8.1 Mind games: acanthocephalan infection and behavioral modification  (Page 4/7)

Acanthocephalan host specificity

Acanthocephalans are highly successful in causing elaborate changes in the behavior of their intermediate host; however, their mechanisms of manipulation are often highly specific to that particular host and do not work on other species. Consequently, parasites that are adapted to a local host may have little to no influence on an invasive host (i.e. one that has recently arrived to the area). For instance, the P. laevis parasite, whose local host is the amphipod G. pulex , fails to alter the behavior and immune system of a recent invader which is also an amphipod, the crustacean G. roeseli (Moret et al 2006). The invading population’s resistance to parasitism gives it a selective advantage over the native population, and helps it invade the area successfully. This finding suggests that the two amphipod species have evolved due to parasite-mediated competition with each other, which demonstrates a profound effect a parasite’s specificity may have on its host in terms of its species evolution (Bauer 2000).

Two acanthocephalans may have different behavioral effects on the same intermediate host. For instance, P. laevis and P. minutus have the same intermediate host, G. pulex . However, the definitive host of P. laevis is freshwater fish, while the definitive host of P. minutus is a waterbird. In the presence of a fish predator, the amphipods infected with P. laevis remained outside of refuge more and spend more time in areas with the predator ‘odor’ than the amphipods infected with P. minutus , (Kaldonski 2007).

Physiological effects of an acanthocephalan infection

Acanthocephalan infection can cause drastic changes of the host’s physiology and distribution of resources. In particular, the parasites often decrease the reproductive capability of the intermediate host because their transmission to the definitive host does not depend on the survival of the intermediate. The significant amount of resources and time that the host spends on finding a mate and producing offspring could be used instead to maximize the parasite’s own fitness and chances of advancing to the next host.

For example, the acanthocephalan Pomphorhynchus laevis causes its intermediate amphipod host, Gammarus pulex , to have lower lipid content in pregnant females. Females need lipids for yolk synthesis, and cannot successfully reproduce without them. The parasites instead use the energetic resources to increase the intermediate host’s survivorship until it is eaten by the parasite’s definitive host (Plaistow 2001).

The parasites can also reduce reproductive capability of its male hosts. When the isopod, Caecidotea intermedius, is parasitized by Acanthocephalus dirus, the host males still develop sperm and have the same fertilization ability as noninfected males. However, they are less likely to initiate mating in noncompetitive situations and are less responsive to females (see [link] ) (Bierbower 2007). Likewise, acanthocephalan infection causes the amphipod G. pulex males to have significantly lower pairing success and to be less likely to engage in competition with other males for females (Bollance 2000). The parasite can use the energy that the host would spend on competition and mating for its own growth.