parasites. It contains the smallest animals ever known to have lived. Over 2,180 species have been described and some estimates have suggested at least 30,000 undiscovered species.[3] Many have a two-host lifecycle, involving a fish and an annelid worm or a bryozoan. The average size of a myxosporeanspore usually ranges from 10 μm to 20 μm,[4]
whereas that of a malacosporean (a subclade of the Myxozoa) spore can be up to 2 mm. Myxozoans can live in both freshwater and marine habitats.
Myxozoans are highly
aerobic respiration. The genomes of some myxozoans are now among the smallest genomes of any known animal species.[5][6]
Life cycle and pathology
Myxozoans are endoparasitic animals exhibiting complex life cycles that, in most of the documented cases, involve an
ectoproct
.
Myxozoans Life Cycle
Only about 100 life cycles have been resolved and it is suspected that there may be some exclusively terrestrial.
sporoplasms to penetrate into the epithelium. Subsequently, the parasite undergoes reproduction and development in the gut tissue, and finally produces usually eight actinosporean spore stages (actinospores) within a pansporocyst. After mature actinospores are released from their hosts they float in the water column.[13] Upon contact with skin or gills of fish, sporoplasms penetrate through the epithelium, followed by development of the myxosporean stage. Myxosporean trophozoites are characterized by cell-in-cell state, where the secondary (daughter) cells develop in the mother (primary) cells. The presporogonic stages multiply, migrate via nervous or circulatory systems, and develop into sporogonic stages. At the final site of infection, they produce mature spores within mono- or di-sporic pseudoplasmodia, or poly-sporic plasmodia.[14]
Relationships between myxosporeans and their hosts are often highly evolved and do not usually result in severe diseases of the natural host. Infection in
parasites can be severe, especially where prevalence
rates are high; they may also have a severe impact on wild fish stocks.
The diseases caused by myxosporeas in cultured fish with the most significant economic impact worldwide are
common carp
.
Anatomy
Myxozoans are very small animals, typically 10–300
Like other cnidarians they possess cnidocysts, which were referred to as "polar capsules" before the discovery that myxozoans are cnidarians. These cnidocysts fire tubules as in other cnidarians; some inject substances into the host. However, the tubules lack hooks or barbs, and in some species are more elastic than in other cnidarians.
Myxozoans have secondarily lost
binary fission is rare, and cells divide instead via endogeny.[15]
In 2020, the myxozoan
aerobic respiration; it was the first animal to be positively identified as such. Its actual metabolism is currently unknown.[17]
Phylogenetics
Myxozoans were originally considered to be
metazoa. Detailed classification within the metazoa was however long hindered by conflicting rDNA evidence: although 18S rDNA suggested an affinity with Cnidaria,[20] other rDNA sampled,[21][22] and the HOX genes of two species,[23] were more similar to those of the Bilateria
.
The discovery that
bryozoans up to 2 mm in length, is a myxozoan[21] initially appeared to strengthen the case for a bilaterian origin, as the body plan is superficially similar. Nevertheless, closer examination reveals that Buddenbrockia's longitudinal symmetry is not twofold, but fourfold, casting doubt on this hypothesis
.
Further testing resolved the genetic conundrum by sourcing the first three previously identified discrepant HOX genes (Myx1-3) to the
nematocysts had been drawn for a long time, but were generally assumed to be the result of convergent evolution
.
Taxonomists now recognize the outdated subgroup Actinosporea as a life-cycle phase of Myxosporea.[25]
Molecular clocks suggest that myxozoans and their closest relatives, the
polypodiozoa, shared their last common ancestor with medusazoans about 600 million years ago, during the Ediacaran period.[3]
Taxonomy
Myxozoan taxonomy has undergone great and important changes in its levels of generic, family and suborder classification. Fiala et al. (2015) proposed a new classification based on spores.[26]
^
J. Zrzavy & V. Hypsa (April 2003). "Myxozoa, Polypodium, and the origin of the Bilateria: The phylogenetic position of "Endocnidozoa" in light of the rediscovery of Buddenbrockia". Cladistics. 19 (2): 164–169.
