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Elongate plasmodia with myxosporean spores belonging to the genus Unicapsula, Davis, 1924 were found in the skeletal muscle of the striped seabream, Lithognathus mormyrus (L.), a candidate for the mediterranean aquaculture. The only species of Unicapsula described from the Mediterranean is Unicapsula pflugfelderi Schubert et al. 1975, which occurs in the picarel, Spicara smaris (L.). For morphological and molecular comparison of U. pflugfelderi from S. smaris with Unicapsula sp. from L. mormyrus measurements of plasmodia and spores, ultrastructural details and 18S and 28S rDNA sequences were analysed. Whereas plasmodia were 2-3 times larger in S. smaris than in L. mormyrus (length 2.47-0.81 mm; width 0.22-0.09 mm; P = 0.000), spore morphology showed minor differences and both 18S and 28S rDNA sequences were 100% identical identifying the myxozoan as U. pflugfelderi. Scanning electron microscopy of the spores revealed a different shell valve distribution than the one used for the diagnosis of the genus Unicapsula. This resulted in a review of the genus Unicapsula dividing it into two morphological groups of different spore valve arrangement. TEM revealed the presence of a yet undescribed crystalline structure in the sporoplasm of the spores.

A new species of myxozoan, Kudoa trifolia sp. n., was found in various organs of the golden grey mullet, Liza aurata (Risso), and the thinlip mullet, L. ramada (Risso), from the western Mediterranean. Spores developed in subspherical plasmodia of 0.28-1 mm diameter within connective tissue, predominantly in the spleen, the outer wall of the gall bladder and the gut, the mesenteries and occasionally also in the gills. The spores of K. trifolia differ from the commonly known shape of Kudoa by considerable enlargement of one of the four valve cells, thus forming a 'spore body', which contains the major part of the binucleate sporoplasm. Scanning electron microscopy of the spores revealed the presence of grape-like appendages, which occur in bundles terminally on the valve cells. Phylogenetic analysis based on the 18S rDNA sequence of K. trifolia showed that this species is deeply embedded in the genus Kudoa despite its aberrant morphology and host tissue location. This suggests important amendments to the morphological diagnosis of the genus Kudoa.

The life cycle of the histozoic myxozoan parasite Henneguya nuesslini was investigated in two salmonid host species. Naive brown trout, Salmo trutta, and brook trout, Salvelinus fontinalis, were experimentally infected in two trials by triactinomyxon type actinospores from naturally infected Tubifex tubifex. In exposed common carp, Cyprinus carpio, no myxospore production was detected. The parasite formed cysts with mature myxospores in the connective tissue of the fish 102 days post-exposure. The morphology of both actinosporean and myxosporean stages was described by light microscopy and a 1417-bp fragment of the 18S rDNA gene was sequenced. Sequence analysis confirmed the absolute congruence of the two developmental stages and assisted in determining species identity. Host range, tissue specificity and myxospore measurements provided sufficiently distinctive features to confirm species validity and were thus crucial for identification. The triactinomyxon spores had 16 secondary germ cells, unique dimensions, a very opaque sporoplasm matrix and three conspicuously protruding, pyriform polar capsules. This is the first record of a Henneguya sp. life cycle with a triactinomyxon-type actinospore, which suggests a close relationship with the Myxobolus group and a polyphyletic origin of the genus Henneguya.

The initial site of infection in the fish host for Tetracapsula bryosalmonae, causative agent of proliferative kidney disease (PKD) is poorly understood. Following the recent recognition that freshwater bryozoans harbour the infective stages to salmonid fish, experimental transmission studies were undertaken to investigate (1) the route of entry of the parasite into the fish host and (2) the minimum exposure time required to induce clinical signs of PKD. In-situ hybridization (ISH) studies were carried out on naive rainbow trout exposed to the naturally infected bryozoan Fredericella sultana for up to 90 min. The sporoplasm of T. bryosalmonae was detected entering the fish via mucous cells in the skin epithelium within the first minute of exposure. In addition, T. bryosalmonae cells were infrequently detected in the skeletal musculature of exposed experimental fish up to 72 h post-exposure. The route of migration through the fish to the kidney and spleen was not determined. All fish exposed to infected, disrupted bryozoans for 10, 30 and 90 min and maintained for up to 8 weeks developed clinical PKD.

