Life History and Life Cycle of Fish Parasites: Trichodinids, Nematodes, Cestoda, Acanthocephalan, and Leeches

Fish parasites exhibit a variety of life cycle types. Those that are transmitted from one definitive host (in which they reach sexual maturity) to another are described as having direct life cycles.

Aquatic habitats offer ideal conditions for the maintenance and evolution of parasite life cycles. Water provides a physiologically stable, buffered environment, and its viscosity facilitates the dispersal and survival of eggs and fragile free-living stages.

Infecting organisms may live inside a host, in which case they are termed endoparasites, or on the external surface as ectoparasites. In fishes, the gill surfaces are usually classified as external environment since they are in constant contact with the external medium, even in species where they are enclosed within opercula.

Parasites that use at least one intermediate host (which harbors sexually immature forms of the parasite) to transfer between definitive hosts are said to have indirect or complex life cycles.

Life cycle of Trichodinids

Trichodinids have a simple direct life cycle. That is, they have a single host and do not use alternation of generations or mass asexual replication off the host. The reproduce by binary fission, literally cell- splitting.

This produces daughter cells with half the number of denticles of the parent cell. The full complement of denticles is restored by synthesis of new denticles from the outer edge of the cell, working inwards.

Trichodinids are typically found on the gills, skin and fins of fishes, though some species parasitize the urogenital system. A range of invertebrates is also host to trichodinid infections, including the surfaces of copepods and the mantle cavity of molluscs.

Transmission occurs by direct contact of infected and uninfected hosts, and also by active swimming of trichodinids from one host to another. Trichodina cells swim with the adoral surface facing forwards. On surfaces, they move laterally, with the adoral surface facing the substrate.

Life Cycle of nematodes

A.crassus requires an intermediate arthropod host in its life reduced survival of the host. Transmission to the definitive host is through the food chain and all sizes of eels from cycle to become infective to the definitive eel host.

Intermediate host specificity is wide with Diaptomus gracilis, juvenile Gammarus and the brackish species Eurytemoraaffinis all acting as possible intermediates but only Cyclopsvicinus and C.albidus allowing the parasite to develop into the infective stage.

Higher densities of infection can lead to significantly glass eels upwards can be infected. Transmission can also occur from eel to eel through predation. A. crassus is found in the swim-bladder of eels and gravid adults can contain up to 0.5×10^6 eggs each.

Released eggs pass into the intestinal tract and hatch during or after passage through the digestive system. Free-living larvae tend to aggregate in clumps of 10-30 individuals, attaching firmly to the substrate by their hooked tails.

Life Cycle of Cestoda

The life cycle is normally completed within 12 months and involves a free-living stage as well as two intermediate hosts and a definitive host.

This parasite is hermaphroditic and the structure of reproductive system in all species of this genus is identical except for the shape of the internal seminal vesicle.

In T. nodulosus the internal seminal vesicle is large (330-440µm in length and 282-385µm in width), and oval in shape. Tapeworms usually release eggs that pass out of the definitive host and complete embryonation in water.

They hatch into ciliated, motile and short-lived larvae (coracidia), which are light-sensitive and swim towards the surface of the water body thereby enhancing the chances of the coracidium being ingested by the first intermediate copepod host.

The coracidium migrates to the haemocoel (body cavity) of the copepod host before it differentiates and develops into a procercoid stage.

The procercoid stage is fully developed in around 9 days and it is transmitted to the second intermediate (fish) host when the infected copepod is ingested. Once ingested it migrates into the muscle and viscera of the fish and differentiates into the plerocercoid stage.

The Transmission to the definitive host, in this case a species of pike is through the ingestion of the infected intermediate host. There are thought to be at least 78 species of intermediate host for T.nodulosus.

Adult tapeworms grow and mature in the definitive host during late spring and early summer and release eggs; one sexually mature worm being capable of producing as many as 1,750,000 eggs.

Most new infections occur during summer and early autumn but the differentiation into procercoids can be rapid due to high temperatures and there can be sufficient time for the tapeworm to re-infect fish prior to winter.

