Effect of parasite-induced behavioral alterations on juvenile development

Like many trophically transmitted parasites, the trematode Microphallus papillorobustus alters the behavior of its intermediate host, the crustacean gammarid Gammarus insensibilis, in a way that favors its vulnerability to definitive hosts (aquatic birds). Parasitized females still produce eggs, but because juvenile development occurs inside the female marsupial brood pouch, young gammarids are subject to the same risk of predation as their mothers until they exit the marsupium. We explored the idea that developing juveniles can adjust their developmental schedule in a state-dependent manner according to the parasitic status of the mother. We predicted that juveniles from parasitized females would accelerate their development, or exit the marsupium at an earlier stage, to avoid predation by birds. Contrary to our expectations, we observed the opposite, that is, juveniles from parasitized females exited the marsupial brood pouch significantly later than those from uninfected mothers. We discuss these results in relation to current ideas on host manipulation by parasites in ecosystems.