selection has retained such a favourization process, it means that the
advantages are greater than the inconveniences. Such imprecision can
also provide opportunities for lateral transfers (host switching) during the
course of evolution (Combes and Théron, 2000).
In many cases, selection has been advocated as a result more of
intuition than of experiments. However, limited predation tests have
demonstrated that infected vectors are more prone to consumption than
uninfected individuals. For instance, Helluy (1982) estimated that the
probability of gammarids being ingested by gulls was multiplied by
a factor of four when they harboured metacercariae ofMicrophallus
papillorobustus. Additional convincing results were obtained by Lafferty
and Morris (1996) withEuhaplorchis californiensis. The metacercariae
of this trematode accumulate (up to 1500) in the brain of the killifish
Fundulus parvipinnis. The authors have shown in semi-open conditions
that heavily infected fishes exhibit several conspicuous abnormal
behaviours and were 40 times more susceptible to predation by final host
birds than uninfected controls.
It is difficult to be certain that favourization always results from
natural selection processes. However, it is also difficult to attribute only to
chance the fact that metacercariae often occupy ‘strategic locations’ in the
host. In the examples cited above, one concerns an invertebrate and the
other a fish, but, in both cases, the metacercariae are located in the brain.An Example: Trematode Strategies in a Marine Ecosystem
In this section, we focus our attention on cercarial transmission because,
as stated above, this is the trematode life stage at which transmission
strategies are most diversified. As suggested by various authors (see, for
instance, Combes, 2001), the signals involved in cercarial transmission
can be classified as follows: (i) the information is received by the cercaria
and originates from either the environment of the DSH or from the DSH
itself; and (ii) the information is received by the DSH and comes either
from the cercaria itself or from an intermediate host manipulated by the
encysted cercaria (the cercaria may remain unencysted in some cases). We
shall illustrate these exchanges of information by a series of little-known
examples taken from a marine ecosystem.Information originating from the environment received by the cercaria
Bartoli and Combes (1986) have shown that cercariae of various species of
trematodes, all emitted by benthic molluscs at the bottom of a lagoon,
were sensitive to different signals of the environment (light, gravity,
current, temperature). They respond with different behaviours (positive
or negative phototactism, photokinetism, geotactism, rheotactism) as a8 C. Combeset al.