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Echinoplectanum laeve body.jpg
Echinoplectanum laeve
Scientific classification Red Pencil Icon.png
Kingdom: Animalia
Phylum: Platyhelminthes
Class: Monogenea
Carus, 1863

See text.

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. [1] Monogeneans have a series of hooks which are used to attach onto fish, and as a result, could lead to infections.


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. [1]

Signs and symptoms

Diplozoon paradoxum Dobbeltdyr.gif
Diplozoon paradoxum

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. [2]

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. [2]


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. [3]

Monogeneans lack respiratory, skeletal, and circulatory systems and have no or weakly developed oral suckers. [4] 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).

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.

Systematics and evolution

The ancestors of Monogenea were probably free-living flatworms similar to modern Turbellaria. 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". [1]

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:

All of these can cause epizootics in freshwater fish when raised in aquaculture.

Polyopisthocotylea include:

Ecology and lifecycle

Eggs of the monogenean Protopolystoma xenopodis, a parasite of the African clawed frog Xenopus laevis Parasite130103-fig1 Protopolystoma xenopodis (Monogenea, Polystomatidae) egg.tif
Eggs of the monogenean Protopolystoma xenopodis , a parasite of the African clawed frog Xenopus laevis

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.

See also

Gastrocotylinae   Family of worms

Related Research Articles

Entobdella soleae is a monogenean (Platyhelminth) skin parasite of the common sole, Solea solea, an important food fish. They are approximately 2 to 6 mm in length. It is flat, translucent, and has a large, disc-shaped haptor, a posterior organ used for semi-permanent attachment to the host. Typically, 2-6 parasites are found on wild sole, but in intensive fish farms this can rise to 200-300 parasites per fish, causing skin inflammation and sometimes death of the sole. E. soleae can live up to 120 days in seawater.

<i>Dactylogyrus</i> genus of worms

Dactylogyrus is a genus of monogeneans in the Dactylogyridae family.

<i>Dactylogyrus vastator</i> species of worm

Dactylogyrus vastator is a species of hermaphroditic flatworms of class Monogenea. It is an ectoparasite of fish which infests the gills. It is problematic on fish farms. It is otherwise non-hazardous to humans.

Polyopisthocotylea A subclass of parasitic flatworms in the class Monogenea

Polyopisthocotylea is a subclass of parasitic flatworms in the class Monogenea.

<i>Microcotyle</i> genus of worms

Microcotyle is a genus which belongs to the phylum Platyhelminthes and class Monogenea. Species of Microcotyle are ectoparasites that affect their host by attaching themselves as larvae on the gills of the fish and grow into adult stage. This larval stage is called oncomiracidium, and is characterized as free swimming and ciliated.

Monopisthocotylea Subclass of parasitic flatworms in the class Monogenea

The Monopisthocotylea are a subclass of parasitic flatworms in the class Monogenea.

Protomicrocotylidae family of worms

Protomicrocotylidae is a family of monogenean parasites in the order Mazocraeidea.

Lethacotyle is a genus of polyopisthocotylean monogeneans, included in the family Protomicrocotylidae.
The genus includes only two species: Lethacotyle fijiensisManter & Price, 1953 , the type-species of the genus, and Lethacotyle veraJustine, Rahmouni, Gey, Schoelinck, & Hoberg, 2013 . Both species are parasitic on the gills of jacks in the Pacific Ocean. They are known only from three localities: off Fiji, Andaman Islands, and New Caledonia.
The genus Lethacotyle is special in that its members have no clamps on their posterior attachment organ or haptor, in contrast to most polyopisthocotylean Monogenean which have clamps. This is reflected in the etymology of the name, which, according to Manter & Price is "from letha = forgetting, and cotyle = cup, and refers to the absence of clamps".

