Monogenea

(invertebrate zoology) A diverse subclass of the Trematoda which are principally ectoparasites of fishes; individuals have enlarged anterior and posterior holdfasts with paired suckers anteriorly and opisthaptors posteriorly.

(Monogeneans)

Phylum: Platyhelminthes

Class: Monogenea

Number of families: 53

Thumbnail description
Flatworm parasites that live mainly on fish skin and gills, with a haptor (posterior attachment organ) containing hooks, and a direct (single host) life cycle

Evolution and systematics

The origins of the monogeneans are obscure, but it is assumed that they evolved from free-living ancestors similar to modern turbellarians (aquatic flatworms). Their nearest relatives are the cestodes (tapeworms). The naming of their upper-level taxa (biological categories) is controversial. "Monogenea" is widely accepted as the name of the class itself, as is the subdivision of the class into two orders, namely the Monopisthocotylea and the Polyopisthocotylea. More recently, the name "Monogenoidea" has been proposed for the class, with the class being subdivided in turn into three subclasses. The first-mentioned, older, and more widely supported scheme is followed here. Some believe that the Monopisthocotylea and the Polyopisthocotylea have separate origins; that is, they think that Monogenea is not a monophyletic (descended from a common ancestor) class. As of 2003, 53 families of monogeneans are recognized.

Physical characteristics

Like other platyhelminths, monogeneans are acoelomate animals; that is, they do not have a body cavity lying between the body wall and the digestive tract. Their bodies are covered by a living syncytial tegument. The digestive system consists of a pharynx, which is a muscular tube used to suck in food, and a saclike or branched intestine with no anus. The pharynx may or may not be glandular. Monogeneans range in length from about 0.04–0.08 in (1–2 mm), as in some gyrodactylids, to more than 0.75 in (2 cm), as in some capsalids. Large monogeneans tend to be flat and leaf-shaped, but the smaller parasites are usually cylindrical. In general, these flatworms are colorless and semitransparent. When on fish skin some may be virtually invisible to the human eye, either by virtue of their glass-like transparency or because they contain scattered pigment that matches pigment in the host's skin. These features may protect the parasites from being eaten by predatory fishes or crustaceans ("cleaner" organisms). The brown/black coloration of the polyopisthocotyleans is associated with their digestive system and derived from their blood meals.

The most significant anatomical feature of monogeneans is their possession of a posterior attachment organ or haptor armed with hooks. The hooks usually fall into two groups: small hooklets, which are often called marginal hooklets, and larger hooks called hamuli or anchors. The hooklets are essentially found in larvae although they often persist, usually without further growth, in adult monogeneans. The hooklets are specialized for attachment to the upper layer of cells (epidermal cells or Malpighian cells) in the host's skin; they fasten themselves in the web of filaments made of keratin known as the terminal web, which lies beneath the apimal membrane of the host epidermal cell. A basic and maximum number of 16 hooklets occurs in monopisthocotyleans and in polyopisthocotyleans, although they may be reduced to 14, 10, or lost altogether. In many but not in all monogeneans, the hooklets are supplemented as the parasite grows by one or two pairs of hamuli, which are usually large enough to penetrate through the epidermis (the outer layer of the host's skin) into the dermis, which is the thicker layer of skin just below the epidermis. Some monopisthocotyleans have haptors that become more elaborate during their development, either by acquiring glands or by subdividing into separate small compartments that function as suckers. As typical polyopisthocotyleans grow, they develop three or four pairs of muscular suckers or clamps; each one is at the site of a hook-let. Many monopisthyocotyleans are able to move like leeches since they have suckers or glands that secrete sticky material on the lateral borders of the head.

The ovary consists of a germarium, which produces egg cells, and an extensive vitellarium that produces vitelline cells. The vitelline cells do not contain genetic material; they secret substances that form a chemically and physically resistant eggshell of tanned protein (sclerotin) and provide food for the growing embryo. Gyrodactylids are exceptional viviparous. Monogeneans are hermaphrodites; many have hard structures (sclerites) supporting the penis, while others have an eversible cirrus. There may be one or more vaginae, often with supporting sclerites, but hypodermic impregnation (through the skin) also takes place.

Distribution

Monogeneans are found worldwide in freshwater and marine environments.

