Monogenea
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 Dermophthirius, a microbothriidmonogenean parasitic on elasmobranchs
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Monogenea (adj. monogenean) are a group of largely ectoparasitic members of the flatworm phylum Platyhelminthes, class Monogenea.
[edit] Characteristics
Monogenea are small bilaterally symmetrical (i.e., the right and left sides are similar) flatworms mainly found on skin or gills of fish. They are flat and oval, and rarely longer than about 2cm except few that are larger such as marine forms which are generally larger than those found on fresh water hosts. Monogeneans lack respiratory, skeletal and circulatory systems and have no oral suckers or are weakly developed in most[1]. They attach to host using hooks and are capable of stretching and compressing their bodies, therefore the need to ensure a completely relaxed form before measurements are taken[2].
Monogeneans have a collection of various attachment structures. The anterior structures are collectively termed the prohaptor, while the posterior ones are collectively termed the opisthaptor. The posterior end evolved into a fancy holdfast structure.
Like other flatworms, Monogenea have no true body cavity (coelom). They have a simple digestive system consisting of a mouth opening with a muscular pharynx and an intestine with no terminal opening (anus). Generally, they also are hermaphroditic—functional reproductive organs of both sexes occurring in one individual— and are the lowest invertebrates to possess three embryonic layers—endoderm, mesoderm, and ectoderm. In addition,they have a head region that contains concentrated sense organs and nervous tissue (brain).
[edit] Systematics and evolution
The ancestors of Monogenea were probably free-living flatworms similar to modern Turberllaria. According to the more widely accepted view, "rhabdocoel turbellarians gave rise to monogeneans; these, in turn, gave rise to digeneans, from which the cestodes were derived. Another view is that the rhabdocoel ancestor gave rise to two lines; one gave rise to monogeneans, who gave rise to digeneans, and the other line gave rise to cestodes" [1] .
There are about 50 families and thousands of described and undescribed species.
Some parasitologists divide Monogenea into two (or three) subclasses based on the complexity of their haptor: Monopisthocotylea have one main part to the haptor, often with hooks or a large attachment disc, whereas Polyopisthocotylea have multiple parts to the haptor, typically clamps. These groups are also known as Polyonchoinea and Heteronchoinea, respectively. Polyopisthocotyleans are almost exclusively gill-dwelling blood feeders, whereas Monopisthocotyleans may live on the gills, skin and fins.
Monopistocotylea include:
- Genus Gyrodactylus, which has no eyespots and is viviparous.
- Genus 'Dactylogyrus, which has four eyespots and lays eggs. This is one of the largest metazoan genera, with at least 970 species.
- Genus Neobenedenia, which is much larger and lives on the skin of many tropical marine species, causing problematic disease in marine aquaria.
All of which can cause epizootics in freshwater fish when raised in aquaculture.
Polyopisthocotylea include:
- Genus Diclidophora, which is primarily found in marine fish and primitive freshwater fish like sturgeons and paddlefish.
[edit] Ecology and life cycle
Monogeneans pocess the simplest life cycle among the platyhelminths. They have no intermediate hosts and are ectoparasitic on fish (seldom in the urinary bladder and rectum of cold-blooded vertebrates). Although they are hermaphrodites, the male reproductive system becomes functional before the female part. They lay their eggs which hatch in water forming a larva stage known as an oncomiracidium and is heavily ciliated. The oncomiracidium has numerous posterior hooks and is responsible for transmission from host to host.
No known monogeneans infect birds, but one (Oculotrema hippopotami) infects mammals, parasitizing the eye of a hippopotamus.
