Trematodes
Trematoda is a class of flatworms known as flukes. They are obligate internal parasites with a complex life cycle requiring at least two hosts. The intermediate host, in which asexual reproduction occurs, is usually a snail. The definitive host ...
are parasitic
flatworms of the class ''Trematoda'', specifically parasitic flukes with two
suckers: one
ventral
Standard anatomical terms of location are used to unambiguously describe the anatomy of animals, including humans. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position prov ...
and the other
oral
The word oral may refer to:
Relating to the mouth
* Relating to the mouth, the first portion of the alimentary canal that primarily receives food and liquid
**Oral administration of medicines
** Oral examination (also known as an oral exam or or ...
. Trematodes are covered by a
tegument, that protects the organism from the environment by providing secretory and absorptive functions.
The life cycle of a typical trematode begins with an
egg
An egg is an organic vessel grown by an animal to carry a possibly fertilized egg cell (a zygote) and to incubate from it an embryo within the egg until the embryo has become an animal fetus that can survive on its own, at which point the a ...
. Some trematode eggs hatch directly in the environment (water), while others are eaten and hatched within a host, typically a
mollusc. The hatchling is called a ''
miracidium
The Miracidium is the second stage in the life cycle of trematodes. When trematode eggs are laid and come into contact with fresh water, they hatch and release miracidium. In this phase, miracidia are ciliated and free-swimming. This stage is com ...
,'' a free-swimming, ciliated larva. Miracidia will then grow and develop within the intermediate host into a sac-like structure known as a sporocyst or into rediae, either of which may give rise to free-swimming, motile cercariae larvae. The cercariae then could either infect a vertebrate host or a second intermediate host. Adult metacercariae or mesocercariae, depending on the individual trematode's life cycle, will then infect the vertebrate host or be rejected and excreted through the rejected host's faeces or urine.
Typical life cycle stages
While the details vary with each
species
In biology, a species is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of the appropriate s ...
, the general life cycle stages are:
Egg
The egg is found in the faeces, sputum, or urine of the definitive host. Depending on the species, it will either be non-embryonated (immature) or embryonated (ready to hatch). The eggs of all trematodes (except
schistosomes
''Schistosoma'' is a genus of trematodes, commonly known as blood flukes. They are parasitic flatworms responsible for a highly significant group of infections in humans termed ''schistosomiasis'', which is considered by the World Health Organi ...
) are operculated. Some eggs are eaten by the intermediate host (snail) or they are hatched in their habitat (water).
Miracidium
Miracidia
Trematodes are parasitic flatworms of the class ''Trematoda'', specifically parasitic flukes with two suckers: one ventral and the other oral. Trematodes are covered by a tegument, that protects the organism from the environment by providing secr ...
hatch from eggs either in the environment or in the intermediate host. They do not have a mouth; therefore they cannot eat and need to find a host quickly if they hatch in the environment. Energy is needed to develop into a sporocyst. The first intermediate host can differ for different trematodes.
Sporocyst
Sporocysts are elongated sacs that produce either more sporocysts or rediae. This is where larvae can develop.
*''Mother Sporocyst'': These have loose plates (cilia) and migrate to gonads.
*''Daughter Sporocyst'': These are an asexual production of cercariae; they absorb nutrients while having no mouth.
Redia (plural: rediae)
After the sporocyst form the larva. The first development from it forms the redia. They have a mouth which allows them to have an advantage to their competitors because they can just consume them and will either produce more rediae or start to form cercariae.
Parasite Competition in Snail Hosts
Co-infections of different parasite species within the same host could occur and cause competition between the rediae and sporocysts. Not all trematode species have a redia stage; some may just have a sporocyst stage depending on the life cycle. The rediae are dominant over sporocysts because they have mouths and are able to either eat their competitors' food or their competitors.
Cercaria (plural: cercariae)
The larval form of the parasite develops within the germinal cells of the sporocyst or redia.
A cercaria has a tapering head with large penetration glands.
[ It may or may not have a long swimming "tail", depending on the species.][ The motile cercaria finds and settles in a host where it will become either an adult, or a mesocercaria, or a metacercaria, according to species.
*Mesocercaria: They are involved in an encysted stage either on vegetation or in a host tissue on the second intermediate host. They have a hard shell and are also involved in the trophic transmission. This is where the parasite is able to infect the definitive host because it consumes the second intermediate host that has metacercariae on/in it.
*Metacercaria: A cercaria encysted and resting. They are only involved when there are 3 intermediate host life cycles.
'']Cercaria
A cercaria (plural cercariae) is the larval form of the trematode
Trematoda is a class of flatworms known as flukes. They are obligate internal parasites with a complex life cycle requiring at least two hosts. The intermediate host, in w ...
'' is also used as a genus of trematodes, when adult forms are not known. The usage dates back to Müller, in 1773.[Vermium terrestrium et fluviatile seu animalium infusorium, helminthicorum et testaceorum, non marinorum, succinct historia. OF Müller, 1773]
Adult
The fully developed mature stage. As an adult, it is capable of sexual reproduction.
Deviations from the typical life cycle
Not all trematodes follow the typical sequence of eggs, miracidia, sporocysts, rediae, cercariae, and adults. In some species, the redial stage is omitted, and sporocysts produce cercariae. In other species, the cercaria develops into an adult within the same host.
Many digenean trematodes require two hosts; one (typically a snail) where asexual reproduction occurs in sporocysts, the other a vertebrate (typically a fish) where the adult form engages in sexual reproduction to produce eggs. In some species (for example ''Ribeiroia
''Ribeiroia'' () is a genus of trematode parasites that sequentially infect freshwater snails in the family Planorbidae (ramshorn snails) as first intermediate hosts, fish and larval amphibians as second intermediate hosts, and birds and mammals ...
'') the cercaria encyst
A microbial cyst is a resting or dormant stage of a microorganism, usually a bacterium or a protist or rarely an invertebrate animal, that helps the organism to survive in unfavorable environmental conditions. It can be thought of as a state of ...
s, waits until their host is eaten by a third host, in whose gut it emerges and develops into an adult.
Most trematodes are hermaphroditic
In reproductive biology, a hermaphrodite () is an organism that has both kinds of reproductive organs and can produce both gametes associated with male and female sexes.
Many taxonomic groups of animals (mostly invertebrates) do not have s ...
, but members of the family Schistosomatidae
Schistosomatidae is a family of digenetic trematodes with complex parasitic life cycles. Immature developmental stages of schistosomes are found in molluscs and adults occur in vertebrates. The best studied group, the blood flukes of the genus ...
are dioecious. Males are shorter and stouter than the females.
Representations of life cycles of several different trematode species
See also
* Bucephalus polymorphus
''Bucephalus polymorphus'' is a type of flatworm. This species is within the Bucephalidae family of Digenea, which in turn is a subclass of Trematodes within the phylum Platyhelminthes (i.e. flatworm). It is characterized by having a mouth near ...
* Trematode infection
Trematoda is a class of flatworms known as flukes. They are obligate internal parasites with a complex life cycle requiring at least two hosts. The intermediate host, in which asexual reproduction occurs, is usually a snail. The definitive host ...
* Apicomplexa lifecycle stages
References
External links
Life cycle of ''Schistosoma mansoni''
{{Use dmy dates, date=April 2017
Reproduction in animals
Digenea