The development of Myxobolus dispar Thelohan, 1895, a myxosporean parasite of the gills of common carp (Cyprinus carpio L.) was studied in experimentally infected oligochaetes Tubifex tubifex Muller. After infection of uninfected tubificids with mature spores of M. dispar, development of actinosporean stages was first observed light microscopically 21 days after initial exposure. In histological sections, early pansporocysts were located in the gut epithelium of experimental oligochaetes, while advanced stages occupied mostly the outer layers of the gut and the coelozoic space. Mature pansporocysts, each containing 8 raabeia spores, appeared 199 days after initial exposure. Following damage of the intestinal wall and rupture of the pansporocysts, free actinosporean stages were found in the gut lumen of the oligochaetes. Actinospores of M. dispar emerged from the worms after 217 days of intra-oligochaete development. They were floating in the water and showed a unique raabeia form. Each raabeia spore had three pyriform polar capsules and a cylindrical-shaped sporoplasm with approximately 32 secondary cells. The spore body joined the three caudal projections without a style. Caudal projections were bifurcated at the end and the two main branches had further small bifurcations. The total length of the raabeia spore was approximately 158 µm. The prevalence of infection in 240 experimentally infected Tubifex specimens was 99.2%. No infection was found in the control oligochaetes.

A new myxosporean, Myxobolus jiroveci sp. nov. has been found to infect the brain of bullhead, Cottus gobio L., in several localities in Czechoslovakia and England. Trophozoites, up to about 200 μm in size, were encountered in various regions of the brain where they occupied the space within the distended myelin sheath of the axons. Trophozoites provoke local compression and atrophy of the neighbouring nervous tissue. Host reaction — mostly glial cell proliferation, less often perivascular lymphocytic infiltration — can be associated with heavy infections. The sporoplasmosomes, spherical, double membrane-bound dense bodies in the sporoplasm of mature spores, are described and compared with similar bodies of other myxosporeans and with the haplosporosomes found in representatives of the phylum Haplosporea.

Guidelines for the preparation of a species description of myxosporean species are proposed. The establishment of a new myxosporean species has to be based on the morphology of both spores and vegetative stages as observed in a fresh state. All important diagnostic features are listed and diagrams for size measurements are presented.

In this second study of the myxozoan Zschokkella russelli Tripathi from Wales, infection occurred in 71-0% of five-bearded rocklings, Ciliata mustela L.; all infected fish were mature and immature fish were uninfected. Histologieal studies showed ehanges to gallbladder and liver that mostly conformed with earher findings, but sinusoidal dilation, which had not been seen previously, was present in three of 22 infected livers. Thickening around hepatic ducts was shown by transmission electron microscopy (TEM) to comprise fibroblasts and associated collagen bundles, while the attenuated epithelium of hepatic ducts bore few mierovilli. Zschokkella russelli plasmodia were seen by TEM to have fine tubular structures, particularly adjacent to developing sporonts. Spores showed long extensions to the sporoplasm, while spore valves had up to 18 surface ridges each and depressions between ridges were supported by microtubules. Otherwise, in its general ultrastructure and sporogenesis Z. russelli resembled other myxosporida.

This ultrastructural study examines Sporogenesis of Kudoa paniformis, a monosporous myxosporean that causes a softened muscle texture in cooked Pacific whiting, Merluccius productus. Capsulogenesis and valvogenesis occur in a manner similar to that of other myxosporeans, while some aspects of generative cell interaction and sporoplasm ultrastructure differ from most other myxosporeans that have been examined with electron microscopy. Capsulogenesis of K. paniformis includes the formation of an external tubule which is continuous with the polar capsule, followed by the apparent internalization and modification of the external tubule to form a polar filament winding within the polar capsule. Valvogenesis occurs with the accumulation of electron-dense material on the cytoplasmic side of the outer membrane of each valvogenic cell to form a valve wall, Sporogenesis of K. paniformis differs from other myxosporeans in two aspects: (1) spore development does not appear to begin as a result of one generative cell engulfing another, and (2) the sporoplasm of developing and mature spores contains two morphologically distinct cells, one of which surrounds the other.

The stages of development leading to sporogenesis of Ceratomyxa shasta (Noble) were investigated by light and electron microscopy. Salmonid fishes were infected by exposing them to water containing the infectious stage and intestinal material was fixed at weekly intervals. Signs of intestinal infection were barely detectable by 7 days following exposure where trophozoites and later developmental forms were present, but by 14 days a large number of pansporoblasts could be detected in varying stages of development. By 21 days the majority of caeca were completely occluded and infection had spread throughout the connective tissues attached to the caeca. The early developing trophozoites contained two or more nucleated cells within a mother cell. There was some evidence of multiplication of nuclei by fission. The sporoblasts usually contained twelve nucleated cells that gave rise to two groups of six cells (sporonts) and resulted in the formation of two spores in each mother cell. Each spore was formed by two sets of bilaterally arranged cells consisting of the main germinative cell or sporoplasm, the anteriorly placed capsule cells and the outer envelope or spore valve cell that surrounded the others and formed the spore covering. As the spore matured the two germinative cells interacted with each other by pseudopodial extensions and appeared to fuse to form a diploid cell. The position of the cells laterally and slightly posteriorly to the central suture line formed a bilaterally curved spore. Mature spores when examined with the electron microscope were condensed, dark staining and relatively featureless, with a lateral measurement of 15 μm and an anterior-posterior measurement of 7μm.