It is vitally important that good quality descriptions of actinosporean stages of myxosporeans are produced since these may prove to have a very important role in the taxonomy and classification of the combined rnyxozoan taxon. Guidelines are therefore presented for the preparation of accurate descriptions of actinosporean stages, and the terminology to be employed in such descriptions is outlined and defined. Actinospore and myxospore should be promoted as terms for the myxosporean and actinosporean spore stages in the myxozoan life cycle.

Tetracapsula bryosalmonae, previously referred to as PKX, causes proliferative kidney disease (PKD) in salmonids and is an economically important myxozoan pathogen in salmonid culture. A variety of molecular and immunological tools have been developed to detect the parasite. To determine the specificity of four monoclonal antibodies (MAbs) raised against T. bryosalmonae, archive material of fish infected with various myxosporean species was obtained and immunostained. Wild fish were also collected from enzootic waters and examined for T. bryosalmonae infection using immunohistochemistry and the polymerase chain reaction (PCR). Three of the MAb probes appear to be specific for T. bryosalmonae while only two of the five sets of primers tested appeared to specifically amplify T. bryosalmonae DNA. The results of the immunostaining and the PCR demonstrate that T. bryosalmonae occurs in the tubules of grayling Thymallus thymallus L., brown trout, Salmo trutta L. and Atlantic salmon, Salmo salar L. outside of the PKD season (June‐September) in the UK. This confirms the results of previous studies that these species are the preferred fish hosts for the parasite in the UK.

Five different myxozoan actinosporean spore forms of 4 collective groups are described from oligochaete worms obtained from the sediments of a Scottish salmon hatchery where Sphaerospora truttae infections recur annually. Synactinomyxon 'A', synactinomyxon 'B', raabeia, triactinomyxon and aurantiactinomyxon stages are illustrated, characterised and the stimuli for their polar filament release investigated. Each actinosporean is compared wlth actinosporean 'species' descnbed in the literature and actinosporean stages described from reports of 2 host myxozoan life cycles. Three of the forms reported appear to be previously undescribed.

Light and electron microscope observations on extrasporogonic and sporogonic stages of a myxosporean parasite of the genus Sphaerospora Thelohan, 1892 from Atlantic salmon, Salmo salar L., in Scotland Abstract. Juvenile Atlantic salmon from a number of freshwater hatcheries in Scotland were found to be infected by a myxosporean parasite of the genus Sphaerospora. Fish first became infected in June by extrasporogonic stages which could be found in the blood and kidney interstitium. These consisted of a primary cell containing one to over one hundred secondary cells. Some secondary cells contained one or two tertiary cells. Sporogony was disporous and occurred later in the kidney tubules. The development of both extrasporogonic and sporogonic stages was studied by light and transmission electron microscopy.

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