As adult Fasciola hepatica lives in the liver and bile ducts of sheep as an endoparasite, it is very well adapted for its parasitic mode of life.
In fact, on one hand adult fluke exhibits certain adaptive features and on the other a number of adaptive features may also be accounted from the various stages of its life history.
So, the adaptive features of Fasciola hepatica can be discussed in the following two headings:A. Adaptations of the Adult Fluke.
These can be accounted as under
- Its body is dorsoventrally flattened, leaf-shaped which increases surface area of the body for increased diffusion of substances through fluid of the body.
- Its body is covered with a thick cuticle which protects it from host’s antitoxins.
- Cilia are absent in adult flukes.
- Adhesive organs like suckers (anterior sucker and ventral sucker) well developed which provide it firm attachment with the host tissue. Many cuticular spines over its body erode the host tissue forming its food and also serve in saving the fluke from being pushed away in the ducts with bile.
- Its mouth is situated anteriorly and the muscular pharynx serves for sucking the nutrients from the host body. Since it feeds on predigested and digested substances of the host body, hence, its alimentary canal is not well developed and digestive glands are not found.
- Since process of digestion does not occur, anus is absent and, hence, circulatory system is wanting because the various organs of alimentary canal (intestine and its various branches) distribute the already digested food substances to the different parts of its body.
- Since it lives in an environment which is devoid of oxygen, hence, anaerobic mode of respiration occurs; respiratory organs are completely wanting.
- Its nervous system is very simple and the sense organs are completely wanting, as the flukes are endoparasites.
- Locomotory organs are not found as the flukes lead a well protected life.
- The excretory system consists of a complicated arrangement of branched tubules so as to facilitate the collection of various metabolic excretory wastes of the body.
- The reproductive system is well developed and best suited for its parasitic mode of life.
- Since adult fluke lives in the body of the sheep, hence, it may die with the death of the sheep. Therefore, there is a need of secondary host for the transference of the parasite from one host to the other, so that its race may be continued. Hence, snail is the secondary host.
Adaptations in Life History
The various parasitic adaptive features in the life history of Fasciola hepatica can be accounted as under.
- Production of enormous number of eggs to overcome their wastage during transference.
- The eggs are to pass down the bile duct into the intestine of sheep and then to the outside with its faeces, hence, the fertilised eggs are enclosed in a chitinous covering, the shell, which protects the zygote from the enzymes of the host. The shelled eggs are called capsules.
- Miracidia are the first larvae to come out of the capsules; miracidia are well adapted to lead free swimming life (it has ciliated body to help in swimming, eye spots are developed) and also for entering into the body of the secondary host, Limnaea, Planorbis, etc. (penetration glands help them to enter into snail’s body).
- The sporocyst leads parasitic life; its body is covered in a cyst-like structure to protect it from digestive enzymes of the snail. The germ balls in sporocyst give rise to rediae which may further produce either large number of daughter rediae or cercariae.
- The locomotory organs of rediae (lappets or procruscula) and cercariae (tail) enable them to move and find their way into the fresh tissues of the snail.
- Cercariae find their way out of the body of snail and lead a very short free life and then get enclosed in a cyst secreted by them itself on vegetations.
- The cysted cercariae called metacercariae on vegetation make sure of their entry into the sheep’s body due to herbivorous habit of the sheep.
- Metacercariae can live for a longer period waiting for entry into sheep’s body as they are well protected in cyst to overcome climatic hazards.
- Mode of parthenogenetic reproduction of larvae further ensures the continuity of their race.
However, the high rate of reproduction, adaptations of the larvae, sexual and asexual mode of reproduction and adaptations in the morphology and physiology of the fluke are to ensure the survival and continuity of the race.