Orthonectida () is a small

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 ...

of poorly known

parasite 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 ...

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, plural cilia (), is a membrane-bound organelle found on most types of eukaryotic cell, and certain microorganisms known as ciliates. Cilia are absent in bacteria and archaea. The cilium has the shape of a slender threadlike projecti ...

ted outer cells surrounding a mass of sex cells. They swim freely within the bodies of their hosts, which include

flatworm The flatworms, flat worms, Platyhelminthes, or platyhelminths (from the Greek πλατύ, ''platy'', meaning "flat" and ἕλμινς (root: ἑλμινθ-), ''helminth-'', meaning "worm") are a phylum of relatively simple bilaterian, unsegment ...

polychaete Polychaeta () is a paraphyletic class (biology), class of generally marine invertebrate, marine annelid worms, common name, commonly called bristle worms or polychaetes (). Each body segment has a pair of fleshy protrusions called parapodia that ...

worms,

bivalve Bivalvia (), in previous centuries referred to as the Lamellibranchiata and Pelecypoda, is a class of marine and freshwater molluscs that have laterally compressed bodies enclosed by a shell consisting of two hinged parts. As a group, bival ...

molluscs, and

echinoderm An echinoderm () is any member of the phylum Echinodermata (). The adults are recognisable by their (usually five-point) radial symmetry, and include starfish, brittle stars, sea urchins, sand dollars, and sea cucumbers, as well as the sea ...

s. Most are

gonochoristic In biology, gonochorism is a sexual system where there are only two sexes and each individual organism is either male or female. The term gonochorism is usually applied in animal species, the vast majority of which are gonochoric. Gonochorism c ...

, 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 Internal fertilization is the union of an egg and sperm cell during sexual reproduction inside the female body. Internal fertilization, unlike its counterpart, external fertilization, brings more control to the female with reproduction. For inter ...

. The resulting

zygote A zygote (, ) is a eukaryotic 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 individual organism. In multicellula ...

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 A syncytium (; plural syncytia; from Greek: σύν ''syn'' "together" and κύτος ''kytos'' "box, i.e. cell") or symplasm is a multinucleate cell which can result from multiple cell fusions of uninuclear cells (i.e., cells with a single nucleus) ...

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 blood-feeding insect host which then injects parasites into a vert ...

larva. This, in turn, breaks up into numerous individual cells called agametes (ameiotic generative cells) which grows 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 The Mesozoa are minuscule, worm-like parasites of marine invertebrates. Generally, these tiny, elusive creatures consist of a somatoderm (outer layer) of ciliated cells surrounding one or more reproductive cells. A recent study recovered Mesoz ...

, recent study has suggested that orthonectids are quite different from the

rhombozoa Dicyemida, also known as Rhombozoa, is a phylum of tiny parasites that live in the renal appendages of cephalopods. Taxonomy Classification is controversial. Traditionally, dicyemids have been grouped with the Orthonectida in the Mesozoa, an ...

ns, the other group in Mesozoa. The genome of one orthonectid species, ''

Intoshia linei ''Intoshia linei'' is a species of parasitic worms belonging to the family Rhopaluridae. It is the only species from the phylum Orthonectida which genome In the fields of molecular biology and genetics, a genome is all the genetic infor ...

'', 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 taxa. The term ''Spiralia'' is applied to those phyla that exhibit canonical spiral cleavage, a ...

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 and, within the Annelida, finds them most closely allied to the

Clitellata The Clitellata are a class of annelid worms, characterized by having a clitellum - the 'collar' that forms a reproductive cocoon during part of their life cycles. The clitellates comprise around 8,000 species. Unlike the class of Polychaeta, the ...

. On the other hand, a 2022 study compensating for

long-branch attraction In phylogenetics, long branch attraction (LBA) is a form of systematic error whereby distantly related lineages are incorrectly inferred to be closely related. LBA arises when the amount of molecular or morphological change accumulated within a lin ...

has recovered the traditional grouping of Orthonectida with rhombozoans in a monophyletic Mesozoa placed close to

Platyhelminthes The flatworms, flat worms, Platyhelminthes, or platyhelminths (from the Greek language, Greek πλατύ, ''platy'', meaning "flat" and ἕλμινς (root: ἑλμινθ-), ''helminth-'', meaning "worm") are a Phylum (biology), phylum of relati ...

