Myxozoa
Encyclopedia
The Myxozoa are a group of parasitic animal
s of aquatic environments. Over 1300 species have been described and many have a two-host lifecycle, involving a fish
and an annelid
worm or bryozoa
n. The average size of a Myxosporea spore usually ranges from 10 μm to 20 μm and Malacosporea up to 2 mm. Infection occurs through valved spores. These contain one or two sporoblast cells and one or more polar capsules that contain filaments which anchor the spore to its host. The sporoblasts are then released as a motile form, called an amoebula, which penetrates the host tissues and develops into one or more multinucleate
plasmodia. Certain nuclei later pair up, one engulfing another, to form new spores.
n, and were included among other non-motile forms in the group Sporozoa. As their distinct nature became clear through 18S ribosomal DNA
(rDNA) sequencing, they were relocated in the metazoa. Further classification was hindered by conflicting evidence: although 18S rDNA suggested an affinity with Cnidaria
, other rDNA sampled, and the HOX genes of two species, were more similar to those of the Bilateria
.
The discovery that Buddenbrockia plumatellae
, a worm-like parasite up to 2 mm in length, is a myxozoan appeared to strengthen the case for a bilaterian origin, as the body plan is superficially similar. Nevertheless, closer examination reveals that Buddenbrockia is not longitudinally symmetrical by two ways, but four, casting doubt on this hypothesis
.
Further testing has sourced the first three HOX genes found in previous research (Myx1-3) to the bryozoan Cristatella mucedo, and the fourth (Myx4) to Northern pike
. This explained the confusion; original experiments had used contaminated tissue from host organisms, leading to false positives for a position among the Bilateria. More careful cloning of 50 coding genes from Buddenbrockia established the clade as severely modified members of the phylum Cnidaria
, with medusozoans as their closest relatives. Similarities between myxozoan polar capsules and cnidarian nematocysts (stinging cells) had been drawn for a long time, but were generally assumed to be the result of convergent evolution
(Still these are insufficient information).
Taxonomists now recognize the outdated subgroup Actinosporea as a life-cycle
phase of Myxosporea
.
hosts can be extremely long-lasting, potentially persisting for the lifetime of the host, however, an increasing number of myxosporeans have been recognised as commercially important pathogens of fish, largely as a result of the recent increase in aquaculture
.
The economic impact of such parasites can be severe, especially where prevalence
rates are high; they may also have a severe impact on wild fish stocks
The most significant diseases worldwide caused by myxosporeas in cultured fishes are PKD-Proliferative Kidney Disease, caused by a Malacosporea member,Tetracapsuloides bryosalmonae
, and whirling disease, caused by a Myxosporea
member Myxobolus cerebralis
; both diseases affect salmonids.
Furthermore, Enteromyxosis is caused by Enteromyxum leei in cultured marine sparids, while "Hamburger disease” or Proliferative Gill Disease is caused by Henneguya ictaluri in catfish
and Sphaerosphora renicola infections occur in common carp
.
Animal
Animals are a major group of multicellular, eukaryotic organisms of the kingdom Animalia or Metazoa. Their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their life. Most animals are motile, meaning they can move spontaneously and...
s of aquatic environments. Over 1300 species have been described and many have a two-host lifecycle, involving 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...
and 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...
worm or bryozoa
Bryozoa
The Bryozoa, also known as Ectoprocta or commonly as moss animals, are a phylum of aquatic invertebrate animals. Typically about long, they are filter feeders that sieve food particles out of the water using a retractable lophophore, a "crown" of tentacles lined with cilia...
n. The average size of a Myxosporea spore usually ranges from 10 μm to 20 μm and Malacosporea up to 2 mm. Infection occurs through valved spores. These contain one or two sporoblast cells and one or more polar capsules that contain filaments which anchor the spore to its host. The sporoblasts are then released as a motile form, called an amoebula, which penetrates the host tissues and develops into one or more multinucleate
Multinucleate
Multinucleate cells have more than one nucleus per cell, which is the result of nuclear division not being followed by cytokinesis. As a consequence, multiple nuclei share one common cytoplasm. This can be the consequence of a disturbed cell cycle control Multinucleate (also multinucleated,...
plasmodia. Certain nuclei later pair up, one engulfing another, to form new spores.
