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Phylum Platyhelminthes.
Class Monogenea
The taxonomy is still controversial but, at present the Monogenea are described as a Class.
The monogeneans are one of the totally parasitic groups of platyhelminths and have typical platyhelminth features: dorso-ventrally flattened, acoelomate, bilaterally symmetrical, protonephridial excretory system (flame cells), no definite anus, no respiratory or circulatory system and are usually hermaphrodite.
The monogeneans live as ectoparasites on the gills or general body surface of marine and freshwater fish, with a few species occurring on amphibians and reptiles. One species is known from mammals, Oculotrema hippopotami from the eye of the hippopotamus.
Some species of monogenean have become, to a certain extent, endoparasitic invading such sites as the buccal cavity, cloaca and bladder (mesoparasites). The most striking feature of the monogeneans is the large posterior sucker called a haptor, which is armed with a series of hooks. There are 10 to 14 marginal hooks and a pair of large median hooks called hamuli (sing. hamulus). In many gill parasites the haptor is adapted to holding on to the gill lamellae and is divided into a number of sucker-like organs. The suckers are strengthened with sclerites and clamp round the gill filaments.
Body Plan.
The tegument of monogeneans is like that of cestodes and digeneans, consisting of a syncitial surface layer, the cell bodies of which are sunken into the parenchyma and remain connected to the tegument via cytoplasmic bridges. The surface of the tegument is covered with short, scattered microvilli. Musculature is provided by outer circular, oblique and inner longitudinal muscle fibres.
The digestive system of monogeneans consists of a mouth, pharynx and bifurcate intestine (there is no anus). The two main branches of the intestine are often linked by an intercaecal network. Nutrients can also be taken up via the tegument, but it is not known if this has any physiological significance. Monogeneans do not move about on their hosts to any great extent, nor do the adults transfer from one host to another. So the adults often inhabit very specific microhabitats on their host. Species living on the skin and some gill forms feed on mucous, most gill parasites feed on blood, which they ingest after rupturing the brachial capillaries.
The hermaphrodite reproductive system of monogeneans is fairly typical of other platyhelminths with vitellaria, ovary, ootype + Mehlis' gland and uterus to store eggs. The male system usually has a single testis, seminal vesicle and muscular penis. As in the Digenea there may be extra openings into the female reproductive system from the outside. Monogeneans often have a pair of vaginas opening on either side; also, a number of monogeneans have a connection between the reproductive system and the gut - the genito-intestinal canal.
LIFE-CYCLE.
The development of monogeneans is direct, i.e., there is no intermediate host such as a snail. "Monogenea" means one generation as one egg gives rise to one adults worm. Eggs are passed out into water where they hatch. The eggs of monogeneans are often remarkably large and are frequently equipped with long filaments, which are extensions of the egg envelope. Some species use the filaments to stick the eggs on to the skin of fish. In others, the eggs form a mass which may float or sink and get caught in fish respiratory currents. The hatched egg is called an oncomiracidium. This is a ciliated larva that can swim actively, has eyespots and a larval haptor. The oncomiracidia attach to their host by means of the haptor and promptly loose their ciliated coat. Growth to the adult stage is usually accompanied by an increase in the complexity of the haptor.
Because the life span of oncomiracidium is short & the spatial distribution of the host makes chance contact unlikely, these parasites have evolved variety of mechanisms to increase chances of host location.
These mechanisms include:
i. The production of eggs at the right time of year when the fish are shoaling and so potential hosts are close together.
ii. Hatching of the oncomiracidia during daytime (light stimulus) when the fish are resting on the bottom, so getting into a host is less of a problem.
iii. Response to chemical (e.g. presence of mucus) and physical stimuli (e.g. respiratory currents around gills), the oncomiracidia are able to locate their hosts over short distances.
Polystoma intergerrimum is one of the few endoparasitic monogeneans. The adult form lives in bladder of Old World frogs and the life-cycle of particular interest because its reproductive cycle is synchronized with that of the host. These worms take 4 to 5 years to mature and lie dormant until the frog enters its reproductive cycle. With the increased levels of gonadotrophic hormones in the frog the parasites begin to copulate and produce eggs which pass out in the frog's urine. The eggs take from 20 -50 days to mature and hatch, releasing the oncomiracidia. During this period the frogs have spawned and young tadpoles have hatched out. The tadpoles have two phases, the first in which they breathe with external gills which later transform into internal gills. The newly hatched oncomiracidium attaches to the external gills of the tadpole and quickly transform into a neotenic larva, i.e. a larva which is sexually mature. The neotenic larva is capable of producing eggs within 20-25 days. This gill form is morphologically and physiologically different from the adult that resides in the bladder. The eggs from this form hatch after a further 15 -20 days and the larvae attach, once again, to the tadpole gills.
