Hitch-hiking parasite: a dark horse may be the real rider. 
Many parasites engaged in complex life cycles manipulate their hosts in a way that facilitates transmission between hosts.  Recently, a new category of parasites (hitch-hikers) has been identified that seem to exploit the manipulating effort of other parasites with similar life cycle by preferentially infecting hosts already manipulated.  Thomas et al. ( Evolution 51 (1997) 1316) showed that the digenean trematodes Microphallus papillorobustus (the manipulator) and Maritrema subdolum (the hitch-hiker) were positively associated in field samples of gammarid amphipods (the intermediate host), and that the behaviour of Maritrema subdolum rendered it more likely to infect manipulated amphipods than those uninfected by M. papillorobustus.  Here I provide experimental evidence demonstrating that M. subdolum is unlikely to be a hitch-hiker 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 subdolum does not express the appropriate cercarial behaviour for hitch-hiking.