Phylogenetics
The Myxozoa were originally considered protozoaProtozoa
Protozoa are a diverse group of single-cells eukaryotic organisms, many of which are motile. Throughout history, protozoa have been defined as single-cell protists with animal-like behavior, e.g., movement...
n, and were included among other non-motile forms in the group Sporozoa. As their distinct nature became clear through 18S ribosomal DNA
DNA
Deoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms . The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in...
(rDNA) sequencing, they were relocated in the metazoa. Further classification was hindered by conflicting evidence: although 18S rDNA suggested an affinity with Cnidaria
Cnidaria
Cnidaria is a phylum containing over 9,000 species of animals found exclusively in aquatic and mostly marine environments. Their distinguishing feature is cnidocytes, specialized cells that they use mainly for capturing prey. Their bodies consist of mesoglea, a non-living jelly-like substance,...
, other rDNA sampled, and the HOX genes of two species, were more similar to those of the Bilateria
Bilateria
The bilateria are all animals having a bilateral symmetry, i.e. they have a front and a back end, as well as an upside and downside. Radially symmetrical animals like jellyfish have a topside and downside, but no front and back...
.
The discovery that Buddenbrockia plumatellae
Buddenbrockia plumatellae
Buddenbrokia plumatellae is a parasitic worm whose taxonomic placement long puzzled biologists. It is now classified as one of the only two myxozoans of class Malacosporea on the basis of both genetic and ultrastructural studies...
, a worm-like parasite up to 2 mm in length, is a myxozoan appeared to strengthen the case for a bilaterian origin, as the body plan is superficially similar. Nevertheless, closer examination reveals that Buddenbrockia is not longitudinally symmetrical by two ways, but four, casting doubt on this hypothesis
Hypothesis
A hypothesis is a proposed explanation for a phenomenon. The term derives from the Greek, ὑποτιθέναι – hypotithenai meaning "to put under" or "to suppose". For a hypothesis to be put forward as a scientific hypothesis, the scientific method requires that one can test it...
.
Further testing has sourced the first three HOX genes found in previous research (Myx1-3) to the bryozoan Cristatella mucedo, and the fourth (Myx4) to Northern pike
Northern Pike
The northern pike , is a species of carnivorous fish of the genus Esox...
. This explained the confusion; original experiments had used contaminated tissue from host organisms, leading to false positives for a position among the Bilateria. More careful cloning of 50 coding genes from Buddenbrockia established the clade as severely modified members of the phylum Cnidaria
Cnidaria
Cnidaria is a phylum containing over 9,000 species of animals found exclusively in aquatic and mostly marine environments. Their distinguishing feature is cnidocytes, specialized cells that they use mainly for capturing prey. Their bodies consist of mesoglea, a non-living jelly-like substance,...
, with medusozoans as their closest relatives. Similarities between myxozoan polar capsules and cnidarian nematocysts (stinging cells) had been drawn for a long time, but were generally assumed to be the result of convergent evolution
Convergent evolution
Convergent evolution describes the acquisition of the same biological trait in unrelated lineages.The wing is a classic example of convergent evolution in action. Although their last common ancestor did not have wings, both birds and bats do, and are capable of powered flight. The wings are...
(Still these are insufficient information).
Taxonomists now recognize the outdated subgroup Actinosporea as a life-cycle
Biological life cycle
A life cycle is a period involving all different generations of a species succeeding each other through means of reproduction, whether through asexual reproduction or sexual reproduction...
phase of Myxosporea
Myxosporea
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 and an ectothermic vertebrate, usually a fish. Each host releases a different type of spore...
.
Species
Some species of myxozoa include:- Class Malacosporea
- Buddenbrockia plumatellaeBuddenbrockia plumatellaeBuddenbrokia plumatellae is a parasitic worm whose taxonomic placement long puzzled biologists. It is now classified as one of the only two myxozoans of class Malacosporea on the basis of both genetic and ultrastructural studies...