Further development is delayed until the tadpoles begin to metamorphose. With the resorption of the gills the worms migrate over the ventral surface of the tadpole and enter the bladder via the urinary tract. The whole migration reportedly takes only about a minute. This migration appears to be endogenously programmed because even if newly hatched larvae are released near the metamorphosing tadpole, they will first attach to the gills then migrate to the bladder.
In North America a similar species, P. nearcticum, occurs in tree frogs. This species also has a neotenic form but the larvae do not undergo the slow developmental phase or the migration from the gills to the bladder observed in P. intergerrimum, instead they enter the urogenital tract directly.
Other monogeneans.
In Diplozoon paradoxum (a parasite of freshwater fish), the adult body is made up of two individuals fused together, neither of which can survive alone. The larva of this species is called a diporpa. There is no development unless two diporpa larvae come together, then each grasps the dorsal button of the other by means of their ventral sucker. This triggers a metamorphosis that leaves the two larvae fused. The intestine ramifies through both individuals and the male and female reproductive ducts become reciprocally fused, so they are in a state of permanent copulation and cross-fertilisation is assured.
Another genus of gill parasitic monogeneans is Gyrodactylus sp. (a parasite of freshwater and marine fish). This group is characterised by the absence of larvae, the adults being viviparous. An unusual form of polyembryony complicates the vivipary. Each zygote gives rise to four groups of cells, each of which gives rise to a separate larva, but with the peculiarity that the larvae become enclosed one inside the other, a bit like stacking Russian dolls.
The adult worm measures 0.5 to 0.8 mm long. Its opisthapter has 16 marginal hooks and 2 large hamuli.
This parasite is of particular importance in aquaculture where large populations of fish are confined to relatively small volumes of water, forcing more frequent contact between fish than would normally occur in a natural habitat. Therefore, rapid increases in populations of this worm can occur in this controlled environment. These parasites have a wide host range, infecting small tropical fish like guppies through to large commercial fish such as trout.
One of the most interesting features of this parasite, & one which also makes it so devastating, is its reproductive cycle. The entire reproductive cycle can occur on the host. This is because the larvae develop within the uterus of the adult worm. However, in addition to this, polyembryony, produces 4 individuals from a single zygote. This leads to rapid increase in parasite numbers. When fish come close together the adult worms can easily move from one fish to another, spreading the infection. In addition, the worms can also survive for short periods in the absence of the host.
Phylum Acanthocephala (Spiny- or thorny- headed worms).
The name Acanthocephala comes from the Greek words Acanthus meaning a prickle and Kephale meaning a head. Acanthocephalans are not encountered as commonly as parasitic flatworms (trematodes and tapeworms) or nematodes. They are found in many species of fishes, amphibians, birds, and mammals.
There are several morphological characteristics serve to separate acanthocephalans from other parasitic worms, but probably the most notable is the presence of an anterior, protrusible proboscis that is usually covered with hooks. It is this characteristic that gives the acanthocephalans their common name, the spiny- or thorny-headed worms.
The taxonomy of the Acanthocephala has been somewhat controversial. The Acanthocephala were not recognised until beginning of the 18th century, and were not distinguished until Koelreuther (1771) proposed the name Acanthocephala . Muller (1776) unaware, of Koelreuther's work called them Echinorhynchus. Rudolphi (1809) formally gave them the name Acanthocephala. Many species were described in the 19th century and all were placed under one generic name- Echinorhynchus. The phyolgenetic position of Acanthocephala was very uncertain; variously they had been placed among flatworms, roundworms, chaetognatha, and finally were placed in the Aschelminthes. Hyman eventually removed the Acanthocephala from the Aschelminthes and made them into a separate phylum.
This is a small, but interesting and quite important group of parasites. The Acanthocephala are all endoparasites and have no gut at any stage of their life cycle. They are usually just a few millimetres in length. Adult acanthocephalans are all parasitic in the intestines of vertebrates (fish, rodents and birds) with arthropods (one case of a mollusc) serving as intermediate hosts. In these two characteristics -(1) no gut and (2) parasitic in the vertebrate intestine, the acanthocephalans resemble cestodes, but morphologically they are very different.
The diagnostic characteristics of acanthocephalans, as shown by the adult are:
i. A spiny retractable proboscis for attachment.
ii. No gut.
iii.Pseudocoelomate.
iv.A pair of lemnisci.
v. The sexes are separate (dioecious).
Morphology of the adult.
