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The conflict is exacerbated when multiple parasites infect a single host and find themselves in competition for the host’s resources. One might expect that each would try to consume as many of host’s nutrients as possible and grow as much as possible to maximize its own fitness. Instead, the growth of parasites in crowded conditions appears to slow down over time. One possible explanation may be that the parasites are related and they are sharing the resources equally to ensure that all the relatives have maximum possible fitness and pass on the genes (Benesh 2007).
If another type of parasite, like the microspordium Dictyocoela (roeselum), infects the same host as the acanthocephalan (e.g. P. minutus ), a competition between two survival strategies may ensue (Haine 2005). The microspordia are transmitted vertically, from infected parent to offspring, so they are highly invested in the reproductive fitness of their host. On the other hand, the acanthocephalans are transmitted horizontally, that is between unrelated hosts, and thus are not invested in their hosts’ reproductive success. In fact, they sometimes manipulate the behavior or physiology of their host in order to use the resources like time, energy, and nutrients that would normally go toward reproductive activities to increase their own fitness. So, when these two parasites infect the same host, an obvious conflict of interest occurs. In these situations, the microspordia are not above sabotage, and cause P. minutus to have less success in manipulating the behavior of the host (Haine 2005).
In order to inhabit the host’s body for significant amount of time, the parasite has to bypass the immune system defenses (Rigaud 2000). The fact that they often manage to do so may indicate co-evolution of the two organisms. If the host’s immune system were able to kill the parasites, then there would be strong selective pressures for the acanthocephalans to develop anti-immune system defenses, because only those parasites that have these defenses would be able to survive and pass on their genes. However, a growing prevalence of immune system resistant parasites would put selective pressures on the host to develop new mechanisms against infection, especially since the parasite dramatically decreases the host’s reproductive fitness. Thus, the two organisms are in an arms race. Furthermore, the parasite’s ability to suppress an immune response is specific to a particular host species, and does not work on invasive species (Rigaud 2000).
| Parasite Species | Intermediate Host | Definitive Host |
| Acanthocephalus dirus | Isopod: Caecidotea intermedius | Chub: Semotilus atromaculatus |
| Acanthocephalus lucii | Isopod: Asellus aquaticus | Perch: Perca fluviatilis |
| Pomphorhynchus laevis | Amphipod: Gammarus pulex | Perch: Perca fluviatilis |
| Corynosoma constrictum | Amphipod: Hyalella azteca | Variety of waterfowl |
| Echinorhynchus borealis | Amphipod: Pallasea quadrispinosa | Burbot: Lota lota |
| Plagiorhynchus cylindraceus | Isopod: Armadillidium vulgare | Starling: Sturnus vulgaris |
| Polymophus paradoxus | Crustacean: Gammarus lacustris | Mallard ducks, muskrats, and beavers |
| Polymorphus minutus | Crustacean: Gammarus roeseli | Waterbird |
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