The Ascetosporea are a group of protist

Protist

Protists are a diverse group of eukaryotic microorganisms. Historically, protists were treated as the kingdom Protista, which includes mostly unicellular organisms that do not fit into the other kingdoms, but this group is contested in modern taxonomy...

s that are parasites of animals, especially marine invertebrate

Invertebrate

An invertebrate is an animal without a backbone. The group includes 97% of all animal species – all animals except those in the chordate subphylum Vertebrata .Invertebrates form a paraphyletic group...

s. There are two groups, the haplosporids and paramyxids, which are not particularly similar morphologically but consistently group together on molecular trees, which place them near the base of the Cercozoa

Cercozoa

The Cercozoa are a group of protists. They are sometimes described as a kingdom.-Characteristics:The group includes most amoeboids and flagellates that feed by means of filose pseudopods. These may be restricted to part of the cell surface, but there is never a true cytostome or mouth as found in...

. Both produce spore

Spore

In biology, a spore is a reproductive structure that is adapted for dispersal and surviving for extended periods of time in unfavorable conditions. Spores form part of the life cycles of many bacteria, plants, algae, fungi and some protozoa. According to scientist Dr...

s without the complex structures found in similar groups (such as polar filaments or tubules).

Haplosporid spores have a single nucleus

Cell nucleus

In cell biology, the nucleus is a membrane-enclosed organelle found in eukaryotic cells. It contains most of the cell's genetic material, organized as multiple long linear DNA molecules in complex with a large variety of proteins, such as histones, to form chromosomes. The genes within these...

 and an opening at one end, covered with an internal diaphragm or a distinctive hinged lid. After emerging, it develops within the cells of its host, usually a marine mollusc

Mollusca

The Mollusca , common name molluscs or mollusksSpelled mollusks in the USA, see reasons given in Rosenberg's ; for the spelling mollusc see the reasons given by , is a large phylum of invertebrate animals. There are around 85,000 recognized extant species of molluscs. Mollusca is the largest...

 or annelid

Annelid

The annelids , formally called Annelida , are a large phylum of segmented worms, with over 17,000 modern species including ragworms, earthworms and leeches...

, although some infect other groups or freshwater species. The trophic cell is generally multinucleate

Multinucleate

Multinucleate cells have more than one nucleus per cell, which is the result of nuclear division not being followed by cytokinesis. As a consequence, multiple nuclei share one common cytoplasm. This can be the consequence of a disturbed cell cycle control Multinucleate (also multinucleated,...

. Paramyxids develop within the digestive system of marine invertebrates, and undergo internal budding to produce multicellular spores.

A 2009 study concluded that Haplosporidium species form a paraphyletic group and that the taxonomy of the haplosporidians needs a thorough revision.