Nearly all organisms bear parasites

The most common parasite found in the whelk are several specie of flukes (trematoda), but even certain specie of sporozoans and flatworms (turbellaria) are found. These parasites probably enter the whelk via its food or by boring through its skin. If a whelk should be infected by a tremtod larva, there is a great chance of it becaming sterile, for example an infected whelk has generally a smaller penis. Therefore, penis size can function as an indicator of how long a whelk has been infected. Both sexes are at risk and the risk of infection does not vary during the year. All sizes and ages are exposed to risk of infection, but it has not been observed that older whelks carry more parasites. It is therefore likely that most snails do not survive very long after they have been infected.

Parasites expose water quality

The presence of parasites in whelks can be a sign that the surrounding water is relatively clean. Parasites are much more sensitive for pollution and the absence of parasites indicates that the water is very polluted. By regularly studying the presence of parasites, it is possible to see any unfavourable environmental changes before less sensitive organisms are effected. In Scotland the association between the presence of parasites in whelks and the degree of water pollution has been tested. The number of infected whelks was very low close to an area where sewage dumping occured, but increased in number with increased distance from the source. This is probably due to that certain base metals are poisonous for fluke larvae, which reduces the transfer of parasites to the whelks. Using a Biological indicator by looking at the presence of parasites in whelks is easy when they are common and move over relatively small areas on the sea bottom.


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Bo Johannesson | Martin Larsvik | Lars-Ove Loo | Helena Samuelsson