Significant features in the life cycle of fasciola are listed below:
1. Complex life cycle:
Development is a long and complicated process, involving five different larval stages. Further, it is digenetic i.e., completes its life cycle in two alternating hosts. Sheep is the primary definitive host and the secondary host is freshwater snail.
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2. More chances of death:
Due to prolonged and complex life cycle different larval forms face more chances of death because:
(i) The encapsulated embryos, after coming out of the host faeces, can’t survive if they don’t find water medium with suitable pH and temperature.
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(ii) The miracidium larva will die soon if it fails to reach the suitable intermediate host within 24 hrs. Out of thousands of larvae only a few are able to find the host (snail).
(iii) The cercaria also faces uncertainty of life during cystment.
(iv) Metacercaria faces chances of death in quest of geting eaten by its primary host, sheep.
3. High rate of reproductiony:
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To overcome all the risks of death during life cycle Fasciola has a high rate of reproduction. A sheep may have about 200 flukes.
Each fluke in its life cycle produces 200,000 eggs, Further, each sporocyst produces 8-12 rediae of second generation. 14-20 cercaria are produced by each redia. Thus from a single egg thousands of cercariae larva are formed.
4. Heterogamy Vs polyembryony:
Grobben (1882) and some others believed that the germ balls in the sporocyst and redia were eggs and they develop parthenogenetically in larval forms which is known as hetrogamy.
But now this view is considered to be wrong and a new concept of polyembryony has been introduced.
Germ cells are actually the direct descendants of propagatory cells which are produced by first division of embryo.
These germ cells multiply mitotically and produce subsequent larval stage within sporocyst and radia.
In doing so they behave as embryo and this phenomenon is described as polyembryony (after Ishit (1934) Chen (1937) Rees (1940) and Cart (1944).
5. Metagenesis Vs. alternation of generation:
In Fasciola asexual reproduction by germ cells of sporocyst and redia followed by sexual reproduction in the adult stage.
Stenstrup et.al (1942) interprets this as the alternation of generation in the life cycle of Fasciola.
Others call this metagenesis. But Hymen is of the view that it is a continuous ontogeny (life history) involving asexual multiplication in larval stages.
6. Advanced larval stages:
Adult Fluke is endoparasite and exhibits degenerative characters, but larvae especially miracidium and cercaria are free swimming possess advanced characters and considered to be more advanced stages.