The ''Eugregarinorida'' are the most large and diverse order of

gregarine The gregarines are a group of Apicomplexan alveolates, classified as the Gregarinasina or Gregarinia. The large (roughly half a millimeter) parasites inhabit the intestines of many invertebrates. They are not found in any vertebrates. However, gr ...

s —

parasitic Parasitism is a close relationship between species, where one organism, the parasite, lives on or inside another organism, the host, causing it some harm, and is adapted structurally to this way of life. The entomologist E. O. Wilson has c ...

protists belonging to the

phylum In biology, a phylum (; plural: phyla) is a level of classification or taxonomic rank below kingdom and above class. Traditionally, in botany the term division has been used instead of phylum, although the International Code of Nomenclature f ...

Apicomplexa The Apicomplexa (also called Apicomplexia) are a large phylum of parasitic alveolates. Most of them possess a unique form of organelle that comprises a type of non-photosynthetic plastid called an apicoplast, and an apical complex structure. T ...

. 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

Eugregarinorida has 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. There are ~900 species of septate gregarines and ~400 of aseptate.

Proposed revision

A revision to the taxonomy of this group has been proposed. 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, Stylocephaloidea s.str. (not in the sense of Clopton, 2009 ), Ancoroidea, Cephaloidophoroidea, Gregarinoidea and Lecudinoidea. These proposed superfamilies have yet to reach general approval.

Description

The trophozoites have lost the apical complex and instead possess a

mucron A ''mucron'' is an attachment organelle found in archigregarines - an order of epicellular parasitic Conoidasida.Simdyanov TG, Guillou L, Diakin AY, Mikhailov KV, Schrével J, Aleoshin VV. (2017) A new view on the morphology and phylogeny of eugreg ...

(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. However, a revision of the morphology of eugregarines has been proposed: they possess only the epimerite and epicytic crests (unlike the genuine

and longitudinal folds of

Archigregarinorida The ''Archigregarinorida'' are an order of parasitic alveolates in the phylum Apicomplexa. Species in this order infect marine invertebrates — usually annelids, ascidians, hemichordates and sipunculids. Taxonomy This order was redefined by L ...

). Intestinal eugregarines are separated into septate (mostly terrestrial) and aseptate (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 forms 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

oocyst Apicomplexans, a group of intracellular parasites, have life cycle stages that allow them to survive the wide variety of environments they are exposed to during their complex life cycle. Each stage in the life cycle of an apicomplexan organism i ...

s in both aquatic and terrestrial environments. Transmission along with egg laying is also known. The life cycle is completed in one host (

monoxenous Monoxenous development, or monoxeny, characterizes a parasite whose development is restricted to a single host species. The etymology of the terms monoxeny / monoxenous derives from the two ancient Greek words (), meaning "unique", and (), m ...

) with the exception of the Porosporicae whose life cycles involve a

mollusc Mollusca is the second-largest phylum of invertebrate animals after the Arthropoda, the members of which are known as molluscs or mollusks (). Around 85,000 extant species of molluscs are recognized. The number of fossil species is esti ...

and a

crustacean Crustaceans (Crustacea, ) form a large, diverse arthropod taxon which includes such animals as decapods, seed shrimp, branchiopods, fish lice, krill, remipedes, isopods, barnacles, copepods, amphipods and mantis shrimp. The crustacean group ...

. After the oocysts are ingested by a suitable host, the sporozoites are released. These are haploid and usually number 8 per oocyte but may be as many as 16 or as few as 4 depending on the species. The sporozoite attaches to or penetrates the intestinal wall. In the latter case the sporozoite may remain within the intestinal wall or enter the body cavity. The sporozoites increase in size and develop into trophozoites. The trophozoites develop into gamonts which associate with each other. The gamonts may not be equal in size: in this case the smaller is referred to as the satellite and the larger as the primite. These are usually the largest stage of the life cycle. The paired gamonts form a membrane around themselves (encystment) — a process known as syzygy. The resulting gametocyst is usually spherical in shape. Within the gametocyst the gamonts give rise to gametes which may be of similar size (isogametes) or different sizes (anisogametes). The gametes fuse in pairs and give rise to a zygote — the only diploid stage in the life cycle. The zygote forms an oocyst and within the oocyst, the sporozoites develop. The oocysts leave the gametocyst either via the latter's rupture or via sporoducts.

References

{{Taxonbar, from=Q12379691 Apicomplexa orders Conoidasida

Taxonomy

Proposed revision

Description

Life cycle

See also

References