Eugregarinorida

The Eugregarinorida are an order of parasitic protozoa in the phylum Apicomplexa. Eugregarines are found in marine, freshwater and terrestrial habitats. These species possess large trophozoites that are significantly different in morphology and behavior from the sporozoites. This taxon contains most of the known gregarine species.

These protozoa are common parasites of many invertebrates including insects and polychete worms.

Taxonomy

There are three recognised suborders: Aseptatorina, Blastogregarinorina and Septatorina. The intestinal eugregarines are separated into septate - suborder Septatina - and aseptate - suborder Aseptatina - depending on whether the trophozoite is superficially divided by a transverse septum. The marine gregarines are the most poorly studied members of this order.

The eugregarines have been classified into 27 families with ~244 genera, 14 of which have more than 25 species each.^[1]

There are ~900 species of sepate gregarines and ~400 of aseptate.

Proposed revision

A revision to the taxonomy of this group has been proposed.^[2] Instead of the division into aseptate and septate taxa five new superfamilies have been proposed on the basis of 18s RNA studies. These are Actinocephaloidea (which is an ancestral and paraphyletic taxon), Ancoroidea, Cephaloidophoroidea, Gregarinoide and Lecudinoidea.

These proposed superfamilies have yet to reach general approval.

Description

The trophozoites have lost the apical complex and instead possess a mucron (aseptate species) or an epimerite (septate species). The surface is inscribed by 90-300 epicytic folds resulting in stiff cells that are capable of gliding motility.

Intestinal eugregarines are separated into septate (mostly terrestrial) and aspetate (mostly marine) gregarines, depending on whether the trophozoite cell is superficially marked by a transverse septum.

In the aseptate (or acephalate) species the cell is not divided. In the septate (or cephalate) species the body is divided into at least 2 parts: a posterior portion which contains the nucleus (the deutomerite) and an anterior portion (the protomerite) which contains the epimerite. In some species the deutomerite may be further subdivided.

Life cycle

The transmission of the parasite to new host usually takes place by oral ingestion of oocysts in both aquatic and terrestrial environments. Transmission along with egg laying is also known.

References