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), worm-like, acoelomate animals, and are widely distributed and abundant in freshwater and marine environme ...

s).

Known species

Phylum Orthonectida * Family

Rhopaluridae Rhopaluridae is a family of worms belonging to the phylum Orthonectida Orthonectida () is a small phylum of poorly known parasites of marine invertebrates that are among the simplest of multi-cellular organisms. Members of this phylum are known ...

Stunkard, 1937 ** ''

Ciliocincta ''Ciliocincta'' is a genus of worms belonging to the family Rhopaluridae. Species: *''Ciliocincta akkeshiensis ''Ciliocincta'' is a genus of worms belonging to the family Rhopaluridae Rhopaluridae is a family of worms belonging to the phyl ...

'' *** ''

Ciliocincta akkeshiensis ''Ciliocincta'' is a genus of worms belonging to the family Rhopaluridae Rhopaluridae is a family of worms belonging to the phylum Orthonectida Orthonectida () is a small phylum of poorly known parasites of marine invertebrates that are among ...

'' Tajika, 1979 – Hokkaido, Japan; in flatworms (

Turbellaria The Turbellaria are one of the traditional sub-divisions of the phylum Platyhelminthes (flatworms), and include all the sub-groups that are not exclusively parasitic. There are about 4,500 species, which range from to large freshwater forms more ...

) *** ''

Ciliocincta julini ''Ciliocincta'' is a genus of worms belonging to the family Rhopaluridae. Species: *''Ciliocincta akkeshiensis ''Ciliocincta'' is a genus of worms belonging to the family Rhopaluridae Rhopaluridae is a family of worms belonging to the phylum ...

'' (Caullery and Mesnil, 1899) – E North Atlantic, in polychaetes *** ''

Ciliocincta sabellariae ''Ciliocincta'' is a genus of worms belonging to the family Rhopaluridae. Species: *''Ciliocincta akkeshiensis'' *''Ciliocincta julini ''Ciliocincta'' is a genus of worms belonging to the family Rhopaluridae. Species: *''Ciliocincta akkeshi ...

'' Kozloff, 1965 – San Juan Islands, WA (USA); in polychaete (''

Neosabellaria cementarium ''Neosabellaria cementarium'' is a species of marine tube worm in the family Sabellariidae, perhaps better known by its previous name, ''Sabellaria cementarium''. It is found in the North Pacific Ocean. Description ''Neosabellaria cementarium'' ...

'') ** ''

Intoshia ''Intoshia'' is a genus of worms belonging to the family Rhopaluridae. Species: *'' Intoshia leptoplanae'' *''Intoshia linei ''Intoshia linei'' is a species of parasitic worms belonging to the family Rhopaluridae. It is the only species from ...

'' *** ''

Intoshia leptoplanae ''Intoshia'' is a genus of worms belonging to the family Rhopaluridae. Species: *'' Intoshia leptoplanae'' *''Intoshia linei ''Intoshia linei'' is a species of parasitic worms belonging to the family Rhopaluridae. It is the only species from ...

'' Giard, 1877 – E North Atlantic, in flatworms ('' Leptoplana'') *** ''

'' Giard, 1877 – E North Atlantic, in nemertines (''

Lineus ''Lineus'' is a genus of nemertine worms, including the bootlace worm The bootlace worm (''Lineus longissimus'') is a species of ribbon worm and one of the longest known animals, with specimens up to long being reported, although this has ...

'') = ''Rhopalura linei'' *** ''

Intoshia major ''Intoshia'' is a genus of worms belonging to the family Rhopaluridae. Species: *''Intoshia leptoplanae'' *''Intoshia linei ''Intoshia linei'' is a species of parasitic worms belonging to the family Rhopaluridae. It is the only species from ...

'' Shtein, 1953 –

Arctic Ocean The Arctic Ocean is the smallest and shallowest of the world's five major oceans. It spans an area of approximately and is known as the coldest of all the oceans. The International Hydrographic Organization (IHO) recognizes it as an ocean, a ...

