Orthonectida

The non-feeding adults, which are the sexual stage, are microscopic wormlike animals, consisting of a single layer of ciliated outer cells surrounding a mass of sex cells. Their musculature and nervous system is very reduced. In Intoshia variabili, the adult females have 32 being muscle cells and 4 nerve cells, and the smaller male only halv of that, with 16 muscle cells and 2 nerve cells.^[5] They swim freely within the bodies of their hosts, which include flatworms, polychaete worms, bivalve molluscs, echinoderms, ribbon worms, acoels, Xenoturbellida, and chordates^[6]. Most are gonochoristic, with separate male and female individuals, but a few species are hermaphroditic.^[7][8]

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 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 its multinucleated and plasmodial stage with a double membrane.^[9] This, in turn, breaks up into numerous individual cells called agametes (ameiotic generative cells) which grow into the next generation of adults.^[7][10]

The phylum consists of about 20 known species, of which Rhopalura ophiocomae is the best-known.^[4] The phylum is not divided into classes or orders, and contains just two families.

Although originally described in 1877 as a class,^[11] 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.^[4] The genome of one orthonectid species, Intoshia linei, has been sequenced.^[12] These animals are simplified spiralians. The genome data confirm earlier findings which allocated these organisms to Spiralia based on their morphology.^[13]

Their position in the spiralian phylogenetic tree has yet to be determined. Some work appears to relate them to the Annelida^[14][10] and, within the Annelida, finds them most closely allied to the Clitellata.^[15] 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.^[16] 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).^[17]

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; 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)
    • 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