gyration, the cercaria emits a succession of flashes, which attract the
attention of microphagous fishes.Information originating in the manipulated USH received by the DSH
This is an example where an extension of the parasite’s phenotype into
the host’s phenotype occurs (see Dawkins, 1982). The exchange of signals
between the parasitic stage and the target organism is thus indirect. By
their localization around the nerves of the locomotory legs of the sand
crab (Carcinus mediterraneus), the metacercariae ofMicrophallus bittii
handicap the movements of the USH, facilitating its predation by gulls,
the definitive host (Prévot, 1974). Interestingly, two other microphallid
species parasitize the same crab species, but the localization of their
metacercariae is different. Those ofMegalophallus carciniaccumulate
in the digestive gland and those ofGynaecotyla longiintestinatain the
urinary lobules. There is no evidence thatM. carciniorG. longiintestinata
manipulate their hosts, but they obviously benefit from the extended
phenotype ofM. bittii, since they seek the same DSH. Associations of
manipulative and non-manipulative species in the same host have been
described by Helluy (1984) and more recently by Thomaset al. (1998).
The latter discuss to what extent the non-manipulative species are ‘hitch-
hikers’ or simply ‘lucky passengers’.Concluding Remarks and Future Directions
The above examples show that, in a single ecosystem, the circulation of
information that characterizes trematode cercarial transmission is highly
diversified. If we try to compare this diversity with the taxonomy and
phylogenetic relationships either of trematodes or of hosts, it is almost
impossible to find a correlation. For instance, the response of cercariae to
vibratory cues, the location of metacercariae in a particular organ, etc.,
have been selected in many different trematode families. Conversely,
closely related species of trematodes can use different strategies and even
different types of hosts. A striking example is that ofMeiogymnophallus
nereicola, whose cercariae settle in annelids, andMeiogymnophallus
fossarum, whose cercariae settle in molluscs (Bartoli, 1972). Both are
parasites of oyster-catcher definitive hosts. This suggests that natural
selection has frequently modified trematode transmission strategies in the
course of evolution. Obviously, using an annelid or a mollusc constitutes
equivalent strategies in terms of the fitness of these parasites. It is difficult
to account for the fact that each of two related trematode species
(classified into a single genus) uses only one of these strategies, but
avoiding competition at this stage of the life cycle (metacercariae) could
be a working hypothesis. Also, an OTS is vital for the species and, because
the environmental conditions, composition of faunas, densities and10 C. Combeset al.