Suppression of predation on the intermediate host by two trophically-transmitted parasites when uninfective.

Trophically-transmitted parasites generally need to undergo a period of development in the intermediate host before reaching infectivity. During this vulnerable period, manipulation of the host to reduce susceptibility to predation would be advantageous for parasites, because it increases the probability of surviving until infectivity and thus the probability of transmission. We tested this 'predation suppression' hypothesis in 2 parasite species that use copepods as first hosts: the tapeworm Schistocephalus solidus and the nematode Camallanus lacustris. In a series of prey choice experiments, we found that copepods harbouring uninfective, still-developing worm larvae were less frequently consumed by stickleback predators than uninfected copepods. The levels of predation suppression were similar in the two parasite species, suggestive of convergent evolution. Additionally, copepods harbouring 2 worms of a given species were not more susceptible to predation than those with 1 worm, suggesting that excessive larval parasite growth does not increase host susceptibility to predation. Our results support the idea that parasites can suppress intermediate host susceptibility to predation while uninfective, but we also note that the available studies suggest that this effect is weaker than the frequently observed enhancement of host predation by infective helminth larvae.