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Orthonectida () is a small
phylum In biology, a phylum (; : phyla) is a level of classification, or taxonomic rank, that is below Kingdom (biology), kingdom and above Class (biology), class. Traditionally, in botany the term division (taxonomy), division has been used instead ...
of poorly known
parasite Parasitism is a Symbiosis, close relationship between species, where one organism, the parasite, lives (at least some of the time) on or inside another organism, the Host (biology), host, causing it some harm, and is Adaptation, adapted str ...
s of marine invertebrates that are among the simplest of multi-cellular organisms. Members of this phylum are known as orthonectids.


Biology

The adults, which are the sexual stage, are microscopic wormlike animals, consisting of a single layer of
cilia The cilium (: cilia; ; in Medieval Latin and in anatomy, ''cilium'') is a short hair-like membrane protrusion from many types of eukaryotic cell. (Cilia are absent in bacteria and archaea.) The cilium has the shape of a slender threadlike proj ...
ted outer cells surrounding a mass of sex cells. They swim freely within the bodies of their hosts, which include
flatworm Platyhelminthes (from the Greek language, Greek πλατύ, ''platy'', meaning "flat" and ἕλμινς (root: ἑλμινθ-), ''helminth-'', meaning "worm") is a Phylum (biology), phylum of relatively simple bilaterian, Segmentation (biology), ...
s,
polychaete Polychaeta () is a paraphyletic class of generally marine Annelid, annelid worms, common name, commonly called bristle worms or polychaetes (). Each body segment has a pair of fleshy protrusions called parapodia that bear many bristles, called c ...
worms,
bivalve Bivalvia () or bivalves, in previous centuries referred to as the Lamellibranchiata and Pelecypoda, is a class (biology), class of aquatic animal, aquatic molluscs (marine and freshwater) that have laterally compressed soft bodies enclosed b ...
molluscs, and
echinoderm An echinoderm () is any animal of the phylum Echinodermata (), which includes starfish, brittle stars, sea urchins, sand dollars and sea cucumbers, as well as the sessile sea lilies or "stone lilies". While bilaterally symmetrical as ...
s. Most are gonochoristic, with separate male and female individuals, but a few species are hermaphroditic. When they are ready to reproduce, adults leave the host, and sperm from the males penetrate the bodies of the females to achieve internal fertilisation. The resulting
zygote A zygote (; , ) is a eukaryote, eukaryotic cell (biology), cell formed by a fertilization event between two gametes. The zygote's genome is a combination of the DNA in each gamete, and contains all of the genetic information of a new individ ...
develops into a ciliated larva that escapes from the mother to seek out new hosts. Once it finds a host, the larva loses its cilia and develops into a syncytial
plasmodium ''Plasmodium'' is a genus of unicellular eukaryotes that are obligate parasites of vertebrates and insects. The life cycles of ''Plasmodium'' species involve development in a Hematophagy, blood-feeding insect host (biology), host which then inj ...
larva. This, in turn, breaks up into numerous individual cells called agametes (ameiotic generative cells) which grow into the next generation of adults.


Classification

The phylum consists of about 20 known species, of which ''Rhopalura ophiocomae'' is the best-known. The phylum is not divided into classes or orders, and contains just two families. Although originally described in 1877 as a class, and later characterized as an order of the phylum Mesozoa, a 1996 study has suggested that orthonectids are quite different from the rhombozoans, the other group in Mesozoa. The genome of one orthonectid species, '' Intoshia linei'', has been sequenced. These animals are simplified
spiralia The Spiralia are a morphologically diverse clade of protostome animals, including within their number the molluscs, annelids, platyhelminths and other Taxon, taxa. The term ''Spiralia'' is applied to those phyla that exhibit canonical spiral clea ...
ns. The genome data confirm earlier findings which allocated these organisms to Spiralia based on their morphology. Their position in the spiralian phylogenetic tree has yet to be determined. Some work appears to relate them to the
Annelida The annelids (), also known as the segmented worms, are animals that comprise the phylum Annelida (; ). The phylum contains over 22,000 extant species, including ragworms, earthworms, and leeches. The species exist in and have adapted to variou ...
and, within the Annelida, finds them most closely allied to the Clitellata. On the other hand, a 2022 study compensating for long-branch attraction has recovered the traditional grouping of Orthonectida with rhombozoans in a monophyletic Mesozoa placed close to Platyhelminthes or Gnathifera. This supports a previous study which found orthonectids and rhombozoans to make a monophyletic taxon Mesozoa and form a clade with Rouphozoa (platyhelminths and
gastrotrich The gastrotrichs (phylum Gastrotricha), commonly referred to as hairybellies or hairybacks, are a group of microscopic (0.06–3.0 mm), cylindrical, acoelomate animals, and are widely distributed and abundant in freshwater and marine en ...
s).