The body of the adult is divided into two regions, the anterior part or praesoma, which includes the proboscis and neck and the main trunk or metasoma. Between the two regions the body wall is divided by a cuticular partition. At the anterior end the proboscis can be withdrawn into a sheath and the whole anterior end withdrawn into the worm. The body wall has both longitudinal and circular muscles, as does the proboscis sheath. So there are two hydrostatic systems, the main body cavity and the cavity of the proboscis sheath. Eversion of the proboscis is brought about hydrostatically, by the action of both of these systems that is by the contraction of both the body wall and the proboscis sheath musculature. Two fluid-filled lemnisci (invaginations of the body wall at the base of the neck) are involved in movement of the proboscis. The sort of internal pressures developed in everting the proboscis are low compared with for example the pressure developed in the nematode pseudocoelom. Pressures in acanthocephala reach about 0.5 mm Hg compared with 100 mm Hg in Ascaris.
Body Plan.
Structure of the body wall.
Like cestodes, the Acanthocephala have no digestive system and must absorb their nutrients through the body wall. The body wall of acanthcephalans is a complex syncitial tissue composed of FIVE major layers - the epicuticle, cuticle, striped layer, felt layer and radial layer. The surface area of the tegument is increased by the presence of pore canals. Contained within the radial layer is the lacunar canal system. This is a series of channels that anastomose throughout the tegument. The canals appear to be filled with a liquid lipid and it has been suggested that they form yet another hydrostatic system.
The Acanthocephala show cell constancy or eutely, that is, members of the same species always have the same number of cells in their different organ systems. The number and arrangement of the nuclei in the body wall is used in acanthocephalan taxonomy. During development these nuclei increase greatly in size, up to one tenth of a millimetre in diameter. In some species of Acanthocephala, these 'giant nuclei' fragment and disappear, in others they persist. Polyploidy is also common in nuclei with up 343n (n = haploid) being recorded!.
Excretory system.
Usually absent although some species have flames cells (protonephridia).
Reproduction.
The sexes are separate and the reproductive organs of the Acanthocephala are unique and have no resemblance to any other phylum. The male has a pair of testes, a sperm duct, a small penis and an eversible copulatory bursa, all held in a ligament sac. The unicellular cement glands help keep the acanthocephalans together during copulation and also seal the vagina of the female copulation has taken place.
In the female, the ovaries (one or two) develop initially in the ligament sac, but as the worm matures the ovaries fragment and in the adult, the ovaries are represented by ovarian balls lying free in the pseudocoelom. The eggs (Acanthors) are fertilised whilst they are still in the ovarian ball, the fertilised eggs are then shed into the pseudocoelom, where they complete their development. So in the mature female the pseudocoelom is full of fragments of ovaries and developing eggs. When the eggs are mature they are passed into the uterus by a remarkable and unique structure called the uterine bell. Eggs enter the bell where the immature eggs are sorted (apparently on the basis of length) from the mature ones. The immature eggs are returned to the pseudocoelom to complete development and the mature eggs are passed into the uterus and on into the vagina, which is guarded by a sphincter.
Life cycle.
All Acanthocephala require an invertebrate intermediate host, and this is always an arthropod, with one exception-a mollusc. Many acanthocephalan life cycles involve paratenic hosts. There are no human infections with acanthocephalans, but they cause major problems in fish farms. Acanthocephalan infections are particularly difficult to treat.
Complete life cycles are known for only about 25 species of acanthocephalans, but all species seem to follow the same basic pattern. The adult acanthocephalans occur in the intestine of the definitive host. The sexes are separate (i.e., they're dioecious), and the females produce eggs that are passed in the host's faeces. The eggs are ingested by an intermediate host (an arthropod), and in the intermediate host the parasite goes through several developmental (juvenile) stages. The definitive host is infected when it eats an intermediate host containing the infective juvenile stage (called a cystacanth).
Impact of Acanthocephalan infection on host.
Number of worms per host is large: 1000 in a duck intestine.
Reproductive capacity is high, up to 10,000,000 eggs per female.
Very injurious to host as proboscis hooks cause damage to intestinal tissues.
In Polymorphus. botulus infections recent research has suggested that there is no evidence of pathogenicity of P. botulus to intermediate crab hosts. However, the eider duck (Somateria mollissima) the definitive host of this parasite can accumulate infections of 100-750 P. botulus which have been associated with seasonal "outbreaks" of disease & mortalities in the eider ducks. The cystacanth is long lived and probably remains infective throughout the life of the crab.
