Myxosporea
Encyclopedia
The Myxosporea are a class
of microscopic parasites, belonging to the Myxozoa
. They have a complex life cycle which comprises vegetative forms in two hosts, an aquatic invertebrate
(generally an annelid
) and an ectotherm
ic vertebrate
, usually a fish
. Each host releases a different type of spore
. The two forms of spore are so different that until recently they were treated as belonging to different classes within the Myxozoa.
s composed of several cells
, contained within between 1 and 7 shell "valves". These cells include 1 or two amoeboid
infective germ cell
s, and 2 to 7 nematocyst-like polar capsule
s. During some stages of the life cycle, the germ cells are completely contained within cells of the host.
The myxosporean species are typically defined by the size and shape of the spores released by vertebrate hosts. For instance, the genus Ceratomyxa is a common parasite of the gallbladder
of many fish species; they have "boomerang-like" spores with two polar capsules resembling eyes in the middle of the spore. Most species within the myxosporea are sized between 10 µm and 20 µm, however Myxidium giganticum is up to 98 µm long.
The spore shell consists of shell "valves", which are joined together along "suture lines". Some species contain polysaccharide
reserves in the form of β-glycogen
particles, which are concentrated in a central "iodinophillous
vacuole".
The shell valves may have smooth or ridged surfaces, may be drawn out into lateral "alate" projections, and may or may not have a mucous
coating. These adaptions probably serve to increase the buoyancy of the spore in the water column, aiding dispersal. The valves are formed of resistant, non-keratinaceous
protein.
or polychaete
worm, typically resembles three or four hooks united by the base, and can also be used for identification.
These organisms were classified in the class
Actinosporea, until careful experimentation supported by analysis of the 16S Ribosomal Subunit RNA
sequence in the early 1990s allowed matching of several actinosporeans with their myxosporean equivalent.
of both actinosporeans and myxosporeans was originally based on spore morphology. Recently Kent et al. (1994) redefined the phylum Myxozoa
to solve the taxonomic and nomenclatural
problems arising from the two host life cycle of myxozoans as first described by Wolf and Markiw (1984). The distinction between the two previously recognised classes Actinosporea and Myxosporea disappeared and the class Actinosporea was suppressed, becoming a synonym
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. Actinosporeans, for which the myxosporean stage is not known, are to be retained as species inquerandae
until their specific identity is established.
failed to produce infections when fed to salmonids (Wolf & Markiw, 1984). 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. Ceratomyxa shasta
, an economically important parasite of salmonids, has been shown to use a polychaete
worm as an alternate host. Surprisingly, however, direct transmission between fish has also been demonstrated, so far in three species of Enteromyxum.
Examples of Myxosporean genera
are Kudoa, which attacks fish muscle
, and Myxobolus
, which attacks the heart
s of freshwater fishes.
Class (biology)
In biological classification, class is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, order, family, genus, and species, with class fitting between phylum and order...
of microscopic parasites, belonging to the Myxozoa
Myxozoa
The Myxozoa are a group of parasitic animals of aquatic environments. Over 1300 species have been described and many have a two-host lifecycle, involving a fish and an annelid worm or bryozoan. The average size of a Myxosporea spore usually ranges from 10 μm to 20 μm and Malacosporea up...
. They have a complex life cycle which comprises vegetative forms in two hosts, an aquatic invertebrate
Invertebrate
An invertebrate is an animal without a backbone. The group includes 97% of all animal species – all animals except those in the chordate subphylum Vertebrata .Invertebrates form a paraphyletic group...
(generally an annelid
Annelid
The annelids , formally called Annelida , are a large phylum of segmented worms, with over 17,000 modern species including ragworms, earthworms and leeches...
) and an ectotherm
Ectotherm
An ectotherm, from the Greek εκτός "outside" and θερμός "hot", refers to organisms that control body temperature through external means. As a result, organisms are dependent on environmental heat sources and have relatively low metabolic rates. For example, many reptiles regulate their body...
ic vertebrate
Vertebrate
Vertebrates are animals that are members of the subphylum Vertebrata . Vertebrates are the largest group of chordates, with currently about 58,000 species described. Vertebrates include the jawless fishes, bony fishes, sharks and rays, amphibians, reptiles, mammals, and birds...
, usually a fish
Fish
Fish are a paraphyletic group of organisms that consist of all gill-bearing aquatic vertebrate 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...