Egg laying occurs during the raining season and although T.nodulosus can be found in the intestines of pike throughout the year, only young parasites are found during the second half of June-September, and these have no trace of gonadal formation.

This ‘young’ parasite are usually 10-90mm in length during this stage but by the end of September they can range from 90-104mm and the initiation of gonadal development is apparent.

Development continues until December/early January at which time growth usually ceases with an average length of 255mm.

Life Cycle of Acanthocephalan

Copulation within the definitive host can occur within 24 hours of infection but egg production starts between 4 and 8 weeks after infection and lasts for approximately 2 months.

Males tend to have shorter life spans than females and may die shortly after copulation. Eggs develop in the fluid-filled body cavity of the female by a process of spiral cleavage which produces an elongated embryo, the acanthor.

The number of eggs is dependent upon the size of the female. Acanthors usually bear hooks or spines arranged in spirals or circles at the anterior end and the entoblast (a group of germinal cells within the acanthor) gives rise to most of the organs of the adult form.

The eggs, containing a fully developed acanthor, are passed out of the definitive host in the feces and will only hatch when ingested by the intermediate host. The acanthor penetrates the intestine wall with its hooks and enters the haemocoel.

Read Also : Morphology and Taxonomy of Fish Parasites

These then migrate from intestine and develop through series of acanthella stages in the dorsal haemocoel of the gammarid until an infective juvenile known as a cystacanth (enveloped in a capsule) is formed. P.laevis has been known to change the appearance and behavior of the intermediate host so as to maximize their chances of being eaten by the next host.

This can include appearing more visible to the host predator due to the orange coloring of the cystacanths or altering the behavior so that the intermediate host swims near the surface of the water (possibly a photophilic response), or clings to the surface of vegetation.

The life cycle is completed when the final infective cystacanth stage is eaten along with its host by the definitive host although a paratenic host may sometimes be involved.

Leeches

Life History and Biology

Leeches, once engorged with the blood of the host, detach and rest on a protected substrate in the water (under stones or in plant debris) until their next meal. Reproduction takes place after several meals.

B.tricarinata in tropical and subtropical waters breeds throughout the year (Oosthuizen, 1989). Glossiphoniidae exhibit parental care.

The leeches produce a thin walled cocoon and immediately after deposition place their bodies over it and the hatching offspring attach themselves in a brood pouch which forms on the parent’s ventral wall.

They remain in the pouch until the first feeding stage when they are often brought to their first host by the parent. The entire life cycle may last, depending on availability of hosts, from 24 days to several months.

B.tricarinata feeds on fish, and if fish are unavailable will also feed on tadpoles and adult anurans (Oosthuisen, 1991).

Some glossiphonids also feed on freshwater molluscs but, although B.tricarinata enters the mantles of molluscs, it does not feed on these hosts. B. tricarinata has been found feeding naturally and induced experimentally to feed on hosts of diverse fish.

Families; Clariasspp., Bagrusspp., Oreochromisspp. Barbusspp. goldfish and Protopterusaethiopicus(Oosthuizen, 1989; Paperna,).

However, populations of a given geographical region or a water system show preference for, or will exclusively occur on, one particular host, for example Clariaslazerain the Jordan system, Israel and Bagrusdocmacin Lake Victoria (Paperna,).

In summary, parasites exhibit a variety of life cycle types. Those that are transmitted from one definitive host (in which they reach sexual maturity) to another are described as having direct life cycles.

Parasites that use at least one intermediate host (which harbors sexually immature forms of the parasite) to transfer between definitive hosts are said to have indirect or complex life cycle.

Trichodinids have a simple direct life cycle. That is, they have a single host and do not use alternation of generations or mass asexual replication off the host. A. crassus requires an intermediate arthropod host in its life reduced survival of the host.

Cestoda life cycle is normally completed within 12 months and involves a free-living stage as well as two intermediate hosts and a definitive host.

Acanthocephalan, copulate within the definitive host can occur within 24 hours of infection but egg production starts between 4 and 8 weeks after infection and lasts for approximately 2 months.

Leech entire life cycle may last, depending on availability of hosts, from 24 days to several months.

Read Also : The Products Derived From Household Waste