infrapopulations, as demonstrated by Mulvey et al. (1991) for
Fascioloides magnainfecting the white-tailed deer.Foraging for the First and Second Intermediate Hosts
Transmission to the first and second intermediate hosts is, in both cases,
accomplished by free-living stages, which are usually aquatic. However,
marked differences characterize these two life-cycle phases depending on
the USH and DSH involved in this process. Regarding transmission to the
first intermediate host, USHs, where trematode eggs are produced and
dispersed, are very diverse and include all classes of vertebrates. They are
also highly mobile and located in a variety of different environments
(aquatic, terrestrial, aerial). The target DSHs are all members of a single
taxonomic group (molluscs) and are sedentary benthic organisms (Fig.
1.2). In contrast, during transmission to the second intermediate host, the
USHs (where cercariae are produced, but not or little dispersed) are
homogeneous (molluscs), while the target DSHs are highly diverse and
include many classes of invertebrates and vertebrates, which are mobile
and spatially dispersed (Fig. 1.3).
This may explain why morphological adaptations and behaviours
differ remarkably between the two infective stages, miracidia and
cercariae: (i) miracidia exhibit narrowly diversified morphology (Fig. 1.2),
whereas cercariae exhibit a fantastic diversity of sizes and shapes (Fig.
1.3); and (ii) it seems to us that, in most species, miracidia explore theTrematode Transmission Strategies 5
USH Foraging for the first intermediate hostDSHFig. 1.2. Transmission to the first intermediate host is characterized by: (i) a highly
diversified pool of mobile USHs, producing and dispersing the infective stages (eggs and
miracidia) within the environment; (ii) poorly morphologically diversified free-living miracidia
exploring the host space transversally; and (iii) target DSHs that are taxonomically
homogeneous (molluscs) with little mobility. USH, upstream host; DSH, downstream host.