Myxosporea is a class of microscopic

parasite Parasitism is a Symbiosis, close relationship between species, where one organism, the parasite, lives on or inside another organism, the Host (biology), host, causing it some harm, and is Adaptation, adapted structurally to this way of lif ...

s, belonging to the

Myxozoa Myxozoa (etymology: Greek: μύξα ''myxa'' "slime" or "mucus" + thematic vowel o + ζῷον ''zoon'' "animal") is a subphylum of aquatic cnidarian animals – all obligate parasites. It contains the smallest animals ever known to have lived. O ...

clade within

Cnidaria Cnidaria () is a phylum under kingdom Animalia containing over 11,000 species of aquatic animals found both in Fresh water, freshwater and Marine habitats, marine environments, predominantly the latter. Their distinguishing feature is cnidocyt ...

. They have a complex life cycle which comprises vegetative forms in two hosts, an aquatic

invertebrate Invertebrates are a paraphyletic group of animals that neither possess nor develop a vertebral column (commonly known as a ''backbone'' or ''spine''), derived from the notochord. This is a grouping including all animals apart from the chordate ...

(generally an

annelid The annelids (Annelida , from Latin ', "little ring"), also known as the segmented worms, are a large phylum, with over 22,000 extant species including ragworms, earthworms, and leeches. The species exist in and have adapted to various ecol ...

but sometimes a

bryozoa Bryozoa (also known as the Polyzoa, Ectoprocta or commonly as moss animals) are a phylum of simple, aquatic invertebrate animals, nearly all living in sedentary colonies. Typically about long, they have a special feeding structure called a ...

n) and an

ectotherm An ectotherm (from the Greek () "outside" and () "heat") is an organism in which internal physiological sources of heat are of relatively small or of quite negligible importance in controlling body temperature.Davenport, John. Animal Life a ...

vertebrate Vertebrates () comprise all animal taxon, taxa within the subphylum Vertebrata () (chordates with vertebral column, backbones), including all mammals, birds, reptiles, amphibians, and fish. Vertebrates represent the overwhelming majority of the ...

, usually a

fish Fish are aquatic, craniate, gill-bearing animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish as well as various extinct related groups. Approximately 95% ...

. Each host releases a different type of

spore In biology, a spore is a unit of sexual or asexual reproduction that may be adapted for dispersal and for survival, often for extended periods of time, in unfavourable conditions. Spores form part of the life cycles of many plants, algae, ...

. The two forms of spore are so different that until relatively recently they were treated as belonging to different classes within the Myxozoa.

Taxonomic status

The

taxonomy Taxonomy is the practice and science of categorization or classification. A taxonomy (or taxonomical classification) is a scheme of classification, especially a hierarchical classification, in which things are organized into groups or types. ...

of both actinosporeans and myxosporeans was originally based on spore morphology. In 1994 the phylum

was redefined to solve the taxonomic and

nomenclatural Nomenclature (, ) is a system of names or terms, or the rules for forming these terms in a particular field of arts or sciences. The principles of naming vary from the relatively informal conventions of everyday speech to the internationally agree ...

problems arising from the two-host life cycle of myxozoans. The distinction between the two previously recognised classes Actinosporea and Myxosporea disappeared and the class ''Actinosporea'' was suppressed, becoming a

synonym A synonym is a word, morpheme, or phrase that means exactly or nearly the same as another word, morpheme, or phrase in a given language. For example, in the English language, the words ''begin'', ''start'', ''commence'', and ''initiate'' are al ...

of the class ''Myxosporea'' (Bütschli, 1881). The generic names of actinosporeans were retained as collective "type" names, and it was proposed that they be used to characterise different morphological forms of actinosporeans. Those remaining actinosporeans whose myxosporean stage is unattested are being retained as species inquerandae until their specific identity is established. It has been hypothesized that myxosporeans might have evolved from a transmissible tumor of ''Polypodium''. This hypothesis is called the "SCANDAL hypothesis", an acronym for ''speciation by cancer development animals'', referencing its radical nature.

Transmission

Until the 1980s, direct transmission of myxosporeans was presumed. In 1984, it was shown experimentally that spores of '' Myxobolus cerebralis'' failed to produce infections when fed to

salmonid Salmonidae is a family of ray-finned fish that constitutes the only currently extant family in the order Salmoniformes . It includes salmon (both Atlantic and Pacific species), trout (both ocean-going and landlocked), chars, freshwater whitefis ...

s. To reproduce successfully, this species requires a tubificid worm as an intermediate host, in which the spores develop into a "species" of the "genus" ''Triactinomyxon''. These spores develop inside the oligochaete into forms which are infective to salmonids. Such a life cycle, with two different sexual stages, resulting in two kinds of resistant spores, is unique amongst the parasitic organisms. This mode of life cycle has been confirmed in several other ''Myxobolus'' species. This mode of infection has also been proved in other families. '' Ceratonova shasta'', an economically important parasite of salmonids, has been shown to use a

polychaete Polychaeta () is a paraphyletic class of generally marine annelid worms, commonly called bristle worms or polychaetes (). Each body segment has a pair of fleshy protrusions called parapodia that bear many bristles, called chaetae, which are ...

worm as an alternate host. Direct transmission between fish has also been demonstrated, so far in three species of ''Enteromyxum''. Examples of Myxosporean

genera Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and below family. In binomial ...

are '' Kudoa'', which attacks fish

muscle Skeletal muscles (commonly referred to as muscles) are Organ (biology), organs of the vertebrate muscular system and typically are attached by tendons to bones of a skeleton. The muscle cells of skeletal muscles are much longer than in the other ...

, and ''

Myxobolus ''Myxobolus'' is a genus of myxozoa that includes important parasites of fish like ''Myxobolus cerebralis''. The genus is polyphyletic, with members scattered throughout the myxozoa. Some stages of ''Myxobolus'' species were previously thought t ...

,'' which attacks the

heart The heart is a muscular organ found in most animals. This organ pumps blood through the blood vessels of the circulatory system. The pumped blood carries oxygen and nutrients to the body, while carrying metabolic waste such as carbon diox ...

s of freshwater fishes.

References

{{Taxonbar, from=Q275657 Myxozoa