. Each host releases a different type of spore
Spore
In biology, a spore is a reproductive structure that is adapted for dispersal and surviving for extended periods of time in unfavorable conditions. Spores form part of the life cycles of many bacteria, plants, algae, fungi and some protozoa. According to scientist Dr...
. The two forms of spore are so different that until recently they were treated as belonging to different classes within the Myxozoa.
Myxosporean stage
In the vertebrate host, organisms belonging to the Myxosporea are characterised by sporeSpore
In biology, a spore is a reproductive structure that is adapted for dispersal and surviving for extended periods of time in unfavorable conditions. Spores form part of the life cycles of many bacteria, plants, algae, fungi and some protozoa. According to scientist Dr...
s composed of several cells
Cell (biology)
The cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos....
, contained within between 1 and 7 shell "valves". These cells include 1 or two amoeboid
Amoeba
Amoeba is a genus of Protozoa.History=The amoeba was first discovered by August Johann Rösel von Rosenhof in 1757. Early naturalists referred to Amoeba as the Proteus animalcule after the Greek god Proteus, who could change his shape...
infective germ cell
Germ cell
A germ cell is any biological cell that gives rise to the gametes of an organism that reproduces sexually. In many animals, the germ cells originate near the gut of an embryo and migrate to the developing gonads. There, they undergo cell division of two types, mitosis and meiosis, followed by...
s, and 2 to 7 nematocyst-like polar capsule
Polar capsule
Polar capsules are structures found in the valves of Myxosporean parasites, which contain the polar filament. The polar capsule is constructed of a proteinaceous and a polysaccharide layer, both layers of which continue into the polar filament....
s. During some stages of the life cycle, the germ cells are completely contained within cells of the host.
Gallbladder
In vertebrates the gallbladder is a small organ that aids mainly in fat digestion and concentrates bile produced by the liver. In humans the loss of the gallbladder is usually easily tolerated....
of many fish species; they have "boomerang-like" spores with two polar capsules resembling eyes in the middle of the spore. Most species within the myxosporea are sized between 10 µm and 20 µm, however Myxidium giganticum is up to 98 µm long.
The spore shell consists of shell "valves", which are joined together along "suture lines". Some species contain polysaccharide
Polysaccharide
Polysaccharides are long carbohydrate molecules, of repeated monomer units joined together by glycosidic bonds. They range in structure from linear to highly branched. Polysaccharides are often quite heterogeneous, containing slight modifications of the repeating unit. Depending on the structure,...
reserves in the form of β-glycogen
Glycogen
Glycogen is a molecule that serves as the secondary long-term energy storage in animal and fungal cells, with the primary energy stores being held in adipose tissue...
particles, which are concentrated in a central "iodinophillous
Iodine
Iodine is a chemical element with the symbol I and atomic number 53. The name is pronounced , , or . The name is from the , meaning violet or purple, due to the color of elemental iodine vapor....
vacuole".
The shell valves may have smooth or ridged surfaces, may be drawn out into lateral "alate" projections, and may or may not have a mucous
Mucus
In vertebrates, mucus is a slippery secretion produced by, and covering, mucous membranes. Mucous fluid is typically produced from mucous cells found in mucous glands. Mucous cells secrete products that are rich in glycoproteins and water. Mucous fluid may also originate from mixed glands, which...
coating. These adaptions probably serve to increase the buoyancy of the spore in the water column, aiding dispersal. The valves are formed of resistant, non-keratinaceous
Keratin
Keratin refers to a family of fibrous structural proteins. Keratin is the key of structural material making up the outer layer of human skin. It is also the key structural component of hair and nails...
protein.
Actinosporean stage
The alternate stage of the life cycle is generally released by an annelidAnnelid
The annelids , formally called Annelida , are a large phylum of segmented worms, with over 17,000 modern species including ragworms, earthworms and leeches...
or 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...
worm, typically resembles three or four hooks united by the base, and can also be used for identification.
These organisms were classified in the class
Class (biology)
In biological classification, class is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, order, family, genus, and species, with class fitting between phylum and order...
Actinosporea, until careful experimentation supported by analysis of the 16S Ribosomal Subunit RNA
Ribosome
A ribosome is a component of cells that assembles the twenty specific amino acid molecules to form the particular protein molecule determined by the nucleotide sequence of an RNA molecule....
sequence in the early 1990s allowed matching of several actinosporeans with their myxosporean equivalent.
