host space ‘transversally’, moving more or less straight on at a relatively
constant speed (Fig. 1.2), whereas cercariae tend to explore it ‘vertically’
moving up and down with frequent resting phases (Fig. 1.3). Transverse
movement appears to be adapted to the exploration of an almost
two-dimensional environment (the habitat of molluscs represented by the
bottom of water bodies). Vertical movement appears to be adapted to the
exploration of a three-dimensional environment (the water column).
There are some exceptions to this scheme, especially when the mira-
cidium is not free and when the cercaria remains attached to the substrate.
Moreover, these two strategies can be explained by the behaviour of the
targets: miracidia have to encounter almost immobile targets, while
cercariae infect mobile hosts, either by active searching or by being con-
sumed by them. In the case of cercariae, it happens that some of these
morphological or behavioural adaptations are signals that make the larval
stages attractive to the DSH (prey mimetism). This is never the case with
the miracidia.Foraging for the Definitive Host
In two-host life cycles, the definitive host takes the place of the vector
and the transmission strategies are of the same type as those described
above.6 C. Combeset al.
Foraging for the second intermediate hostUSHDSHFig. 1.3. Transmission to the second intermediate host is characterized by: (i) a taxo-
nomically homogeneous group of USHs (molluscs) emitting the infective stage (cercaria)
within the environment; (ii) highly morphologically diversified free-living cercariae exploring
the host space vertically; and (iii) target DSHs that are taxonomically diversified, mobile and
spatially dispersed. USH, upstream host; DSH, downstream host.