The Optimal Transmission Strategy
Since there are at least two host resources (and generally three) that are
exploited during trematode development, there is an equivalent number
of optimal foraging strategies that can be positively selected.
In the classic three-host life cycle, the miracidium forages for a
mollusc, the cercaria forages for a vector and the metacercaria, although
not mobile, forages for a vertebrate. A narrow host range is the rule,
especially for the miracidium, which forages for molluscs. Host speci-
ficity is often less marked for the cercaria, which forages for the vector,
and the metacercaria, which forages for the vertebrate host.
Even though natural selection acts separately at each step of the life
cycle, the end result is an ‘optimal transmission strategy’ (OTS). Taken as
a whole, the OTS is selected to maximize the number of individuals that
reach the appropriate definitive host, where reproduction and genetic
recombination occur. It is likely that trade-offs exist in trematode life
cycles. For instance, one may forecast that a limited production of eggs
might be compensated for by a large production of cercariae and vice
versa. Heavy losses at a given stage should be compensated for by better
success elsewhere in the cycle. Regulatory mechanisms may, if necessary,
intervene to reduce the success of transmission.
As Combeset al. (1994) pointed out, behavioural adaptations of
trematode free stages are largely independent of the actual presence of
a potential adequate host. Although there are various reports in the
literature that miracidia and cercariae can to some extent respond to
physical and chemical stimuli originating in their target host (Haas, 1994;
Haaset al., 1995), these processes invariably act at a very short distance.
Responses to host cues do constitute a part of the OTS, but may at most
give the finishing touch to host finding. There is considerable theoretical
literature on the concept of sequential steps in host finding. This is
especially true for parasitoid insects, where host finding is performed by
the adult stages, which possess sophisticated sensory structures. One of
us (Combes, 2001) considers that there are three main origins of stimuli
that may be perceived by parasite infective stages: habitat of the host,
effective presence of the host and the host itself. Once the encounter
between an infective stage and a particular organism occurs, the latter can
be susceptible (the parasite develops), resistant (the host opposes active
processes to infection) or unsuitable (the host simply cannot be exploited
as a resource).
OTS involves selective adaptations that increase the probability of the
free-living stages to localize themselves in what Combeset al. (1994)
called ‘host space’ and ‘host time’. As stated above, selection tends to
increase the fitness of parasites by retaining genes at each step of the life
cycle. Available data mostly concern cercariae. Spectacular examples are
the distribution of cercariae at various depths in water (Bartoli and
Combes, 1986) and cercarial shedding at various times of the circadian
period (Théron, 1984). These cercarial emergence rhythms tend to limit2 C. Combeset al.