The three-spined sticklebacks' shoaling behaviour means that individual fishes within the group will follow the behaviour of the majority. This is well and good, but can have deadly consequences if the majority have been infected with the Schistocephalus solidus tapeworm, according to a new study.
Parasites like the tapeworm alter their host’s behaviour to their own advantage. The Schistocephalus solidus tapeworm, which infects three different animals as part of its life cycle, can sometimes influence the behaviour of healthy animals as well, albeit indirectly.
This is the findings of a study recently published in the journal Proceedings of the Royal Society B.
Three-spined sticklebacks that have been infected with the tapeworm will swim towards the surface of the water, a risky move that runs contrary to their normal behaviour. This exposes them to the possibility of being eaten by birds, something the tapeworm wants: It wants the fish to be eaten by a bird so it can reproduce inside the bird's gut.
However, researchers have discovered that the non-infected individuals within the school of fish will imitate the infected sticklebacks and swim to the surface as well.
“The reason for this ‘wrong’ decision on the part of the non-infected sticklebacks presumably has something to with shoaling behaviour,” said Dr Jörn Peter Scharsack, from University of Münster. “The urge to remain in the group is stronger than exercising caution against any attack by a bird independently.”
The research team discovered that this holds true only if the infected sticklebacks form the majority within the school of fish. If it turns out that the non-infected sticklebacks are the majority, they will stick to their normal behaviour and not follow their infected peers to the water surface.
According to the University's press release, the researchers suspect that the tapeworm's influence on the healthy fish may also have an impact on stickleback and bird populations: "More birds could be lured, for example, because more fish means more attractive prey. The predators’ urge to eat fish could thus increase, and ultimately more tapeworm could get into the birds’ intestines and reproduce there."