Orthonectida

(invertebrate zoology) An order of Mesozoa; orthonectids parasitize various marine invertebrates as multinucleate plasmodia, and sexually mature forms are ciliated organisms.

(Orthonectidans)

Phylum: Orthonectida

Number of families: 1

Thumbnail description
Minute dioecious and dimorphic or hermaphroditic parasites found in tissues of a wide variety of marine invertebrate phyla.

Evolution and systematics

The phylum Orthonectida encompasses one family, four genera, and 20 species. Twenty nominal species have been described and variously placed in four genera. Genera are separated on the basis of body shape and the number and arrangement of egg cells in the axial region of the females. Species are separated on the basis of body size; the number, size, and arrangement of rings of ciliated and unciliated jacket cells; the presence and location of pigment and refringent granules in jacket cells; and host specificity. One family, the Rhopaluridae, contains all four genera: Rhopalura, Intoshia, Ciliocincta, and Stoecharthrum.

The orthonectids were previously placed in the Mesozoa because of their simple body organization, but recent phylogenetic studies suggest the orthonectids are closer to myxozoans and nematodes. Many scientists now classify Orthonectida at the order level. A more traditional classification; with Orthonectida at the phylum level, is followed here.

Physical characteristics

Depending on the species, orthonectids are either dioecious and dimorphic, or hermaphroditic. The adults are minute, ranging in length from 0.002 to 0.031 in (50 to 800 µm). The body of the adult consists of a jacket of ciliated and unciliated somatic cells arranged in rings around an internal axial mass. Contractile muscle cells differentiate to pack the gonad with longitudinal, circular, and oblique orientations.

Distribution

Orthonectids occur in coastal regions of the English Channel, the Dover Strait, the Strait of Kattegat, the Barents Sea, the White Sea, the northwestern Pacific Ocean (Japan), and the northeastern Pacific Ocean (United States).

Habitat

Orthonectids are found in tissues of organisms in the marine invertebrate phyla Platyhelminthes, Nemertea, Annelida, Mollusca, Echinodermata, Bryozoa, and Urochordata. In the host, infective orthonectid germinal cells penetrate a host cell, and embryos develop and grow into adults within the cytoplasm of the host cell (the "plasmodium").

Behavior

The name Orthonectida means "straight swimming," because orthonectids swim in a straight line, but they generally swim in a spiral motion.

Feeding ecology and diet

Orthonectids may absorb nutrients within the host's cytoplasm.

Reproductive biology

During mating, males make brief contact with females when sperm are released. Fertilization is internal in females. Embryos form about 22 hours after the first cleavage of eggs. When the embryos are fully developed, the female ruptures and dies, releasing ciliated larvae that disperse and enter a new host.

Conservation status

No species of orthonectid is listed by the IUCN.

Significance to humans

None known.

Species accounts

Resources

Kozloff, Eugene N. "Phyla Placozoa, Dicyemida, and Orthonectida." In Invertebrates. Philadelphia: Saunders College Publishing, 1990.

Kozloff, Eugene N. "The Genera of the Phylum Orthonectida." Cahiers de Biologie Marine 33 (1992): 377–406.

Pawlowski, J., J. Montoya-Burgos, J. F. Fahrni, J. Wüest, and L. Zaninetti. "Origin of the Mesozoa Inferred from 18S rRNA Gene Sequences." Molecular Biology and Evolution 13 (1996): 1128–1132.

[Article by: Hidetaka Furuya, PhD]

An order of Mesozoa. The orthonectids parasitize various marine invertebrates as multinucleate plasmodia. The plasmodia multiply by fragmentation. Eventually they give rise asexually, by polyembryony, to sexual males and females. Commonly only one sex arises from a given plasmodium.

These sexually mature forms escape as minute ciliated organisms. Structurally they are composed of a single layer of ciliated epithelial cells surrounding an inner mass of sex cells. The ciliated cells are disposed in rings around the body.

After insemination the eggs develop in the female and form ciliated larvaea. When liberated, these larvae invade new individuals of their host and then disaggregate, liberating germinal cells which give rise to new plasmodia. See also Mesozoa.

Orthonectids
Scientific classification
Kingdom: Animalia Phylum: Orthonectida Giard, 1880

Orthonectida (pronounced /ˌɔrθɵˈnɛktɪdə/) is a small phylum of poorly-known parasites of marine invertebrates^[1] that are among the simplest of multi-cellular organisms. Members of this phylum are known as orthonectids.

Characteristics

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.

Originally described in 1880 as a class,^[2] and sometimes characterized as an order of Mesozoa, recent study shows that orthonectids are indeed quite different from rhombozoans, the other group in Mesozoa.^[1]

Species

The phylum consists of about 20 known species, of which Rhopalura ophiocomae is the best-known.^[1]

Known species:

• 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

Wikispecies has information related to: Orthonectida

References

1. ^ ^{a b c} Hanelt B, Van Schyndel D, Adema CM, Lewis LA, Loker ES (November 1996). "The phylogenetic position of Rhopalura ophiocomae (Orthonectida) based on 18S ribosomal DNA sequence analysis". Mol. Biol. Evol. 13 (9): 1187–91. PMID 8896370. http://mbe.oxfordjournals.org/cgi/pmidlookup?view=long&pmid=8896370. 
2. ^ Giard, E., "The Orthonectida, a new class of the phylum of the worms" Quart. J. Microsc. Sci., 1880 n.s. 20: 225-240

v • d • e Opisthokonta: Extant phyla of kingdom Animalia by subkingdom Parazoa Porifera (Calcarea, Demospongiae, Hexactinellida) · Placozoa (Trichoplax) Mesozoa Orthonectida · Rhombozoa Eumetazoa Radiata Ctenophora · Cnidaria (Anthozoa, Hydrozoa, Scyphozoa, Cubozoa, Staurozoa, Myxozoa) Bilateria Protostomia Ecdysozoa Cycloneuralia: Scalidophora (Kinorhyncha, Loricifera, Priapulida) · Nematoida (Nematoda, Nematomorpha) Panarthropoda: Onychophora · Tardigrada · Arthropoda Spiralia Platyzoa Platyhelminthes · Gastrotricha Gnathifera: Rotifera · Acanthocephala · Gnathostomulida · Micrognathozoa · Cycliophora Lophotrochozoa Trochozoa (Sipuncula, Nemertea, Mollusca, Annelida, Echiura) · Brachiozoa Lophophorata (Bryozoa, Entoprocta, Phoronida, Brachiopoda) Deuterostomia Ambulacraria Hemichordata · Echinodermata · Xenoturbellida Chordata Craniata (Vertebrata, Hyperotreti) · Cephalochordata · Tunicata Basal/disputed Acoelomorpha (Acoela, Nemertodermatida) · Chaetognatha

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