Haptor organ of Monogeneans

The haptor is the attachment organ of the monogeneans, a group of parasitic Platyhelminthes. The haptor is sometimes called opisthaptor to emphasize that it is located in the posterior part of the body, and to differentiate it from the prohaptor, a structure including glands located at the anterior part of the body. According to Yamaguti (1963), the chief adhesive organ of the monogeneans, the haptor, is posterior, more or less discoid, muscular, may be divided into alveoli or loculi, is usually provided with anchors, has nearly always marginal larval hooklets, or is in a reduced form with anchors. The haptor may consist of symmetrical or asymmetrical, sessile or pedunculate, muscular suckers or clamps with or without supporting sclerites; accessory adhesive organs may be present in form of armed plaques, lappets or appendices.

<i>Pseudorhabdosynochus</i> genus of worms

Pseudorhabdosynochus is a genus of monopisthocotylean monogeneans, included in the family Diplectanidae.The type-species of the genus is Pseudorhabdosynochus epinepheli .

Capsalidae family of worms

The Capsalidae is a family of monopisthocotylean monogeneans, which includes about 200 species.

Gyrodactylus turnbulli is an ectoparasite from the class Monogenea, is part of the phylum Platyhelminthes, and from the genus Gyrodactylus. It only requires one host to transmit an infection; however, since this parasite lacks oncomiracidium, it must rely on either the adult or subadult for spread of infection. Found in freshwater, this flatworm is commonly found on the gills and fins of the guppy, Poecilia reticulata. G. turnbulli was said to be host specific, but an experiment where parasitologists artificially infected guppies suggests that the parasite can infect a wider range of species. This ability is achievable by host switching, which promotes speciation.

Diclidophora nezumiae is a species of monogenean flatworm that parasitizes the gills of the rattail fish Nezumia bairdii. Due to a highly localized host habitat parasite incidence is relatedly localized to the Hudson Submarine Canyon.

Delane C. Kritsky is an American parasitologist who specialised on the Monogenea, a class of parasitic flatworms which are important ectoparasites of fishes. His research was mainly in the fields of taxonomy, faunistics, and phylogeny of the Monogenea.

<i>Pseudorhabdosynochus lantauensis</i> species of worm

Pseudorhabdosynochus lantauensis is a diplectanid monogenean parasitic on the gills of the longtooth grouper, Epinephelus bruneus. It was described in 1981 as Cycloplectanum lantauensis and later transferred to the genus Pseudorhabdosynochus by Kritsky & Beverley-Burton in 1986.

<i>Microcotyle sebastis</i> Species of worms

Microcotyle sebastis is a species of monogenean, parasitic on the gills of a marine fish. It belongs to the family Microcotylidae.

<i>Pseudaxine</i> genus of worms

Pseudaxine is a genus which belongs to the phylum Platyhelminthes and class Monogenea; all its species are parasites of fish.

<i>Pseudaxine trachuri</i> Species of worms

Pseudaxine trachuri is a species of monogenean, parasitic on the gills of a marine fish. It belongs to the family Gastrocotylidae.

<i>Sibitrema</i> genus of worms

Sibitrema is a genus which belongs to the phylum Platyhelminthes and class Monogenea; the only species included in this genus is parasite of fish.

<i>Sibitrema poonui</i> Species of worms

Sibitrema poonui is a species of monogenean flatworm, which is parasitic on the gills of a marine fish. It belongs to the family Gastrocotylidae.


  1. 1 2 3 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.
  2. 1 2 Reed, Peggy et al. Monogenean Parasites of Fish. Institute of Food and Agricultural Sciences. .
  3. Roberts, S. Larry & John Janovy, Jr. Foundations of Parasitology
  4. flatworm :: Annotated classification - Britannica Online Encyclopedia
  5. Theunissen, M., Tiedt, L. & Du Preez, L. H. 2014: The morphology and attachment of Protopolystoma xenopodis (Monogenea: Polystomatidae) infecting the African clawed frog Xenopus laevis. Parasite, 21, 20. doi : 10.1051/parasite/2014020