Orthonectida (/ˌɔːrθəˈnɛktɪdə,-θoʊ-/[3]) is a small
phylum of poorly known
parasites of marine invertebrates[4] that are among the simplest of multi-cellular organisms. Members of this phylum are known as orthonectids.
Biology
The adults, which are the sexual stage, 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. Most are
gonochoristic, 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 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
syncytialplasmodium 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]
Classification
The phylum consists of about 20 known species, of which Rhopalura 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 phylum
Mesozoa, a 1996 study has suggested that orthonectids are quite different from the
rhombozoans, the other group in Mesozoa.[4] The genome of one orthonectid species, Intoshia linei, has been sequenced.[9] These animals are simplified
spiralians. 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 the
Annelida[11][7] and, within the Annelida, finds them most closely allied to the
Clitellata.[12] On the other hand, a 2022 study compensating for
long-branch attraction has recovered the traditional grouping of Orthonectida with rhombozoans in a monophyletic Mesozoa placed close to
Platyhelminthes or
Gnathifera.[13] This supports a previous study which found orthonectids and rhombozoans to make a monophyletic taxon Mesozoa and form a clade with
Rouphozoa (platyhelminths and
gastrotrichs).[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.