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Digenetic parasites and their amazing life cycle

© Esther Wullschleger

Some parasites have astonishing life cycles: their young grow up in different host species than the adults. One group of parasites, the digenetic trematodes (also called ?flukes?) use mostly aquatic snails as their first hosts, as sort of a ?kindergarten? where the larvae grow. The second hosts in the life cycle are a number of animals which eat snails or occur (at least part of the time) in habitats with snails - mostly fish, frogs, rats, or water birds. Snail-eating hosts guarantee, as an expert once said, that all the larval parasites from a large pond area are ?directly canalized into the gullet of a heron? (for example). Otherwise, the parasites would have a much lower chance to find their second host. In some cases, even a third host may be used. This third host in the life cycle is, again, often a species which eats the second host.

An infected bird will release the eggs of this parasite into the water. The first larvae, tiny organisms called miracidia, will hatch from the egg and swim through the water in search of a snail. It burrows into the snail, eats the snail?s tissues, and grows to become an amazing creature: a large sack-like creature, within which further larval stages grow. Although it seems to produce ?young?, this stage is not an adult parasite, as there is no exchange of genes with other individuals. It reproduces in a manner similar to vegetative reproduction, resulting in the ?birth? of a large number of genetically identical larval stages. These larval stages will also grow to become similar sack-like creatures, and produce even more larvae inside their bodies!

After many rounds of sack-like creatures giving ?birth? to more sack-like creatures, what finally gets released from the sack-like creatures are another type of larvae. These are built differently, looking a little bit more like the adults, but with a tail that facilitates swimming. This stage is called a cercaria. The cercaria?s purpose of existence is to leave the snail, swim very efficiently through the water, and very rapidly find a suitable second host ? before its energy to swim is exhausted.

A number of such cercariae have been found in snails, described, drawn, and examined by modern laboratory techniques. Although the cercariae are fairly well-known, the adults of many species have never been found. This is because it is very difficult to follow the life cycle of the parasites through all of their hosts. After all, trematode parasites occur in a large variety of host species, and an array of more or less unknown trematode species inhabit most freshwater bodies where snails are found. For example, a detailed scientific study in Germany revealed eight trematode species within a single pond!

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