Known species

Phylum Orthonectida * Family Rhopaluridae Stunkard, 1937 ** '' Ciliocincta'' *** '' Ciliocincta akkeshiensis'' Tajika, 1979 – Hokkaido, Japan; in flatworms ( Turbellaria) *** '' Ciliocincta julini'' (Caullery and Mesnil, 1899) – E North Atlantic, in polychaetes *** '' Ciliocincta sabellariae'' Kozloff, 1965 – San Juan Islands, WA (USA); in polychaete ('' Neosabellaria cementarium'') ** '' Intoshia'' *** '' Intoshia leptoplanae'' Giard, 1877 – E North Atlantic, in flatworms ('' Leptoplana'') *** '' Intoshia linei'' Giard, 1877 – E North Atlantic, in nemertines ('' Lineus'') = ''Rhopalura linei'' *** '' Intoshia major'' Shtein, 1953
Arctic Ocean The Arctic Ocean is the smallest and shallowest of the world's five oceanic divisions. It spans an area of approximately and is the coldest of the world's oceans. The International Hydrographic Organization (IHO) recognizes it as an ocean, ...
; in gastropods ('' Lepeta'', '' Natica'', '' Solariella'') = ''Rhopalura major'' *** '' Intoshia metchnikovi'' (Caullery & Mesnil, 1899) – E North Atlantic, in polychaetes and nemertines *** '' Intoshia paraphanostomae'' (Westblad, 1942) – E North Atlantic, in flatworms ('' Acoela'') *** '' Intoshia variabili'' (Alexandrov & Sljusarev, 1992) – Arctic Ocean, in flatworms ('' Macrorhynchus'') ** '' Rhopalura'' *** '' Rhopalura elongata'' Shtein, 1953 – Arctic Ocean, in bivalves ('' Astarte'') *** '' Rhopalura gigas'' (Giard, 1877) *** '' Rhopalura granosa'' Atkins, 1933 – E North Atlantic, in bivalves ('' Pododesmus'') *** '' Rhopalura intoshi'' Metchnikoff – Mediterranean, in nemertines *** '' Rhopalura litoralis'' Shtein, 1953 – Arctic Ocean, in gastropods ('' Lepeta'', '' Natica'', '' Solariella'') *** '' Rhopalura major'' Shtein, 1953 *** '' Rhopalura murmanica'' Shtein, 1953 – Arctic Ocean, in gastropods ('' Rissoa'', ''
Columbella ''Columbella'' is a genus of small sea snails, marine (ocean), marine gastropod mollusks in the family (biology), family Columbellidae, the dove snails. Species Species within the genus ''Columbella'' include: * ''Columbella adansoni'' Karl The ...
'') *** '' Rhopalura ophiocomae'' Giard, 1877 – E North Atlantic, in ophiuroids (usually '' Amphipholis'') *** '' Rhopalura pelseneeri'' Caullery & Mesnil, 1901 – E North Atlantic, polychaetes and nemertines *** '' Rhopalura philinae'' Lang, 1954 – E North Atlantic, in gastropods *** '' Rhopalura pterocirri'' de Saint-Joseph, 1896 – E North Atlantic, in polychaetes *** '' Rhopalura vermiculicola'' ** '' Stoecharthrum'' *** '' Stoecharthrum burresoni'' Kozloff, 1993 *** '' Stoecharthrum fosterae'' Kozloff, 1993 *** '' Stoecharthrum giardi'' Caullery & Mesnil, 1899 – E North Atlantic, in polychaetes *** '' Stoecharthrum monnati'' Kozloff, 1993 – E North Atlantic, in molluscs * Family
Pelmatosphaeridae ''Pelmatosphaera'' is a monotypic genus of worms belonging to the monotypic family Pelmatosphaeridae. The only species is ''Pelmatosphaera polycirri''. References

Orthonectida Monotypic protostome genera {{Protostome-stub ...
Stunkard, 1937 ** '' Pelmatosphaera'' *** '' Pelmatosphaera polycirri'' Caullery and Mesnil, 1904 – E North Atlantic, in polychaetes and nemertines


References

{{Taxonbar, from=Q131032 Protostome phyla Mesozoa