Orthonectida

Orthonectids
Two different female Orthonectids
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
(unranked):	Mesozoa
Phylum:	Orthonectida

Orthonectida (Template:Pron-en) 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 as orthonectids.

Biology

The adults are microscopic wormlike animals, consisting of a single layer of ciliated outer cells surrounding a mass of sex cells. They swim freely within the bodies of their hosts, which include flatworms, polychaete worms, bivalve molluscs, and echinoderms. They are dioecious, with separate male and female individuals.

When they are ready to reproduce, the adults are released from the host, and sperm from the males penetrates 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 a syncitial plasmodium larva. This, in turn, breaks up into numerous individual cells that become 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.

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

Known species:

Phylum Orthonectida

• Family Rhopaluridae
  • 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 (Sabellaria cementarium)
  • 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