Biology Reference
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eggs are laid containing fully developed larvae that are eaten by snails and
hatch immediately. If eggs were laid at an earlier stage of development,
the chances of infecting snails would probably be reduced to zero, because
eggs would not survive in the snail if eaten immediately, and they would
not survive on the sea floor long enough to complete development, due
to periodic exposure at low tides. There is a relatively small number of
sensory receptors, because an intermediate host is not actively searched
for. In Multicotyle purvisi, eggs are laid that have to develop over almost a
month on the bottom of freshwater bodies before they can hatch. This
facilitates production of larger numbers of eggs than if eggs containing
fully developed larvae were produced, increasing chances of infection.
On the other hand, it restricts completion of the life cycle to calm waters,
because in strong currents eggs as well as larvae floating in the water
would be swept away. Hatching occurs in the morning (probably facili-
tated by the eyes) to ensure that snails, which are apparently active during
the day, become infected. Larvae have an astonishing array of sensory
receptors, ciliary tufts, and a dense layer of microfila, to ensure swimming
and floating in the water column and inhalation by a snail host. Larvae,
furthermore, have to find their way to the kidneys, probably also facili-
tated by the receptors. The very great number of receptors in juveniles
and adults is likely to play a role inside the vertebrate host. Mate finding,
feeding, and (not very likely, because the stomach and small intestine can
hardly be missed) finding the way to their microhabitat are the only
possibilities. They may also help in reducing damage to the delicate
snail tissue. Lack of an anterior ''pseudosucker'' in Multicotyle may be an
adaptation to the delicate kidney tissues, in which the juvenile develops.
Damage to the host may thus be avoided. In contrast, the large and
muscular pseudosucker of Lobatostoma may help in feeding, i.e., biting
off pieces of robust host tissue, the digestive gland of snails.
As just mentioned, intermediate hosts are also infected with digenean
trematodes, but there is no evidence whatsoever that interspecific com-
petition has been in any way involved in the evolution of the very
intricate morphological and behavioral adaptations of the two aspido-
gastrean species. In other words, there is no evidence for coevolution
with other, competing species. This is also the case if we consider other
aspidogastreans. Rugogaster infects the caecal glands of chimaeras,
Multicalyx the gall bladder and bile ducts of chimaeras, sharks, and rays,
and Stichocotyle the bile ducts of rays. Each species has adapted to its host
and microhabitat within the host over millions of years, although we do
not know what these adaptations are. There are many more potential host
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