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Trematoda
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The Trematoda is a class within the phylum Platyhelminthes that contains two groups of parasitic worms, commonly referred to as flukes.
Trematoda are estimated to include 18 000 to 24 000 species, and are divided into two subclasses. Nearly all trematodes are parasites of molluscs and vertebrates. The smaller Aspidogastrea, comprising about 100 species, are obligate parasites of molluscs and may also infect turtles and fish, including cartilaginous fish.
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The Trematoda is a class within the phylum Platyhelminthes that contains two groups of parasitic worms, commonly referred to as flukes.
Taxonomy and biodiversity
The Trematoda are estimated to include 18 000 to 24 000 species, and are divided into two subclasses. Nearly all trematodes are parasites of molluscs and vertebrates. The smaller Aspidogastrea, comprising about 100 species, are obligate parasites of molluscs and may also infect turtles and fish, including cartilaginous fish. The Digenea, which constitute the majority of trematode diversity, are obligate parasites of both molluscs and vertebrates, but rarely occur in cartilaginous fish.
Formerly the Monogenea were included in the Trematoda on the basis that these worms are also vermiform parasites, but modern phylogenetic studies have raised this group to the status of a sister class within the Platyhelminthes, along with the Cestoda.
Life cycles
Almost all trematodes infect molluscs as the first host in the life cycle, and most have a complex life cycle involving other hosts. Most trematodes are monoecious and alternately reproduce sexually and asexually. The two main exceptions to this are the Aspidogastrea, which have no asexual reproduction, and the schistosomes, which are dioecious.
In the definitive host, in which sexual reproduction occurs, eggs are commonly shed along with host feces. Eggs shed in water release free-swimming larval forms that are infective to the intermediate host, in which asexual reproduction occurs.
A species that exemplifies the remarkable life history of the trematodes is the bird fluke, Leucochloridium paradoxum. The definitive hosts, in which the parasite multiplies, are various woodland birds, while the hosts in which the parasite grows (intermediate host) are various species of snail. The adult parasite in the bird's gut produces eggs and these eventually end up on the ground in the bird's faeces. Some very fortunate eggs get swallowed by a snail and here they hatch into tiny, transparent larva (miracidium). These larvae grow and take on a sac-like appearance. This stage is known as the sporocyst and it forms an central body in the snail's digestive gland that extends into a brood sac in the snail's head, muscular foot and eye-stalks. It is in the central body of the sporocyst where the parasite replicates itself, producing lots of tiny embryos (redia). These embryos move to the brood sac and mature into cercaria. The young trematodes have come a long way, but they must still get into the digestive tract of a bird and to do this they take over the snail and control its behaviour. The sporocysts in the snail's eye-stalks begin to swell, change colour and pulsate. The parasite then somehow influences the snail to wander out into the open with it eye-stalks flashing like beacons. Such unusual behaviour doesn't go unnoticed for long and the snail is soon snatched and eaten by a bird. The bird, unknowingly, has aided the completion of the parasite's lifecycle and soon after being swallowed the larval parasites will mature and take up residence in the bird's gut.
Infections
Human infections are most common in the Orient, Africa, South America, or the Middle East. However, trematodes can be found anywhere that human waste is used as fertilizer.
Etymology
Trematodes are commonly referred to as flukes. This term can be traced back to the Old English name for flounder, and refers to the flattened, rhomboidal shape of the worms.
The flukes can be classified into two groups, on the basis of the system which they infect in the vertebrate host.
They may also be classified according to the environment in which they are found. For instance, pond flukes infect fish in ponds.
External links
- [Aspidogastrea knol I
- [Aspidogastrea knol II
- [Aspidogastrea knol III
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