Hairworm response to notonectid attacks

Very few parasite species are directly predated but most of them inherit the predators of their host. We explored the behavioural response of nematomorph hairworms when their hosts are preyed upon by one of the commonest invertebrate predators in the aquatic habitat of hairworms, notonectids. The hairworm Paragordius tricuspidatus can alter the behaviour of its terrestrial insect host (the cricket Nemobius sylvestris), causing it to jump into the water; an aquatic habitat is required for the adult free-living stage of the parasite. We predicted that hairworms whose hosts are captured by a notonectid should accelerate their emergence to leave the host before being killed. As predicted, the emergence length of the worm was significantly shortened in cases of notonectid predation, but the exact reason of this response seems to be more complex than expected. Indeed, experimental manipulations revealed that hairworms are remarkably insensitive to a prolonged exposure to predator effluvia which notonectids inject into prey, so accelerated emergence is not a protective response against digestive enzymes. We discuss other possibilities for the accelerated exit observed, ranging from unspecific stress responses to other scenarios requiring consideration of the ecological context.