Urosporidium astomatum n. sp., a haplosporidian infecting the metacercariae of Parvatrema donacis, a trematode parasite of the bivalve.
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The presence of a haplosporidian infecting metacercariae parasitic in a bivalve mollusk allowed development stages of the hyperparasite to be released and studied alive and undamaged. The hyperparasite, a new species of Urosporidium, was described and named Urosporidium astomatum. The developmental cycle of this species differs from that reported from previously described haplosporidians. The plasmodium of U. astomatum divides into a number of smaller plasmodia which are all contained within a single wall formed from the original plasmodial wall. Further divisions of the plasmodia produces a cyst containing a number of prespores. These latter cells appear to undergo a process of rearrangement before they develop into mature spores. Elongation of the cells is followed by cytoplasmic constriction which produces a bilobed structure. The lobes later recombine to form a single oval cell. Donax variabilis were maintained for five months in laboratory aquaria which contained artificial which contained artificial sea water and a 4 cm layer sand. Results of staining procedures and host-parasite relationships are described.