8.1 Mind games: acanthocephalan infection and behavioral modification  (Page 5/7)

Acanthocephalans also use physiological changes to directly make their intermediate hosts more vulnerable to predation by final host. For example, the acanthocephalan P . laevis has a conspicuous yellow-orange color (see [link] ), caused by the pigments called carotenoids. They are readily visible through the transparent cuticle of its host G. pulex , which makes the infected host become more vulnerable to predation due to enhanced visibility (also called oddity selection ) , as well as due to parasite-induced behavioral changes (Bakker 1997).

Parasite-induced behavioral changes result in predation

Acanthocephalans alter the behavior of their hosts in many ways to make them more vulnerable to predation. First, they can manipulate their hosts’ response to various aspects of light stimuli, including light and wavelength. For example, the acanthocephalan Corynosoma constrictum alters its amphipod intermediate host Hyalella azteca’ s response to higher red (600-700 nm) and green (500-550 nm) wavelength regions. The host uses properties of light to identify various microhabitats, and acanthocephalan infection causes the amphipod to become lost and enter foreign microhabitats. The host also becomes less photophobic and begins to prefer dangerous, well-lit areas (Benesh 2005). Uninfected amphipods usually stay away from such sunlit areas, as well as from areas disturbed by duck movement. However, acanthocephalan infection interferes with such self-preserving tactics and increases the chance of their intermediate host being eaten by their final host (Bethel 1976).

Deep under water, the sense of smell is often more important than sight in detecting aquatic predators. Infection by acanthocephalans reduces the ability of the host to respond to olfactory signals that indicate the presence of predator. For example, acanthocephalan Echinorhynchus borealis causes its host, amphipod Pallasea quadrispinosa, to spend less time hiding, and thus is more exposed, in water containing strong chemical cues from its predator (Benesh et al 2008).

Another way in which acanthocephalans like Plagiorhynchus cylindraceus manipulate their host (isopod Armadillidium vulgare ) is by increasing their tolerance to low humidity (Moore 1983). As the result, the infected isopods spend more time in exposed, dry spots, which causes them to be isolated from the group and more visible to predators. The parasites also increase the visibility of the host by increasing its affinity to light-colored areas, so that they stand out more (Moore 1983).

Serotonin and behavior manipulation: a mechanism of action

One mechanism by which acanthocephalan parasites alter the behavior of their host is through manipulation of neuromodulators (Adamo 2002). For example, P. laevis changes the serotonin (5-HT) levels in the brain its host G. pulex, which causes it to have an inverse reaction to light. Laboratory injection of serotonin into an amphipod, such as G. pulex, causes the same inversed reaction to light, which indicates that the change in serotonin levels is the reason for the behavior and not merely byproduct of the invasion (Tain 2006).