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 ...

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 h ...

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 proje ...

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, unsegmen ...

polychaete Polychaeta () is a paraphyletic class of generally marine annelid worms, commonly called bristle worms or polychaetes (). Each body segment has a pair of fleshy protrusions called parapodia that bear many bristles, called chaetae, which are made ...

worms,

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

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 ...

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. 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 multicell ...

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

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

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, th ...

. 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 πλατύ, ''platy'', meaning "flat" and ἕλμινς (root: ἑλμινθ-), ''helminth-'', meaning "worm") are a phylum of relatively simple bilaterian, unsegm ...

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 gastrotrichs).

Known species

Phylum Orthonectida * Family Rhopaluridae Stunkard, 1937 ** '' Ciliocincta'' *** '' Ciliocincta akkeshiensis'' 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 mo ...

) *** '' 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 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 Family (from la, familia) is a group of people related either by consanguinity (by recognized birth) or affinity (by marriage or other relations ...

'', '' 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 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 ...

'') *** '' 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 (''

'', '' Natica'', '' Solariella'') *** '' Rhopalura major'' Shtein, 1953 *** '' Rhopalura murmanica'' Shtein, 1953 – Arctic Ocean, in gastropods ('' Rissoa'', '' Columbella'') *** '' 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 Stunkard, 1937 ** '' Pelmatosphaera'' *** '' Pelmatosphaera polycirri'' Caullery and Mesnil, 1904 – E North Atlantic, in polychaetes and nemertines

References

{{Taxonbar, from=Q131032 Animal phyla