Parasitic fauna of marine molluscs

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Within the phylum mollusca, trematode parthogenetic generations take place (sporocyte and rediae); these molluscs are only the intermediate hosts for these trematode parasites. Development of these stages takes place within the gonad and the digestive gland of the host. This can have devastating results upon the host species because there is a constant battle going on within the host to gain energy and resources and also for the parasite within its habitat (the host) to gain energy and recourses. A consequence of this is the parasite usually wins the battle and the host undergoes histochemical and biochemical changes.

These changes can include stunted growth due to only gaining the energy and recourses that are left after the parasite has gained what it requires. Gigantism can also occur in the host whereby it undergoes rapid growth after the host gains much energy and resources after castration, thus a sacrifice. However the degree of resources and energy that the parasite can gain from the host needs to be kept in check in order for the parasite to be able to produce its own progeny (Sorensen and Minchella 2001). The aim of this investigation is to examine two species of mollusc, Littorina littorea and Littorina saxtilis to determine if they are infected or uninfected with parasitic trematodes.

Null hypothesis.

1. There will be no difference between the parasite and what host it infects, thus no host specificity.

2. There will be no difference in the size of the host and the number of parasites that infect it.

3. There will be no correlation between host habitat and parasite specificity.


Take four samples of each of the two specimens L.littorina and L. saxtilis and measure in mm the height of their shells. Gently take one sample at a time and gently tap with a hammer and remove the shell. Cut of the foot and mount the visceral hump on a glass slide. Using another glass slide gently press down on the other slide to spread out the sample. Under the light microscope examine the sample for infection and record the stage at which infection is occurring and the trematode that is infecting the sample. Carry out the procedure for all of the samples.


With regards to Cercaria littorina saxatilis V and Microphallus pygmaeus the null hypothesis was rejected and thus there was a significant difference in host specificity for L. saxitilis. Therefore there may be an underlying factor influencing the reason for this result beyond the boundaries of this observation that is linking these two particular parasites with the above host species. The null hypothesis was accepted with regards to the size of the two specimens used and their parasite number and also their habitat type. Hence there is no specificity by the parasites found in the size and habitat type of their host. It seems likely that they are not infecting host species based on these two factors.


It seems evident that the two parasite species above would be worthy of further study to determine the reason why in this investigation they seemed to have host specificity for L. saxitilis. (Fig2) Evidence in this observation showed that it was not the size of the host or their particular (Fig3)habitat type that determined the infection. Hence another possible environmental factor may be influencing these particular parasites to infect L. saxitilis. There could possibly be factors such as temperature, desiccation, and salinity of the water that are influencing these results. It may become apparent with further study that there may well be some kind of host-parasite combination with their own set of rules associated with some other ecological and physiological factors. This may have laid the path to their possible co-evolution (Gorbushin and Lerakin 1999).

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