behaviour of potential hosts have been continuously changing on an
evolutionary scale, transmission strategies could be one of the first and
more sensitive targets of selection. Changes imply both host switching
and ‘signal switching’. While it is possible to design phylogenetic trees
of species (trematodes and hosts), it seems impossible to think of a
phylogeny of strategies. That no phylogenetic pattern of strategies
emerges may result either from a lack of research or from the fact
that environment has more influence than developmental or phyletic
constraints. This is a fruitful area for future work.
In most cases, the mechanisms of transmission are better understood
than the determinants of the strategies, and the latter (e.g. why such
a host, why such a signal?) remain to be clarified. Another important
avenue of investigation (Poulin, 2000) will be to examine to what extent
the selection of a given trematode strategy may modify host phenotype in
such a way that host evolution can itself be modified.References
Bartoli, P. (1972) Les cycles biologiques deGymnophallus nereicolaRebecq et
Prévot, 1962 et deG. fossarumBartoli, 1965, espèces jumelles parasites
d’oiseaux de rivages marins (Trematoda, Digenea, Gymnophallidae).Annales
de Parasitologie Humaine et Comparée47, 193–223.
Bartoli, P. and Combes, C. (1986) Stratégies de dissémination des cercaires de
trématodes dans un écosystème marin littoral.Acta Oecologica Oecologia
Generalis7, 101–114.
Bartoli, P. and Gibson, D.I. (1998) A new acanthocolpid cercaria (Digenea) from
Cantharus dorbignyi(Prosobranchia) in the Western Mediterranean.System-
atic Parasitology40, 175–184.
Combes, C. (1991) Ethological aspects of parasite transmission.American Natural-
ist138, 866–880.
Combes, C. (2001)Parasitism. The Ecology and Evolution of Intimate Interactions.
University of Chicago Press, Chicago, 728 pp.
Combes, C. and Théron, A. (2000) Metazoan parasites and resource heterogeneity:
constraints and benefits.International Journal for Parasitology30, 299–304.
Combes, C., Fournier, A., Moné, H. and Théron, A. (1994) Behaviours in trema-
tode cercariae that enhance parasite transmission: patterns and processes.
Parasitology109 (suppl.), S3-S13.
Dawkins, R. (1982)The Extended Phenotype. Oxford University Press, Oxford,
307 pp.
Haas, W. (1994) Physiological analyses of host-finding behaviour in trematode
cercariae: adaptations for transmission success.Parasitology109, S15-S29.
Haas, W., Haberl, B., Kalbe, M. and Körner, M. (1995) Snail-host finding by
miracidia and cercariae: chemical host cues.Parasitology Today11, 468–472.
Haberl, B., Körner, M., Spengler, Y., Hertel, J., Kalbe, M. and Haas, W. (2000)
Host-finding inEchinostoma caproni: miracidia and cercariae use different
signals to identify the same snail species.Parasitology120, 479–486.
Helluy, S. (1982) Relations hôte–parasite du trématodeMicrophallus papillo-
robustus(Rankin, 1940). II. Modifications du comportement desGammarusTrematode Transmission Strategies 11