Habitat

Many monogeneans are strictly host-specific; that is, they are limited to a single or a few closely related hosts. Skin parasites may be widespread on the surface of the host's skin or concentrated in specific areas. Many monopisthocotyleans use two pairs of counter-rotating hamuli, one pair on their ventral surface and one pair on the dorsal surface, to attach themselves between two adjacent secondary gill lamellae on one of the fish's primary lamellae. The monocotylid monopisthocotyleans are parasites of elasmobranch fishes, and must occupy internal sites such as their nasal fossae (cavities), body cavity, and cloaca. Another monopisthocotylean, Amphibdella, spends its early life inside the heart of an electric ray, moving to the gills when it is an adult and releasing its eggs into the ray's gill cavity. Many polyopisthocotyleans use their clamps to grip one or two secondary gill lamellae. Other polyopisthocotyleans (polystomatids) use suckers to attach themselves inside the bladders of frogs and toads, or the bladders or mouths of freshwater turtles. Oculotrema hippopotami is the only monogenean that infests a mammal; it is a polystomatid that lives beneath the eyelids of the hippopotamus. The gyrodactylid Isancistrum lives on the skin of squids and is the only monogenean parasite that infests an invertebrate.

Behavior

Many monogeneans migrate, moving like leeches from their site of initial attachment to the host to the site where they mate and lay their eggs. How they find their way within the host is mostly unknown as of 2003. Many monopisthocotyleans retain the ability to change their location on the host throughout their lives. Most adult polyopisthocotyleans, however, are sedentary. Some skin parasites ventilate their bodies by undulating while some juvenile and adult parasites are able to swim.

Feeding ecology and diet

Most monopisthocotyleans feed on the epidermis of their host, which is eroded by a protrusible glandular pharynx. Polyopisthocotyleans are blood feeders. These parasites accumulate indigestible residues of brown/black hematin (a pigment found in blood) from their blood meals in their digestive tract, which they eject at intervals through their mouths.

Reproductive biology

Monogeneans are hermaphrodites, with the male reproductive system usually first to mature. This characteristic is known as protandry. Mutual or unilateral insemination may occur, although self-insemination also takes place. The tanned eggs are assembled in an egg mold or ootype; the vitelline cells provide the raw material for the shell and resources for the developing embryo. With the exception of the viviparous gyrodactylids, the eggs are released into the environment and produce infective larvae (oncomiracidia) which are able to swim freely with the help of cilia. The larvae of many monogeneans hatch spontaneously at a particular time of day, which often coincides with times when the host is particularly vulnerable to invasion. Hatching may also be triggered by such host-derived cues as chemicals, mechanical disturbance, or shadows. The oncomiracidia do not feed until they reach the host, which means that their survival as free-living organisms and their potential for host infection are limited, usually to a period of several hours. The oncomiracidia throw aside their ciliated cells when they reach the host. Entobdella soleae can infect new hosts by direct transfer of adults or juveniles, as well as by eggs and oncomiracidia. The juveniles of a related parasite can swim and may reach new hosts in this way.

The gyrodactylids are unique among monogeneans. They have abandoned freely deposited eggs and free-swimming oncomiracidia; they are viviparous, producing offspring that are usually full size at birth. They increase their reproductive rate by telescoping generations; their offspring already contain a partly developed embryo at birth and sometimes a second smaller embryo within the first. The appearance of the male reproductive system is delayed until after the female reproductive system is operational. This characteristic, which is known as protogyny, helps to concentrate the organism's resources on embryo development. The first two offspring are probably produced asexually. Gyrodactylids usually spread to new hosts by direct transfer when infected hosts make contact with one another. In addition, however, the hosts may be infected by gyrodactylids drifting freely in the water column or by making contact with parasites attached to the substrate. There is also one record of a gyrodactylid that can swim.

Conservation status

No species of monogeneans are listed by the IUCN.

Significance to humans

In the wild, the number of monogeneans living on an individual host is generally low, and infestations of these parasites do not usually cause disease. In crowded fish farms, however, parasite populations often increase uncontrollably and the hosts may be damaged or killed.

