Monogeneans, members of the class Monogenea, are a group of ectoparasitic

flatworm Platyhelminthes (from the Greek language, Greek πλατύ, ''platy'', meaning "flat" and ἕλμινς (root: ἑλμινθ-), ''helminth-'', meaning "worm") is a Phylum (biology), phylum of relatively simple bilaterian, Segmentation (biology), ...

s commonly found on the skin,

gills A gill () is a respiratory organ that many aquatic organisms use to extract dissolved oxygen from water and to excrete carbon dioxide. The gills of some species, such as hermit crabs, have adapted to allow respiration on land provided they are ...

, 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 Oviparous animals are animals that reproduce by depositing fertilized zygotes outside the body (i.e., by laying or spawning) in metabolically independent incubation organs known as eggs, which nurture the embryo into moving offsprings kno ...

(egg-laying) and some are

viviparous In animals, viviparity is development of the embryo inside the body of the mother, with the maternal circulation providing for the metabolic needs of the embryo's development, until the mother gives birth to a fully or partially developed juve ...

(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.Roberts, S. Larry & John Janovy, Jr. ''Foundations of Parasitology'' Monogeneans lack respiratory, skeletal, and circulatory systems but they do have posterior attachment structures in the form of adhesives, clamps, hamuli and 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, a ...

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

pharynx The pharynx (: pharynges) is the part of the throat behind the human mouth, mouth and nasal cavity, and above the esophagus and trachea (the tubes going down to the stomach and the lungs respectively). It is found in vertebrates and invertebrates ...

and an intestine with no terminal opening (

anus In mammals, invertebrates and most fish, the anus (: anuses or ani; from Latin, 'ring' or 'circle') is the external body orifice at the ''exit'' end of the digestive tract (bowel), i.e. the opposite end from the mouth. Its function is to facil ...

). Monogenea are Platyhelminthes, so are among the lowest invertebrates to possess three embryonic germ layers—

endoderm Endoderm is the innermost of the three primary germ layers in the very early embryo. The other two layers are the ectoderm (outside layer) and mesoderm (middle layer). Cells migrating inward along the archenteron form the inner layer of the gastr ...

mesoderm The mesoderm is the middle layer of the three germ layers that develops during gastrulation in the very early development of the embryo of most animals. The outer layer is the ectoderm, and the inner layer is the endoderm.Langman's Medical ...

, and

ectoderm The ectoderm is one of the three primary germ layers formed in early embryonic development. It is the outermost layer, and is superficial to the mesoderm (the middle layer) and endoderm (the innermost layer). It emerges and originates from the o ...

. 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 haptor is the attachment organ of the monogeneans, a group of parasitic Platyhelminthes. The haptor is sometimes called opisthaptor (from ''opistho-'': behind) to emphasize that it is located in the posterior part of the body, and to diffe ...

. 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 In biology, phylogenetics () is the study of the evolutionary history of life using observable characteristics of organisms (or genes), which is known as phylogenetic inference. It infers the relationship among organisms based on empirical dat ...

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 Digenea (Gr. ''Dis'' – double, ''Genos'' – race) is a class of trematodes in the Platyhelminthes phylum, consisting of parasitic flatworms (known as ''flukes'') with a syncytial tegument and, usually, two suckers, one ventral and one or ...

, from which the

cestodes Cestoda is a class of parasitic worms in the flatworm phylum (Platyhelminthes). Most of the species—and the best-known—are those in the subclass Eucestoda; they are ribbon-like worms as adults, commonly known as tapeworms. Their bodies con ...

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 Family (from ) is a group of people related either by consanguinity (by recognized birth) or affinity (by marriage or other relationship). It forms the basis for social order. Ideally, families offer predictability, structure, and safety as ...

