A group of Protozoa usually regarded as an order within the class Haplosporea. The chief distinguishing characteristic of the Haplosporida, and the one from which their name is derived, is the production of uninucleate spores that lack polar capsules and polar filaments.
Haplosporida are mainly parasites of invertebrates, such as rotifers, annelids, crustacea, insects, and mollusks, but they also occur in the bodies of Ascidia (tunicates), which are primitive chordates. If the genus Icthyosporidium is properly included in the order, Haplosporida also parasitize fish. One species causes “neon” fish disease.
It is of considerable biological interest that some species of Haplosporida are hyperparasites, that is, parasitic on other parasites. They have been found in gregarines parasitizing annelids, and also in flukes. See Sporozoa