Trematodes Trematoda is a class of flatworms known as trematodes, and commonly 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 a moll ...

are parasitic

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 of the class ''Trematoda'', specifically parasitic flukes with two suckers: one

ventral Standard anatomical terms of location are used to describe unambiguously the anatomy of humans and other animals. 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 ora ...

. 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 ...

. Some trematode eggs hatch directly in the environment (water), while others are eaten and hatched within a host, typically a

mollusc Mollusca is a phylum of protostome, protostomic invertebrate animals, whose members are known as molluscs or mollusks (). Around 76,000 extant taxon, extant species of molluscs are recognized, making it the second-largest animal phylum ...

. The hatchling is called a '' miracidium,'' 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 A species () is often defined as the largest group of organisms in which any two individuals of the appropriate sexes or mating types can produce fertile offspring, typically by sexual reproduction. It is the basic unit of Taxonomy (biology), ...

, 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) are operculated. Some eggs are eaten by the intermediate host (snail) or they are hatched in their habitat (water).

Miracidium

Miracidia 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 the larva forms. 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, 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 three intermediate host life cycles. Species of family Syncoeliidae have mesocercariae or metacercariae that are not encysted and can be free floating, but details of the early stages of the life cycles of these marine parasites are not known. Bony and cartilaginous fish are the definitive hosts within at least the gills, oral cavity or skin, and crustaceans like

krill Krill ''(Euphausiids)'' (: krill) are small and exclusively marine crustaceans of the order (biology), order Euphausiacea, found in all of the world's oceans. The name "krill" comes from the Norwegian language, Norwegian word ', meaning "small ...

and

copepod Copepods (; meaning 'oar-feet') are a group of small crustaceans found in nearly every freshwater and saltwater habitat (ecology), habitat. Some species are planktonic (living in the water column), some are benthos, benthic (living on the sedimen ...

s can be paratenic or possibly intermediate hosts. Metacercariae of certain species, such as '' Copiatestes filiferum'', have long filamentous structures termed byssal filaments, which in ''C. filiferum'' have been reported to foul the feet of white-faced storm petrels and cause snagging-related mortality in this accidental host after the metacercariae dry out and form hardened connections between the legs. ''

Cercaria A cercaria (plural cercariae) is a larval form of the trematode class of parasites. It develops within the germinal cells of the Trematode life cycle stages, sporocyst or redia. A cercaria has a tapering head with large penetration glands. It may ...

'' 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'') the cercaria encysts, waits until their host is eaten by a third host, in whose gut it emerges and develops into an adult. Most trematodes are

hermaphroditic 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 ...

, 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 genu ...

are

dioecious Dioecy ( ; ; adj. dioecious, ) is a characteristic of certain species that have distinct unisexual individuals, each producing either male or female gametes, either directly (in animals) or indirectly (in seed plants). Dioecious reproduction is ...

. Males are shorter and stouter than the females.