Taxonomic status
The taxonomyTaxonomy
Taxonomy is the science of identifying and naming species, and arranging them into a classification. The field of taxonomy, sometimes referred to as "biological taxonomy", revolves around the description and use of taxonomic units, known as taxa...
of both actinosporeans and myxosporeans was originally based on spore morphology. Recently Kent et al. (1994) redefined the phylum Myxozoa
Myxozoa
The Myxozoa are a group of parasitic animals of aquatic environments. Over 1300 species have been described and many have a two-host lifecycle, involving a fish and an annelid worm or bryozoan. The average size of a Myxosporea spore usually ranges from 10 μm to 20 μm and Malacosporea up...
to solve the taxonomic and nomenclatural
Binomial nomenclature
Binomial nomenclature is a formal system of naming species of living things by giving each a name composed of two parts, both of which use Latin grammatical forms, although they can be based on words from other languages...
problems arising from the two host life cycle of myxozoans as first described by Wolf and Markiw (1984). The distinction between the two previously recognised classes Actinosporea and Myxosporea disappeared and the class Actinosporea was suppressed, becoming a synonym
Synonym
Synonyms are different words with almost identical or similar meanings. Words that are synonyms are said to be synonymous, and the state of being a synonym is called synonymy. The word comes from Ancient Greek syn and onoma . The words car and automobile are synonyms...
of the class Myxosporea Bütschli, 1881. The generic
Genus
In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...
names of actinosporeans were retained as collective "type" names, and it was proposed that they be used to characterise different morphological forms of actinosporeans. Actinosporeans, for which the myxosporean stage is not known, are to be retained as species inquerandae
Incertae sedis
, is a term used to define a taxonomic group where its broader relationships are unknown or undefined. Uncertainty at specific taxonomic levels is attributed by , , and similar terms.-Examples:*The fossil plant Paradinandra suecica could not be assigned to any...
until their specific identity is established.
Transmission
Until the 1980s, direct transmission of myxosporeans was presumed. In 1984, it was shown experimentally that spores of Myxobolus cerebralisMyxobolus cerebralis
Myxobolus cerebralis is a myxosporean parasite of salmonids that causes whirling disease in farmed salmon and trout and also in wild fish populations...
failed to produce infections when fed to salmonids (Wolf & Markiw, 1984). 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. Ceratomyxa shasta
Ceratomyxa shasta
Ceratomyxa shasta is a myxosporean parasite that infects salmonid fish on the Pacific coast of North America. It was first observed at the Crystal Lake Hatchery, Shasta County, California, and has now been reported from Idaho, Oregon, Washington, British Columbia and Alaska .-Life history:In...
, an economically important parasite of salmonids, has been shown to use a 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...
worm as an alternate host. Surprisingly, however, direct transmission between fish has also been demonstrated, so far in three species of Enteromyxum.
Examples of Myxosporean genera
Genus
In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...
are Kudoa, which attacks fish muscle
Muscle
Muscle is a contractile tissue of animals and is derived from the mesodermal layer of embryonic germ cells. Muscle cells contain contractile filaments that move past each other and change the size of the cell. They are classified as skeletal, cardiac, or smooth muscles. Their function is to...
, and Myxobolus
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 to be different organisms entirely, but are now united in this group....
, which attacks the heart
Heart
The heart is a myogenic muscular organ found in all animals with a circulatory system , that is responsible for pumping blood throughout the blood vessels by repeated, rhythmic contractions...
s of freshwater fishes.
Cited literature
- Bartholomew, J. L., M. J. Whipple, D. G. Stevens and J. L. Fryer. (1997). The life cycle of Ceratomyxa shasta, a myxosporean parasite of salmonids, requires a freshwater polychaete as an alternate host. American Journal of Parasitology. 83:859-868.
- Wolf, K. & Markiw, M.E. (1984). Biology contravenes taxonomy in the Myxozoa: new discoveries show alternation of invertebrate and vertebrate hosts. Science 225:1449-1452
- Kent, M. L., Margolis. L., & Corliss, J. 0. (1994). The demise of a class of protists: Taxonomic and nomenclatural revisions proposed for the protist phylum Myxozoa Grasse, 1970. Canadian Journal of Zoology 72:932-937.