphology of spores and plasmodia, site of infection, and host species (Davis, 1944). Agar preparations (Lom, 1969) of scrapings and contents of urinary bladders containing fresh spores and vegetative stages were viewed and photographed with a Zeiss Photomicroscope II with Nomarsky interference contrast (NIC) optics. Urinary bladder tissue and parasites were also processed for TEM. Specimens were fixed in 3% glutaraldehyde in 0.1 M phosphate buffer or in Kamovsky's (1965) fixative for 4 hr at room temperature. Tissues were then postfixed in OS04, dehydrated in a graded ethanol series, transferred to propylene oxide and embedded in Epon-araldite (Mollenhauer, 1963). One-micrometer thick Epon-araldite sections were stained with 0.5% toluidine blue in 1.0% Na borate and viewed and photographed with a Zeiss Photomicroscope II light microscope. Silver-gray sections were cut with glass knives on a Sorvall Mt-2 ultramicrotome, mounted on 150-mesh Formvarcoated grids, stained with uranyl acetate and lead citrate and viewed and photographed on a Philips EM 300 electron microscope.