Archigregarinorida Eugregarinorida Neogregarinorida The gregarines are a group of Apicomplexan alveolates, classified as the Gregarinasina[1] or Gregarinia.

Both protists use protein complexes similar to those that are formed by the gregarines for gliding motility and for invading target cells.

[2][3] This makes the gregarines excellent models for studying gliding motility, with the goal of developing treatment options for both toxoplasmosis and malaria.

In general, gregarines are regarded as a very successful group of parasites, as their hosts are distributed over the entire planet.

In some species, the sporozoites and trophozoites are capable of asexual replication – a process called schizogony or merogony.

Unusually, they tend to lack attachment structures and form gamont pairs that pulsate freely within the coelomic fluid.

The eugregarines multiply by sporogony and gametogony, while the neogregarines have an additional schizogenic stage – merogony – within their hosts.

The first of them – Gregarinomorphea – comprises Orthogregarinia, Cryptosporidiidae and, additionally, Rhytidocystidae previously considered as divergent coccidians[9] or Apicomplexa incertae sedis.

[12] The Orthogregarinia with two new orders Arthrogregarida and Vermigregarida was created for the gregarines most closely related to Cryptosporidium.

This point of view was challenged in 2017 by Simdyanov and co-authors, who performed the global integrated analysis of available morphological and molecular phylogenetic data and concluded that eugregarines are rather a monophyletic taxon.

[13] Several genera of gregarines are currently not classified: Acuta, Cephalolobus, Gregarina, Levinea, Menospora, Nematocystis, Nematopsis, Steinina, and Trichorhynchus.

The parasites are relatively large, spindle-shaped cells, compared to other apicomplexans and eukaryotes in general (some species are > 850 µm in length).

Most gregarines have longitudinal epicytic folds (bundles of microtubules beneath the cell surface with nematode like bending behaviour): crenulations are instead found in the urosporidians.

The gregarines are able to move and change direction along a surface through gliding motility without the use of cilia, flagella, or lamellipodia.