Orthonectida

Orthonectida is a small phylum of poorly-known parasites of marine invertebrates that are among the simplest of multi-cellular organisms.

Multinucleate plasmodia give rise to sexual males and females, which are free-swimming by means of cilia. They are composed of a single layer of ciliated outer cells surrounding a mass of sex cells. When a female is inseminated, the eggs develop into ciliated larvae that seek out new hosts and then break up into individual cells that become the next generation of plasmodia.

Parasitized organisms include flatworms, polychaete worms, bivalve molluscs, and echinoderms.

The phylum consists of about 20 known species, of which Rhopalura ophiocomae is the best-known.

Originally described in 1880 as a class, and sometimes characterized as an order of Mesozoa, recent study shows that orthonectids are indeed quite different from rhombozoans, the other group in Mesozoa.

Known species:

• Family Rhopaluridae
  • Ciliocincta julini (Caullery and Mesnil, 1899) - E North Atlantic, in polychaetes
  • 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 paraphanostomae (Westblad, 1942) - E North Atlantic, in flatworms (Acoela)
  • 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, 1954) - Arctic Ocean, in gastropods (Lepeta, Natica, Solariella)
  • Rhopalura metschnikowi (Caullery and Mesnil, 1901) - E North Atlantic, in polychaetes and nemertines
  • Rhopalura murmanica (Shtein, 1953) - Arctic Ocean, in gastropods (Rissoa, Columbella)
  • Rhopalura ophiocomae (Giard, 1877) - E North Atlantic, in ophiuroids (usually Amphipholis)
  • Rhopalura pelseeneri (Caullery and Mesnil, 1901) - E North Atlantic, polychaetes and nemertines
  • Rhopalura philinae (Lang, 1951) - E North Atlantic, in gastropods
  • Rhopalura pterocirri (de Saint-Joseph, 1896) E North Atlantic, in polychaetes
  • Rhopalura variabili (Alexandrov and Sljusarev, 1992) - Arctic Ocean, in flatworms (Macrorhynchus)
  • Stoecharthrum giardi (Caullery and Mesnil, 1899) - E North Atlantic, in polychaetes
  • Stoecharthrum monnati (Kozloff, 1993) - E North Atlantic, in molluscs

• Family Pelmatosphaeridae
  • Pelmatosphaera polycirri (Caullery and Mesnil, 1904) - E North Atlantic, in polychaetes and nemertines