- Tetracapsuloides bryosalmonaeTetracapsuloides bryosalmonaeTetracapsuloides bryosalmonae is a myxozoan parasite of salmonid fishes, which causes Proliferative kidney disease , one of the most serious parasitic diseases of salmonid populations in Europe and North America, which causes losses of up to 90% in infected populations.-Taxonomy:Until the late...
- an important salmonSalmonSalmon is the common name for several species of fish in the family Salmonidae. Several other fish in the same family are called trout; the difference is often said to be that salmon migrate and trout are resident, but this distinction does not strictly hold true...
parasite.
- Buddenbrockia plumatellae
- Class MyxosporeaMyxosporeaThe 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 and an ectothermic vertebrate, usually a fish. Each host releases a different type of spore...
- Myxobolus cerebralisMyxobolus cerebralisMyxobolus cerebralis is a myxosporean parasite of salmonids that causes whirling disease in farmed salmon and trout and also in wild fish populations...
- an important parasite of salmon and troutTroutTrout is the name for a number of species of freshwater and saltwater fish belonging to the Salmoninae subfamily of the family Salmonidae. Salmon belong to the same family as trout. Most salmon species spend almost all their lives in salt water...
.
- Myxobolus cerebralis
Pathology
Relationships between myxosporeans and their hosts are often highly evolved and do not usually result in severe diseases. Infection in fishFish
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...
hosts can be extremely long-lasting, potentially persisting for the lifetime of the host, however, an increasing number of myxosporeans have been recognised as commercially important pathogens of fish, largely as a result of the recent increase in aquaculture
Aquaculture
Aquaculture, also known as aquafarming, is the farming of aquatic organisms such as fish, crustaceans, molluscs and aquatic plants. Aquaculture involves cultivating freshwater and saltwater populations under controlled conditions, and can be contrasted with commercial fishing, which is the...
.
The economic impact of such parasites can be severe, especially where prevalence
Prevalence
In epidemiology, the prevalence of a health-related state in a statistical population is defined as the total number of cases of the risk factor in the population at a given time, or the total number of cases in the population, divided by the number of individuals in the population...
rates are high; they may also have a severe impact on wild fish stocks
The most significant diseases worldwide caused by myxosporeas in cultured fishes are PKD-Proliferative Kidney Disease, caused by a Malacosporea member,Tetracapsuloides bryosalmonae
Tetracapsuloides bryosalmonae
Tetracapsuloides bryosalmonae is a myxozoan parasite of salmonid fishes, which causes Proliferative kidney disease , one of the most serious parasitic diseases of salmonid populations in Europe and North America, which causes losses of up to 90% in infected populations.-Taxonomy:Until the late...
, and whirling disease, caused by a Myxosporea
Myxosporea
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 and an ectothermic vertebrate, usually a fish. Each host releases a different type of spore...
member Myxobolus cerebralis
Myxobolus cerebralis
Myxobolus cerebralis is a myxosporean parasite of salmonids that causes whirling disease in farmed salmon and trout and also in wild fish populations...
; both diseases affect salmonids.
Furthermore, Enteromyxosis is caused by Enteromyxum leei in cultured marine sparids, while "Hamburger disease” or Proliferative Gill Disease is caused by Henneguya ictaluri in catfish
Catfish
Catfishes are a diverse group of ray-finned fish. Named for their prominent barbels, which resemble a cat's whiskers, catfish range in size and behavior from the heaviest and longest, the Mekong giant catfish from Southeast Asia and the second longest, the wels catfish of Eurasia, to detritivores...
and Sphaerosphora renicola infections occur in common carp
Common carp
The Common carp is a widespread freshwater fish of eutrophic waters in lakes and large rivers in Europe and Asia. The wild populations are considered vulnerable to extinction, but the species has also been domesticated and introduced into environments worldwide, and is often considered an invasive...
.