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.
Nearly all organisms bear parasites
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.
Parasites expose water quality