The life cycle of Polymorphus botulus normally occurs between sea ducks (e.g.., eiders and scoters) and crabs. Thus, infections found in commercial-sized lobsters reported in Canada were probably acquired from crabs that form an important dietary item of lobsters. Cystacanths occurring in lobsters can cause econmic loss. There are no known methods of prevention or control.
Recommended reading:
Introduction to Animal Parasitology. 3/e. JD Smyth. Cambridge University Press.
1) For monogeneans - pp. 157-162, p. 167 for Gyrodactylus, pp. 171-171 for Diplozoon, and p. 169 for the sketch of these two species.
2) For the Ancanthocephalans - pp. 451- 457.
Useful reading:
The appropriate chapters in 'Invertebrate Zoology' by Ruppert and Barnes. 6/e. Saunders College Publishing.
Review articles of interest:
Monogenea:
Journal of Aquariculture & Aquatic Sciences Article
Tinsley RC, Jackson JA.
Host factors limiting monogenean infections: a case study.
Int J Parasitol. 2002 Mar;32(3):353-65. PMID: 11835975 [PubMed - in process]
Bakke TA, Harris PD, Cable J.
Host specificity dynamics: observations on gyrodactylid monogeneans.
Int J Parasitol. 2002 Mar;32(3):281-308. PMID: 11835970 [PubMed - in process]
Morand S, Simkova A, Matejusova I, Plaisance L, Verneau O, Desdevises Y.
Investigating patterns may reveal processes: evolutionary ecology of ectoparasitic monogeneans.
Int J Parasitol. 2002 Feb;32(2):111-9. Review. PMID: 11812488 [PubMed - indexed for MEDLINE]
Kearn GC.
The survival of monogenean (platyhelminth) parasites on fish skin.
Parasitology. 1999;119 Suppl:S57-88. Review. PMID: 11254148 [PubMed - indexed for MEDLINE]
Tinsley RC.
Parasite adaptation to extreme conditions in a desert environment.
Parasitology. 1999;119 Suppl:S31-56. Review. PMID: 11254147 [PubMed - indexed for MEDLINE]
Whittington ID, Cribb BW, Hamwood TE, Halliday JA.
Host-specificity of monogenean (platyhelminth) parasites: a role for anterior adhesive areas?
Int J Parasitol. 2000 Mar;30(3):305-20. Review. PMID: 10719124 [PubMed - indexed for MEDLINE ]
Scholz T.
Parasites in cultured and feral fish.
Vet Parasitol. 1999 Aug 1;84(3-4):317-35. Review. PMID: 10456421 [PubMed - indexed for MEDLINE]
Cable J, Harris PD, Tinsley RC.
Life history specializations of monogenean flatworms: a review of experimental and microscopical studies.
Microsc Res Tech. 1998 Aug 1;42(3):186-99. Review. PMID: 9764919 [PubMed - indexed for MEDLINE]
Schmahl G.
The chemotherapy of monogeneans which parasitize fish: a review.
Folia Parasitol (Praha). 1991;38(2):97-106. Review. PMID: 1937280 [PubMed - indexed for MEDLINE]
Lebedev BI.
Monogenea in the light of new evidence and their position among platyhelminths.
Angew Parasitol. 1988 Sep;29(3):149-67. Review. PMID: 3059847 [PubMed - indexed for MEDLINE]
Acanthocephala:
Taraschewski H.
Host-parasite interactions in Acanthocephala: a morphological approach.
Adv Parasitol. 2000;46:1-179. Review. PMID: 10761555 [PubMed - indexed for MEDLINE]
Poulin R.
Evolution and phylogeny of behavioural manipulation of insect hosts by parasites.
Parasitology. 1998;116 Suppl:S3-11. Review. PMID: 9695105 [PubMed - indexed for MEDLINE]
Tonguthai K.
Control of freshwater fish parasites: a Southeast Asian perspective.
Int J Parasitol. 1997 Oct;27(10):1185-91. Review. PMID: 9394189 [PubMed - indexed for MEDLINE]
Chubb JC.
Seasonal occurrence of helminths in freshwater fishes. Part IV. Adult Cestoda, Nematoda and Acanthocephala.
Adv Parasitol. 1982;20:1-292. Review. No abstract available. PMID: 6765855 [PubMed - indexed for MEDLINE]
Parshad VR, Crompton DW.
Aspects of Acanthocephalan reproduction. Adv Parasitol. 1981;19:73-138. Review. No abstract available. PMID: 7034492 [PubMed - indexed for MEDLINE]
Nicholas WL.
The biology of the acanthocephala.
Adv Parasitol. 1973;11(0):671-706.
Review. No abstract available. PMID: 4601313 [PubMed - indexed for MEDLINE]