Species accounts

Resources

Boeger, W. A., and D. C. Kritsky. "Phylogenetic Relationships of the Monogenoidea." In Interrelationships of the Platyhelminths, edited by D. T. J. Littlewood and R. A. Bray. London: Taylor & Francis, 2001.

Kearn, G. C. Parasitism and the Platyhelminths. London: Chapman & Hall, 1998.

Bakke, T. A., P. D. Harris, and J. Cable. "Host Specificity Dynamics: Observations on Gyrodactylid Monogeneans." International Journal for Parasitology 32 (2002): 281–308.

Kearn, G. C. "Evolutionary Expansion of the Monogenea." International Journal for Parasitology 24 (1994): 1227–1271. ——. "The Survival of Monogenean (Platyhelminth) Parasites on Fish Skin." Parasitology 119 (1999): S57–S88.

Tinsley, R. C. "Parasite Adaptation to Extreme Conditions in a Desert Environment." Parasitology 119 (1999): S31–S56.

Whittington, I. D., L. A. Chisholm, and K. Rohde. "The Larvae of Monogenea (Platyhelminthes)." Advances in Parasitology 44 (2000): 139–232.

[Article by: Graham Clive Kearn, DSc]

A subclass of the Trematoda which are ectoparasites of the gills, skin, and orifices of fishes and, less frequently, of the esophageal tracts and bladders of amphibians and turtles. They have conspicuous anterior and posterior holdfasts, the latter usually armed. The terminal genitalia are frequently sclerotized. The group is characterized by sexual reproduction, direct development, and a single host in the life cycle.

The most widely used classification employs two orders, the Monophisthocotylea, in which the posthaptor is without discrete multiple suckers or clamps, and the Polyopisthocotylea, with suckers or clamps on the posthaptor.

Body shapes of the various genera are distinctive, sometimes bizarre, as in Vallisia, which is sickle-shaped. Paired external suckers or buccal cavity suckers and adhesive glands occur anteriorly. The posterior holdfast is either solid and armed with central anchors and marginal hooks (Gyrodactyloidea), sucker-shaped with anchors and hooks (Capsaloidea; see illustration), or solid and bearing suckers or clamps (Polyopisthocotylea).

ventral view.">
A monogeneid of the superfamily Capsaloidea, Heterocotyle aetobatis from the spotted eagle ray, ventral view.

Monogenea usually have direct development involving simple metamorphosis from the ciliated larval stage to the nonciliated juvenile. Juvenile anchors and hooks may be retained or replaced by adult suckers or clamps. Cross-fertilization or, perhaps less frequently, self-fertilization of hermaphroditic individuals resulting in egg capsules which hatch on the host or in its environment is most common. See also Trematoda.

A subclass of trematode parasites that infest cold-blooded aquatic or amphibious vertebrates. Includes the genera Benedenia, Dactylogyrus, Diplozoon, Discocotyle, Gyrodactylus.

Monogenea
Dermophthirius, a microbothriidmonogenean parasitic on elasmobranchs
Scientific classification
Kingdom: Animalia Phylum: Platyhelminthes Class: Monogenea Carus, 1863

Monogenea (adj. monogenean) are a group of largely ectoparasitic members of the flatworm phylum Platyhelminthes. They are also known by the name monogenetic trematodes.

Characteristics

Monogenea are small parasitic flatworms. The body is usually flat and oval, and rarely longer than about 2cm. 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).

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.

Systematics and evolution

The ancestors of Monogenea were probably free-living flatworms similar to modern Turbellaria. According to molecular studies, their closest relatives among Platyhelminthes are the tapeworms. 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.

Ecology and life cycle

Monogenea are mainly parasites on the surface of fish. Monogenea are especially common on the skin, fins and gills of fishes. Less commonly, they can be found in the urinary bladder and rectum of cold-blooded vertebrates. None infect birds, but one (Oculotrema hippopotami) infects mammals, parasitizing the eye of a hippopotamus.

Monogenea are usually hermaphrodites (the male reproductive system becoming functional before the female part).They have direct life-cycles with no asexual reproduction (unlike the Digenea) and in those that lay eggs, a larval stage (generally ciliated) called an oncomiracidium that is responsible for transmission from host to host. As adults, they eat the blood, mucus, and epithelial cells of their host.

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