Development of a new myxozoan parasite Tetracapsula bryozoides gen. n. et sp. n. in the coelomic cavities of Cris-latella mucedo Cuvier is described. Uninucleate proliferative cells are formed within well-defined sacs, the wall of which is one cell thick. The sacs, of different sizes according to age, are free floating and are conspicuously moved about within the coelomic fluid by the ciliary movements of the host. Division of the proliferative cells produces spherical cells of different sizes with nuclei of commensurate size. The largest cells enter sporogony by dividing into ten cells. Four of these become capsulogenic cells arranged as an anterior group, each giving rise to a spherical polar capsule containing a polar filament, possibly without prior formation of an external tube or, at most, very transient formation of these. Four valvogenic cells enclose the two sporoplasms and overlie the capsulogenic cells except at the points of exit of the polar filaments from the polar capsules. The two uninucleate sporoplasms are packed with endoplasmic reticulum, numerous mitochondria with tubular cristae and sporoplasmosomes which are distributed peripherally. Both sporoplasms produce secondary cells. Typical myxosporean features of the wall cells of the sac and all stages within the sac are: nuclei with granular nucleoplasm and prominent nucleolus, gap junctions between cells consisting of thickened membranes with cross connections, and haplosporosomes. A new genus is established for the parasite, defined as having development limited to uninucleate pseudoplasmodia within a sac of parasite origin, each uninucleate sporogonie stage giving rise to one spore with tetraradial symmetry, composed of four shell valves, four anterior polar capsules and two uninucleate sporoplasms with secondary cells. No plasmodia are formed. The genus is placed within the order Multivalvuli-da, in a new family Saccosporidae, defined as having development within a sac of parasite origin and sporogony without external tube or microtubules during polar capsule formation.
We undertook a detailed ultrastructural investigation to gain insight into the early stages of development of the vermiform myxozoan, Buddenbrockia plumatellae Schröder, 1910 in two bryozoan hosts. Early cell complexes arise in the peritoneum after division and migration of isolated cells in the host body wall. The development of cell junctions linking the outer (mural) cells of the complex then produces a sac enclosing a mass of inner cells. Elongation to the vermiform stage (myxoworm) occurs during multiplication and reorganisation of the inner cells as a central core within the single-layered sac wall. The core cells develop into muscle and sporogonic cells separated from the mural cells by a basal lamina. Myogenesis occurs along the length of the myxoworm from cells that differentiate from the central core, and is independent of elongation. Four primary sporogonic cells maintain positions close to the basal lamina, between muscle cells, while giving rise to secondary sporogonic cells that eventually become free in the central cavity. At least some secondary sporogonic cells undergo meiosis. In view of the recent confirmation of the phylogenetic affinity of Buddenbrockia with the Cnidaria, we postulate how features observed in Buddenbrockia may be homologous with cnidarian structures. Finally we propose a new family name, Buddenbrockiidae, to replace Saccosporidae which was proposed previously in breach of the International Code of Zoological Nomenclature.
During an ecological investigation of populations of the freshwater bryozoan, Cristatella mucedo Cuvier, parasitic sac-like stages of the myxozoan, Tetracapsula bryozoides Canning, Okamura et Curry, 1996 were discovered within the body cavity of some bryozoan colonies. Subsequent to their detection, data were collected on the incidence and prevalence of myxo-zoans in bryozoan populations in the Thames Valley region and on the effects of myxozoans on their hosts. Notable spatial and temporal variation in incidence and prevalence of myxozoans was documented. The production of statoblasts by bryozoans was significantly compromised by myxozoan infection although the production of larvae was not. Infection by myxozoans resulted in generalized swelling, malformation, degeneration, and slower response times in bryozoan colonies. These findings indicate that myxozoans adversely affect host fitness and may therefore influence population levels. Light microscopy revealed that multiple myxozoan sacs can be present within the continuous body cavity of bryozoan colonies and that sacs apparently undergo fission. Infective spores that develop within sacs are at least on some occasions released into the body cavity of bryozoans. Nothing is known of the subsequent stages in the life cycle of this myxozoan, including whether T. bryozoides infects another species. Spores may be released into the water column to infect new hosts or they may be introduced to new hosts when infected bryozoans are ingested.