Hitch-hiking parasite: a dark horse may be the real rider

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 Science Ltd. All rights reserved.