The threespine stickleback fish plays host to the tapeworm Schistocephulus solidus when the threespine stickleback is in a freshwater environment. These Schistocephulus solidus use their presence in the threespine stickleback fish in order to infect their desired prey: birds. In fact, infection with the Schistocephulus solidus causes the fish to behave in a different manner, which increases the chances that they will be eaten by the birds. In addition to behavioral changes, the Schistocephulus solidus causes the fish to lose melanin, making them more visible to the desired predators. Another physical change is that the Schistocephulus solidus appears to cause an increased rate of growth in infected fish.
Ness and Foster investigated the relationship between infection with the Schistocephulus solidus and the threespine stickleback fish's response to predators. Because the Schistocephulus solidus infects the fish in order to be transmitted to the birds that they target, they expected the fish to exhibit some changes after being infected with the Schistocephulus solidus. However, they were unsure whether those changes would cause the threespine stickleback to change their behavior in regard to all predators, or simply in regard to avian predators (Ness and Foster, 128). In order to determine the extent of the behavioral changes caused by Schistocephulus solidus infection, they simulated predators to determine whether the threespine stickleback's response changed across predators. What they found was that infected threespine sticklebacks were less likely to respond to predators of any type, not just avian predators, than non-infected fish (Ness and Foster, 131-132). Furthermore, the scientists were able to dismiss the idea that positive buoyancy was the agent responsible for the behavioral changes in the infected threespine stickleback, but instead determined that the threespine stickleback moved more slowly after infection (Ness and Foster, 130-133). Finally, as expected, because advanced Schistocephulus solidus infection is characterized by demelanization, they found the most dramatic changes in the demelanized threespine sticklebacks (Ness and Foster, 132).
Based on the work of Ness and Foster, Arnott, et. al., knew that Schistocephulus solidus infection caused physical and behavioral changes in the threespine stickleback, which made them more susceptible to predators. However, Arnott, et. al., believed that infection could have positive effects on the threespine stickleback. They posited that Schistocephulus solidus infection could actually increase the size of the host. In order to determine whether or not the Schistocephulus solidus infection could increase the size of the fish, they isolated infected fish and provided them with the same amount of food in order to determine whether the threespine stickleback infection could cause an increase in growth. They discovered that the infected fish were larger than uninfected fish growing under the same conditions (Arnott, et. al., 662).
However, the positive changes in body size did not correlate with other positive changes in the fish. Bagamian, Heins, and Baker posited that, despite the fact that the Schistocephulus solidus infection increased the size of the threespine stickleback fish, it would have a detrimental effect on the reproductive capacity of the threespine stickleback. In general, reproductive activities had a negative effect on body condition, especially in the female threespine stickleback fish (Bagamian, et. al., 1569). What the scientists discovered was that this decline in body condition increased in infected fish (Bagamian, et. al., 1574-1575). Furthermore, males and females responded differently to infection by the parasites; males were negatively impacted by mere infection; however females responded more negatively as the level of parasites in their system increase (Bagamian, et. al., 1573-1576). Finally, they discovered that there appeared to be a threshold body condition for reproduction, as females with lower body condition were less likely to produce eggs than those with a higher body condition (Bagamian, et. al., 1575).
Barber, Walker, and Svennson returned to the work of Foster and Ness, by once again studying the response of the infected threespine stickleback to the presence of predators. However, while Foster and Ness had investigated the response to predators that occurred in naturally infected wild threespine stickleback fish, Barber et. al., studied whether captured fish responded also changed their responses to predators in the same manner after threespine stickleback infection. What they found supported Foster and Ness's findings. Even in captivity, threespine stickleback fish changed their response to predators. This change in behavior was not the result of some unknown variable, because Barber et. al., used sham-exposed fish as a control. In contrast to the…