Orthonectida (/ˌɔːrθəˈnɛktɪdə,-θoʊ-/[3]) is a smallphylum of poorly knownparasites of marine invertebrates[4] that are among the simplest of multi-cellular organisms. Members of this phylum are known asorthonectids.
The adults, which are the sexual stage, are microscopic wormlike animals, consisting of a single layer ofciliated outer cells surrounding a mass of sex cells. They swim freely within the bodies of their hosts, which includeflatworms,polychaete worms,bivalve molluscs, andechinoderms. Most aregonochoristic, with separate male and female individuals, but a few species are hermaphroditic.[5][6]
When they are ready to reproduce, adults leave the host, and sperm from the males penetrate the bodies of the females to achieveinternal fertilisation. The resultingzygote 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 asyncytialplasmodium larva. This, in turn, breaks up into numerous individual cells called agametes (ameiotic generative cells) which grow into the next generation of adults.[5][7]
The phylum consists of about 20 known species, of whichRhopalura 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,[8] and later characterized as an order of the phylumMesozoa, a 1996 study has suggested that orthonectids are quite different from therhombozoans, the other group in Mesozoa.[4] The genome of one orthonectid species,Intoshia linei, has been sequenced.[9] These animals are simplifiedspiralians. The genome data confirm earlier findings which allocated these organisms to Spiralia based on their morphology.[10]
Their position in the spiralian phylogenetic tree has yet to be determined. Some work appears to relate them to theAnnelida[11][7] and, within the Annelida, finds them most closely allied to theClitellata.[12] On the other hand, a 2022 study compensating forlong-branch attraction has recovered the traditional grouping of Orthonectida with rhombozoans in a monophyletic Mesozoa placed close toPlatyhelminthes orGnathifera.[13] This supports a previous study which found orthonectids and rhombozoans to make a monophyletic taxon Mesozoa and form a clade withRouphozoa (platyhelminths andgastrotrichs).[14]
^Bondarenko, N.; Bondarenko, A.; Starunov, V.; Slyusarev, G. (8 March 2019). "Comparative analysis of the mitochondrial genomes of Orthonectida: insights into the evolution of an invertebrate parasite species".Molecular Genetics and Genomics.294 (3):715–727.doi:10.1007/s00438-019-01543-1.PMID30848356.S2CID71716789.