; in gastropods (''

Lepeta ''Lepeta'' is a genus of sea snails, the true limpets, marine gastropod mollusks in the family Lepetidae. Species Species within the genus ''Lepeta'' include: * ''Lepeta caeca'' (Müller O.F., 1776) * '' Lepeta depressa'' Hedley, 1916 ;Species b ...

'', ''

Natica ''Natica'' is a genus of small to medium-sized predatory sea snails, marine gastropods in the subfamily Naticinae of the family Naticidae, the moon snails. The genus was erected by Giovanni Antonio Scopoli in 1777.Gofas, S. (2011). Natica Scop ...

'', ''

Solariella ''Solariella'' is a genus of small to minute sea snails, marine gastropod molluscs in the family Solariellidae within the superfamily Trochoidea, the top snails, turban snails and their allies. MolluscaBase eds. (2022). MolluscaBase. Solariella ...

'') = ''Rhopalura major'' *** ''

Intoshia metchnikovi ''Intoshia'' is a genus of worms belonging to the family Rhopaluridae. Species: *''Intoshia leptoplanae'' *''Intoshia linei'' *''Intoshia major ''Intoshia'' is a genus of worms belonging to the family Rhopaluridae. Species: *''Intoshia lep ...

'' (Caullery & Mesnil, 1899) – E North Atlantic, in polychaetes and nemertines *** ''

Intoshia paraphanostomae ''Intoshia'' is a genus of worms belonging to the family Rhopaluridae. Species: *''Intoshia leptoplanae'' *''Intoshia linei'' *''Intoshia major'' *''Intoshia metchnikovi ''Intoshia'' is a genus of worms belonging to the family Rhopaluridae. ...

'' (Westblad, 1942) – E North Atlantic, in flatworms (''

Acoela Acoela, or the acoels, is an order of small and simple invertebrates in the subphylum Acoelomorpha of phylum Xenacoelomorpha, a deep branching bilaterian group of animals, which resemble flatworms. Historically they were treated as an order of ...

'') *** ''

Intoshia variabili ''Intoshia'' is a genus of worms belonging to the family Rhopaluridae. Species: *''Intoshia leptoplanae'' *''Intoshia linei'' *''Intoshia major'' *''Intoshia metchnikovi'' *''Intoshia paraphanostomae ''Intoshia'' is a genus of worms belong ...

'' (Alexandrov & Sljusarev, 1992) – Arctic Ocean, in flatworms ('' Macrorhynchus'') ** '' Rhopalura'' *** '' Rhopalura elongata'' Shtein, 1953 – Arctic Ocean, in bivalves (''

Astarte Astarte (; , ) is the Hellenized form of the Ancient Near Eastern goddess Ashtart or Athtart (Northwest Semitic), a deity closely related to Ishtar (East Semitic), who was worshipped from the Bronze Age through classical antiquity. The name i ...

'') *** '' 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 (''

'', ''

'') *** '' Rhopalura major'' Shtein, 1953 *** '' Rhopalura murmanica'' Shtein, 1953 – Arctic Ocean, in gastropods (''

Rissoa ''Rissoa'' is a genus of minute sea snails, marine gastropod mollusks or micromollusks in the family Rissoidae. The genus was first introduced by M. de Freminville for some small shells observed by M. Antoine Risso, a distinguished naturalis ...

'', ''

Columbella ''Columbella'' is a genus of small sea snails, marine gastropod mollusks in the family Columbellidae, the dove snails. Species Species within the genus ''Columbella'' include: * '' Columbella adansoni'' Menke, 1853 * '' Columbella aphthaegera'' ...

'') *** '' Rhopalura ophiocomae'' Giard, 1877 – E North Atlantic, in

ophiuroid Brittle stars, serpent stars, or ophiuroids (; ; referring to the serpent-like arms of the brittle star) are echinoderms in the class Ophiuroidea, closely related to starfish. They crawl across the sea floor using their flexible arms for locomot ...

s (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 Stunkard, 1937 ** '' Pelmatosphaera'' *** '' Pelmatosphaera polycirri'' Caullery and Mesnil, 1904 – E North Atlantic, in polychaetes and nemertines

References

{{Taxonbar, from=Q131032 Animal phyla