Many parasites engaged in complex life cycles manipulate their hosts in a w
ay that facilitates transmission between hosts. Recently, a new category of
parasites (hitch-hikers) has been identified that seem to exploit the mani
pulating effort of other parasites with similar life cycle by preferentiall
y infecting hosts already manipulated. Thomas et al. (Evolution 51 (1997) 1
316) showed that the digenean trematodes Microphallus papillorobustus (the
manipulator) and Maritrema subdolum (the hitch-hiker) were positively assoc
iated in field samples of gammarid amphipods (the intermediate host), and t
hat the behaviour of Maritrema subdolum rendered it more likely to infect m
anipulated amphipods than those uninfected by AL papillorobustus. Here I pr
ovide experimental evidence demonstrating that M. subdolum is unlikely to b
e a hitchhiker in the mentioned system, whereas the lucky candidate rather
is the closely related but little known species, Microphallidae sp. no. 15
(Parassitologia 22 (1980) 1). As opposed to the latter species. Maritrema s
ubdolum does not express the appropriate cercarial behaviour for hitch-hiki
ng. (C) 2001 Australian Society for Parasitology Inc. Published by Elsevier
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