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The phylum Myxozoa falls under the domain Eukarya and were traditionally thought of as protozoa. Within the last two decades however more advanced identifying techniques have led us to believe that they are in fact metazoan's more closely related to Cnidarians, but this will be discussed later in this essay. There are approximately twelve thousand species of Myxozoa, and it is believed that there may be more than this number. Most Myxozoa are parasites which inhabit primarily tissues and organ cavities of ectothermic vertebrates, especially fish, and can cause major problems within fish farms and hatcheries. Myxozoans are extra cellular parasites of ectothermic vertebrates and invertebrates. They mainly infect fish especially salmonoid's, but do also infect some amphibians, reptiles and annelids.
In fish they are found in cavities such as gall bladder, urinary tract, ureters or tissues including cartilage, muscle gills and skin. In Annelids they can be found in the intestinal epithelium. Myxozoans have multicellular spores which contain two or more shell valves which join at a sutra plane, a sporoplasm that is infective to the host, and polar capsules containing polar filaments coiled within. Once ingested these polar filaments are then be expelled and it is thought that they are used as a means of attachment to the host. The most well known species of Myxozoa is the Myxobolus cereals due to its affect on salmonoid's farms and hatcheries as it is responsible foe significant losses of stock. It affects the cartilage and nervous systems of many salmonoid's worldwide.
Nearly all Myxozoa have a typical lifecycle common amongst most organisms of the phylum normally involving two host species. The life cycle of Myxobolus cereals ids the best example as it is the most known about and most others follow essentially the same pattern. Spores released by the host species (salmonoid's usually) are ingested by a tubificid oligochaete. The polar filaments within the polar capsules of the spore are then everted and the spores release the infective sporoplasma.
During development within the oligochaete a form of sexual reproduction occurs along with the production of triactinomyxon spores. These new spores are released into the environment along with the host faeces. These spores were until recently believed to be a different species. When the next host, a salmonoid's fish then ingests these spores the polar capsules again evert and the infective sporoplasm emerges once again and migrates through the blood to the infection site. Once In the tissue it begins to grow and its nucleus divides many times to produce a multi nucleate mass (trophozoite). This mass feeds off of the surrounding tissues.
As this development continues generative nuclei within the trophozoite become surrounded by cytoplasm forming a unique pansporoblast which will produce the spores. The process by which these spores reach the water to gain access to the invertebrate host, but it is thought that when the salmonoid's is ingested by a predator the spores are released along with faeces following digestion. It is also thought that once the salmonoid dies, decomposition releases the spores. The effect of these parasites on the host can be varied but usually affect major tissues and the nervous system. Myxobolus cereals is the cause of a disease called whirling disease in young salmon.
It causes them to have un calcified skeletons or the cartilage is destroyed or consumed by the trophozoites developing within them. When this occurs it interferes with coordination and the equilibrium of the fish causing it to lose its sense of balance making it tumble erratically when it tries to feed (whirling). As the disease progresses, the parasite invades the vertebral column between the anus and the tail forcing the tail and trunk to curve, simultaneously the sympathetic nerves controlling melanocytes in the region are impaired causing a condition called Black tail. This is due to a permanently dark colouring of the caudal region. Myxozoans also cause a disease called PKD. This affects the spleen and kidneys of the infected organism causing the immune cells to multiply out of control, which ultimately destroys these organs leading to anaemia, discoloration, bloating and eventually death.
Very few survive to adult hood once infected, due to severe deformities and reduced coordination leaves them severely handicapped when it comes to avoiding predators. They have been known to wipe out whole stocks of salmon or trout in farms and hatcheries, which make them extremely important to these businesses with respect to finding a way to control or even eradicate them. The control methods being looked into at the moment are based on interruption of the lifecycles to reduce infection. One way is to focus upon the alternative host of the parasite.
It seems that some species of Tubifex are resistant. If these resistant species could be made to replace the susceptible species then the Myxozoans life cycle would be interrupted reducing cases of infection. Another method is based upon the temperature dependant release of infective spores. It has been found that at 15 C tubificids release huge numbers of spores but at 5 C very few spores are released.
So if by natural selection they can force the salmon to spawn early (usually spawn in the warmer month of may) the synchrony of hatching and spore release would be disrupted. The big question surround Myxozoa is to where they belong within the taxonomic hierarchy. Conventionally they were believed to be protozoans, but recent studies have suggested that they are infact metazoan's due to the work produced by Siddal et al in 1995. These showed that their spores are multicellular, and that cell junctions hold some of the cells together.
These characteristics are found in all other metazoan's as well as the fact that they also produce collagen, a protein produced only by metazoan's. There has also been speculation of an affinity between Myxozoa and Cnidarians (Siddal et al. , 1995). Although molecular analysis does not show this affinity (Katyama et al. , 1995, Sch legal et al. , 1996). The Myxozoa is a very interesting and topical subject at the moment, which is very important within the fish farming community. The question of where its place lies within the taxonomic hierarchy will no doubt be solved very soon, and its control as a pest Im sure will be resolved also.
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Parasitism: The diversity and ecology of animal parasites. Canning, E. U. , Okamura, B. & Curry, A. 1996. Development of a myxozoa parasite Tetracapsula bryozoides n. g. , n. sp.
in the body cavity of Cristatella macedon (Bryozoa, Phylactolaemata). Folia Parasitologica 43: 249 - 261. Dossou, C. Kate, G.
Marques, A. Sakiti, N. Gill Diseases Caused by Myxozoa in Fishes of Inland Waters of the South of Benin. 36 th Annual Meeting, May 21 - 25, 1998. Katayama, T. , Wada, H. , Satoh, N. & Yamamoto, M. 1995. Phylogenetic position of dicyemid mesa inferred from 18 s rDNA sequences.
Biological bulletin, 189, 81 - 90. Noble, E. R, Noble, G. A. 1976.
Parasitology: The biology of animal parasites. Siddal, M. E. , Martin, D. S. , Bridge, D. , Cone, D.
M. , Dealer, S. S. 1995. The demise of a phylum of protists: Myxozoa and other parasitic Cnidarians. Journal of Parasitology 81: 961 - 967. Patterson, D. J. , Hedley, S. 1992.
Free living freshwater protozoa: A colour guide. Poindexster, J. S. 1971. Microbiology: An introduction to protests. Ragan, M. A. , Chapman, D.
J. 1978 A biochemical phylogeny of the protists. Sleigh, M. 1989. Protozoa and other protists. web web web anderson.
html web lab/Eukarya/Protest/Myxozoa
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