the temporal dispersion of the short-lived infective cercariae by concen-
trating them in the period of time when the chances of meeting the host
are the greatest. Interspecific variability of cercarial emergence rhythms
correlated with different periods of host activity is well documented
(Combes et al., 1994). Genetic differences in the timing of cercarial
shedding have also been demonstrated between sympatric populations of
parasites belonging to the same species (Théron and Combes, 1988). Such
chronobiological polymorphism might occur when two species of DSH
with contrasted periods of activity are involved in the parasite life cycle.
This is the case forSchistosoma mansonion the island of Guadeloupe,
where humans, with diurnal water contacts, and rats, with nocturnal
behaviour, are both exploited by the parasite (Théron and Pointier, 1995).
A strong disruptive selection maintains distinct populations of schisto-
somes with early cercarial shedding adapted to human behaviour and
with crepuscular cercarial shedding adapted to rat behaviour (Théron and
Combes, 1995). Success in reaching host space and host time depends on
the selection of adaptive responses to stimuli coming from the environ-
ment. This selection is most important when the USH and the DSH are at
different points in space and time (Fig. 1.1).
One may wonder why foraging strategies are more variable among
trematode species compared with predator–prey systems (and probably
most parasites, parasitoids excepted), while targets can be similar. The
reason is not in the signals (visual, chemical or acoustic cues) emitted by
the targets, but in the limited physical ability of the infective stages to
detect and respond to the corresponding information. Infective stages are
minute compared with their hosts. They therefore lack sufficient energy
and morphological adaptations to learn or to search for and pursue their
hosts in the way that predators search for and pursue prey. This explains
why natural selection may act to retain more general responses to
environmental cues: the infective stages do not search for potential hosts
directly, but rather for the space and time where the probability of
meeting them is the highest. Whereas data and models are available
in parasitoid–host systems, the individual probability of a trematode
infective stage to transmit its genes still represents a black box.
For both transmission phases, this ‘generalist’ strategy of dispersal
towards the potential host space is reinforced by the high reproductive
capacity of trematodes, which leads to a quasi-permanent flow of numer-
ous infective stages into the environment (with the exception of seasonal
fluctuations). High productive rates nearly always characterize the mira-
cidial and cercarial phases of the life cycle. This can be seen as a typical
adaptation of parasites. However, in the case of trematodes, this strategy
has two additional advantages.
First, two different processes of multiplication occur in parasitic
stages that exploit independent resource patches, which may act as a
mechanism to avoid competition between larval and adult stages. This
can be compared to the case of many insects and amphibians, where
larvae and adults have a totally different diet in different habitats.Trematode Transmission Strategies 3