Orthonectida
Encyclopedia
Orthonectida 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.
s, polychaete
worms, bivalve
molluscs, and echinoderm
s. They are gonochoristic
, 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.
Although originally described in 1877 as a class, and sometimes characterized as an order of the phylum Mesozoa
, recent study shows that orthonectids are quite different from the rhombozoa
ns, the other group in Mesozoa.
Known species:
Phylum Orthonectida
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 flatwormFlatworm
The flatworms, known in scientific literature as Platyhelminthes or Plathelminthes are a phylum of relatively simple bilaterian, unsegmented, soft-bodied invertebrate animals...
s, polychaete
Polychaete
The Polychaeta or polychaetes are a class of annelid worms, generally marine. Each body segment has a pair of fleshy protrusions called parapodia that bear many bristles, called chaetae, which are made of chitin. Indeed, polychaetes are sometimes referred to as bristle worms. More than 10,000...
worms, bivalve
Bivalvia
Bivalvia is a taxonomic class of marine and freshwater molluscs. This class includes clams, oysters, mussels, scallops, and many other families of molluscs that have two hinged shells...
molluscs, and echinoderm
Echinoderm
Echinoderms are a phylum of marine animals. Echinoderms are found at every ocean depth, from the intertidal zone to the abyssal zone....
s. They are gonochoristic
Gonochorism
In biology, gonochorism or unisexualism describes sexually reproducing species in which individuals have just one of at least two distinct sexes. The term is most often used with animals . The sex of an individual may change during its lifetime, this can for example be found in parrotfish...
, 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
Zygote
A zygote , or zygocyte, is the initial cell formed when two gamete cells are joined by means of sexual reproduction. In multicellular organisms, it is the earliest developmental stage of the embryo...
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.Although originally described in 1877 as a class, and sometimes characterized as an order of the phylum Mesozoa
Mesozoa
The Mesozoa are enigmatic, minuscule, worm-like parasites of marine invertebrates. It is still unclear as to whether they are degenerate platyhelminthes or truly-primitive, basal metazoans. Generally, these tiny, elusive creatures consist of a somatoderm of ciliated cells surrounding one or...
, recent study shows that orthonectids are quite different from the rhombozoa
Rhombozoa
Dicyemida, or Rhombozoa, is a phylum of tiny parasites that live in the renal appendages of cephalopods. Although the name "Dicyemida" precedes "Rhombozoa" in usage, and is preferred by most contemporary authors, "Rhombozoa" still enjoys much popular support.Classification is controversial...
ns, the other group in Mesozoa.
Known species:
Phylum Orthonectida
- Family Rhopaluridae Stunkard, 1937
- Ciliocincta akkeshiensis Tajika, 1979 – Hokkaido, Japan; in flatworms (TurbellariaTurbellariaThe Turbellaria are one of the traditional sub-divisions of the phylum Platyhelminthes , and include all the sub-groups that are not exclusively parasitic. There are about 4,500 species, which range from to in length...
) - 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 (LineusLineusLineus is a genus of nemertine worms, including the bootlace worm, arguably the longest animal alive....
) = Rhopalura linei - Intoshia major Shtein, 1953 – Arctic Ocean; in gastropods (LepetaLepetaLepeta is a genus of sea snails, the true limpets, marine gastropod mollusks in the family Lepetidae.-Species:Species within the genus Lepeta include:* Lepeta antarctica Smith, 1907* Lepeta caeca...
, NaticaNaticaNatica is a genus of small to medium-sized predatory sea snails, marine gastropods in the family Naticidae, the moon snails. The genus is known from the Cretaceous to the Recent periods.- Species :...
, SolariellaSolariellaSolariella is a genus of small sea snails, marine gastropod molluscs in the family Solariellidae.-Species:According to the World Register of Marine Species , the following species with valid names are included within the genus Solariella :...
) = Rhopalura major - Intoshia metchnikovi (Caullery & Mesnil, 1899) – E North Atlantic, in polychaetes and nemertines
- Intoshia paraphanostomae (Westblad, 1942) – E North Atlantic, in flatworms (AcoelaAcoelaAcoela is an order of animals treated either as a group of flatworms or as one of the two classes of the phylum Acoelomorpha, containing the majority of that phylum's species. It contains about 20 families....
) - Intoshia variabili (Alexandrov & Sljusarev, 1992) – Arctic Ocean, in flatworms (Macrorhynchus)
- Rhopalura elongata Shtein, 1953 – Arctic Ocean, in bivalves (AstarteAstarteAstarte is the Greek name of a goddess known throughout the Eastern Mediterranean from the Bronze Age to Classical times...
) - 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 (LepetaLepetaLepeta is a genus of sea snails, the true limpets, marine gastropod mollusks in the family Lepetidae.-Species:Species within the genus Lepeta include:* Lepeta antarctica Smith, 1907* Lepeta caeca...
, NaticaNaticaNatica is a genus of small to medium-sized predatory sea snails, marine gastropods in the family Naticidae, the moon snails. The genus is known from the Cretaceous to the Recent periods.- Species :...
, SolariellaSolariellaSolariella is a genus of small sea snails, marine gastropod molluscs in the family Solariellidae.-Species:According to the World Register of Marine Species , the following species with valid names are included within the genus Solariella :...
) - Rhopalura major Shtein, 1953
- Rhopalura murmanica Shtein, 1953 – Arctic Ocean, in gastropods (RissoaRissoaRissoa is a genus of minute sea snails, marine gastropod mollusks or micromollusks in the family Rissoidae.The genus was first introduced by M. de Freminville for some small shells observed by M. Antoine Risso, a distinguished naturalist of Nice, France. They were described by M. Demarest in 1814...
, ColumbellaColumbellaColumbella is a genus of small sea snails, marine gastropod mollusks in the family Columbellidae, the dove snails.-Species:Species within the genus Columbella include: * Columbella adansoni Menke, 1853...
) - Rhopalura ophiocomae Giard, 1877 – E North Atlantic, in ophiuroids (usually AmphipholisAmphipholisAmphipholis is a large genus of brittle stars found in oceans worldwide from tropics to Arctic and Antarctic regions. Some species have been used to study echinoderm development and bioluminescence .-Systematics:Amphipholis is a large genus with potentially many cryptic species...
) - 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 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
- Ciliocincta akkeshiensis Tajika, 1979 – Hokkaido, Japan; in flatworms (Turbellaria
- Family Pelmatosphaeridae Stunkard, 1937
- Pelmatosphaera polycirri Caullery and Mesnil, 1904 – E North Atlantic, in polychaetes and nemertines