Fisheries & Aquaculture News

Sticklebacks Infected with Parasites Influence Behavior of Healthy Fish

A team of evolutionary biologists around Dr. Jörn Peter Scharsack at the University of Münster (Germany) have demonstrated for the first time that the tapeworm not only influences the behavior of the infected fish -- indirectly, it can also induce equally risky behavior in other healthy fish in the group. The study is published in the current issue of the journal Proceedings of the Royal Society B.

Parasites which are passed on via the food chain often influence the behavior of their host to their own benefit. One example of this is the tapeworm Schistocephalus  solidus, which makes three-spined sticklebacks behave "carelessly." The infected fish venture more often into open waters, making themselves easier prey for piscivorous birds, e.g. kingfishers.

EMBED 1 9647 BAF3o7ES…and his parasite: a tapeworm of the Schistocephalus solidus species © WWU/Jörn Scharsack

In laboratory experiments, the researchers demonstrated that in schools of stickleback in which the number of infected fish exceeds that of healthy fish, this healthy minority imitates the changed behavior of the infected members of their group.

"The reason for this 'wrong' decision on the part of the non-infected sticklebacks presumably has something to with shoaling behavior," says Jörn Scharsack. "The urge to remain in the group is stronger than exercising caution against any attack by a bird independently." The other way around, however, it is different. The infected fish display risk-friendly behavior in any case and do not take their lead from the cautious behavior of the healthy fish when these are in the majority.

The researchers suspect that, in the wild, the ability of the parasite to have an indirect influence on the behavior of healthy sticklebacks could also have an effect 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.

The researchers involved in the study were not only evolutionary biologists from the University of Münster, but also researchers from Berlin: from the Leibniz-Institute of Freshwater Ecology and Inland Fisheries, the Max Planck Institute for Human Development, and Humboldt University.

Tapeworm of the species Schistocephalus solidus have a complex life-cycle. The first tapeworm larva swims freely in the water and has to be eaten by a small copepod. Inside the crustacean, the larva grows until the copepod is itself swallowed, together with the larva, by a three-spined stickleback which grows up to eleven centimeters in length. Inside the stickleback, the tapeworm grows enormously. After it has pierced the intestinal wall of the fish and has migrated into its abdominal cavity, it can attain up to 50% of the weight of its host. Nevertheless, the fish continues to live with the parasite inside it. The tapeworm has reached its aim when the stickleback becomes prey to a bird, because then the tapeworm can reproduce in the bird's intestines. The tapeworm eggs are transferred with the bird's excrement back into the water -- where the cycle begins all over again. 


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