Monogeneans are a group of ectoparasitic flatworms commonly found on the skin, gills, or fins of fish. They have a direct lifecycle and do not require an intermediate host. Adults are hermaphrodites, meaning they have both male and female reproductive structures.L.A. Tubbsa et al. (2005). "Effects of temperature on fecundity in vitro, egg hatching and reproductive development of ''Benedenia seriolae'' and ''Zeuxapta seriolae'' (Monogenea) parasitic on yellowtail kingfish Seriola lalandi". ''International Journal for Parasitology''(35), 315–327. Some monogeneans are oviparous (egg-laying) and some are viviparous (live-bearing). Oviparous varieties release eggs into the water. Viviparous varieties release larvae, which immediately attach to another host. The genus ''Gyrodactylus'' is an example of a viviparous variety, while the genus ''Dactylogyrus'' is an example of an oviparous variety.

Signs and symptoms

Freshwater fish that become infected with this parasite become lethargic and end up swimming towards the surface of the water. In addition, some may be seen rubbing the bottom or sides of their skin where the parasite is located. Infected skin where the parasite is attached may show areas of scale loss and may produce a pinkish fluid. Gills that are infected may appear swollen and pale. "Pipping", which is gulping for air at the water surface, could indicate severe respiratory distress.Reed, Peggy et al. Monogenean Parasites of Fish. ''Institute of Food and Agricultural Sciences''. . http://www.aces.edu/dept/fisheries/education/ras/publications/Update/Monogenean%20Parasites%20of%20Fish.pdf In salt water fish, Monogeneans can infect the skin and gills, resulting in irritations to the host. Heavy infections could result in erratic swimming behavior. Affected gills may become irritated and swollen.

Characteristics

Monogenea are small parasitic flatworms mainly found on skin or gills of fish. They are rarely longer than about 2 cm. A few species infecting certain marine fish are larger, and marine forms are generally larger than those found on freshwater hosts. Monogenea are often capable of dramatically elongating and shortening as they move. Biologists need to ensure that specimens are completely relaxed before measurements are taken. Monogeneans lack respiratory, skeletal, and circulatory systems and have no or weakly developed oral suckers. Like other flatworms, Monogenea have no true

body cavity A body cavity is any space or compartment, or potential space, in an animal body. Cavities accommodate organs and other structures; cavities as potential spaces contain fluid. The two largest human body cavities are the ventral body cavity, and ...

(coelom). They have a simple digestive system consisting of a mouth opening with a muscular

pharynx The pharynx (plural: pharynges) is the part of the throat behind the mouth and nasal cavity, and above the oesophagus and trachea (the tubes going down to the stomach and the lungs). It is found in vertebrates and invertebrates, though its st ...

and an intestine with no terminal opening ( anus). Monogenea are Platyhelminthes, so are among the lowest invertebrates to possess three embryonic germ layers— endoderm, mesoderm, and ectoderm. In addition, they have a head region that contains concentrated sense organs and nervous tissue (brain). Like all ectoparasites, monogeneans have well-developed attachment structures. The anterior structures are collectively termed the prohaptor, while the posterior ones are collectively termed the opisthaptor, or simply haptor. The posterior opisthaptor with its hooks, anchors, clamps etc. is typically the major attachment organ. Generally, monogeneans also are hermaphroditic with functional reproductive organs of both sexes occurring in one individual. Most species are oviparous, but a few are viviparous.

Phylogeny

The following cladogram depicts the phylogenetic relationships of the different monogenean orders:

Systematics and evolution

The ancestors of Monogenea were probably free-living flatworms similar to modern

Turbellaria The Turbellaria are one of the traditional sub-divisions of the phylum Platyhelminthes (flatworms), and include all the sub-groups that are not exclusively parasitic. There are about 4,500 species, which range from to large freshwater forms mo ...

. 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, which gave rise to digeneans, and the other line gave rise to cestodes". About 50 families and thousands of species are described. Some parasitologists divide the Monogenea into two (or three) subclasses based on the complexity of their haptor: the Monopisthocotylea have one main part to the haptor, often with hooks or a large attachment disc, whereas the 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. Monopisthocotylea include: *Genus ''

Gyrodactylus ''Gyrodactylus'' is a genus of parasitic flatworms in the family Gyrodactylidae. Species Species listed in the World Register of Marine Species The World Register of Marine Species (WoRMS) is a taxonomic database that aims to provide an auth ...

'' has no eyespots and is viviparous. *Genus '' Dactylogyrus'' has four eyespots and is oviparous. This is one of the largest metazoan genera, with at least 970 species. *Genus '' Neobenedenia'' much larger and lives on the skin of many tropical marine species, causing problematic infections in marine aquaria. All of these can cause epizootics in

freshwater fish Freshwater fish are those that spend some or all of their lives in fresh water, such as rivers and lakes, with a salinity of less than 1.05%. These environments differ from marine conditions in many ways, especially the difference in levels o ...

when raised in aquaculture. Polyopisthocotylea include: *Genus '' Diclidophora'' is primarily found in marine fish and primitive freshwater fish such as sturgeons and

paddlefish Paddlefish (family Polyodontidae) are a family of ray-finned fish belonging to order Acipenseriformes, and one of two living groups of the order alongside sturgeons (Acipenseridae). They are distinguished from other fish by their titular elong ...

. *Genus '' Protopolystoma'' is found in aquatic clawed toads ('' Xenopus'' species).

Ecology and lifecycle

Monogeneans possess the simplest lifecycle among the parasitic 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. The eggs hatch releasing a heavily ciliated larval stage known as an oncomiracidium. The oncomiracidium has numerous posterior hooks and is generally the life stage responsible for transmission from host to host.
No known monogeneans infect birds, but one ('' Oculotrema hippopotami'') infects mammals, parasitizing the eye of the

hippopotamus The hippopotamus ( ; : hippopotamuses or hippopotami; ''Hippopotamus amphibius''), also called the hippo, common hippopotamus, or river hippopotamus, is a large semiaquatic mammal native to sub-Saharan Africa. It is one of only two exta ...

References

External links

Monogenea – ectoparasitic flukes (flatworms)
- Clinical Sciences {{Taxonbar, from=Q575927 Protostome classes