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 The Monopisthocotylea are an inferior class of parasitic flatworms in the class Monogenea.WoRMS (2019). Monopisthocotylea. Accessed at: http://www.marinespecies.org/aphia.php?p=taxdetails&id=119219 on 2019-02-08 Yamaguti, S. (1963). Systema Helmi ...

have one main part to the haptor, often with hooks or a large attachment disc, whereas the

Polyopisthocotylea Polyopisthocotylea is a subclass of parasitic flatworms in the class Monogenea.WoRMS (2023). Polyopisthocotylea. Accessed at: http://www.marinespecies.org/aphia.php?p=taxdetails&id=119220 on 2023-12-21Yamaguti, S. (1963). Systema Helminthum Volu ...

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.

include: *Genus '' Gyrodactylus'' 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

epizootic In epizoology, an epizootic (or epizoötic, from Greek: ''epi-'' "upon" + ''zoon'' "animal") is a disease event in a nonhuman animal population analogous to an epidemic in humans. An epizootic disease (or ) may occur in a specific locale (an ...

s in

freshwater fish Freshwater fish are fish species that spend some or all of their lives in bodies of fresh water such as rivers, lakes, ponds and inland wetlands, where the salinity is less than 1.05%. These environments differ from marine habitats in many wa ...

when raised in

aquaculture Aquaculture (less commonly spelled aquiculture), also known as aquafarming, is the controlled cultivation ("farming") of aquatic organisms such as fish, crustaceans, mollusks, algae and other organisms of value such as aquatic plants (e.g. Nelu ...

include: *Genus '' Diclidophora'' is primarily found in marine fish and primitive freshwater fish such as

sturgeon Sturgeon (from Old English ultimately from Proto-Indo-European language, Proto-Indo-European *''str̥(Hx)yón''-) is the common name for the 27 species of fish belonging to the family Acipenseridae. The earliest sturgeon fossils date to the ...

s 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 elongated rost ...

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

Xenopus ''Xenopus'' () (Gk., ξενος, ''xenos'' = strange, πους, ''pous'' = foot, commonly known as the clawed frog) is a genus of highly aquatic frogs native to sub-Saharan Africa. Twenty species are currently described with ...

'' 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 The bladder () is a hollow organ in humans and other vertebrates that stores urine from the Kidney (vertebrates), kidneys. In placental mammals, urine enters the bladder via the ureters and exits via the urethra during urination. In humans, the ...

and

rectum The rectum (: rectums or recta) is the final straight portion of the large intestine in humans and some other mammals, and the gut in others. Before expulsion through the anus or cloaca, the rectum stores the feces temporarily. The adult ...

of cold-blooded

vertebrates Vertebrates () are animals with a vertebral column (backbone or spine), and a cranium, or skull. The vertebral column surrounds and protects the spinal cord, while the cranium protects the brain. The vertebrates make up the subphylum Vertebra ...

). Although they are

hermaphrodite A hermaphrodite () is a sexually reproducing organism that produces both male and female gametes. Animal species in which individuals are either male or female are gonochoric, which is the opposite of hermaphroditic. The individuals of many ...

s, 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 Birds are a group of warm-blooded vertebrates constituting the class (biology), class Aves (), characterised by feathers, toothless beaked jaws, the Oviparity, laying of Eggshell, hard-shelled eggs, a high Metabolism, metabolic rate, a fou ...

, but one ('' Oculotrema hippopotami'') infects

mammals A mammal () is a vertebrate animal of the class Mammalia (). Mammals are characterised by the presence of milk-producing mammary glands for feeding their young, a broad neocortex region of the brain, fur or hair, and three middle e ...

, parasitizing the eye of the

hippopotamus The hippopotamus (''Hippopotamus amphibius;'' ; : hippopotamuses), often shortened to hippo (: hippos), further qualified as the common hippopotamus, Nile hippopotamus and river hippopotamus, is a large semiaquatic mammal native to sub-Sahar ...

References

External links

Monogenea – ectoparasitic flukes (flatworms)
- Clinical Sciences {{Authority control Protostome classes