Experimental infection with the directly transmitted parasite Gyrodactylus influences shoaling behaviour in sticklebacks

Animals usually benefit from joining groups, but joining a group can also come at a cost when members expose themselves to competition and the risk of contracting a contagious disease. Therefore, individuals are expected to adjust grouping behaviour to the ecological circumstances, their own competitiveness and the composition of the group. Here, we used experimental infections and classic binary choice tests to test whether the monogenean flatworm Gyrodactylus spp. has the potential to influence shoaling behaviour in the three-spined stickleback, Gasterosteus aculeatus, a model organism in behavioural ecology and evolutionary biology. Gyrodactylus spp. is a genus of widespread and rather inconspicuous, small (<0.5 mm) ectoparasites on fishes with the ability to cause severe damage to its host. Gyrodactylus species infecting sticklebacks have short generation times and those species typically residing on the skin or fins of their hosts are easily spread via body contact. In our experiments uninfected sticklebacks significantly preferred a group of uninfected fish over a group of Gyrodactylus-infected fish, while Gyrodactylus-infected sticklebacks did not discriminate between the two stimulus shoals with regard to their Gyrodactylus infection status. As infected fish were in poorer condition, were less likely to shoal and had a relatively heavy spleen, we suggest a generally reduced health state caused by the infection as a possible indirect mechanism of the altered shoaling preference. Although parasitism has been shown to play an important role in group formation, only a few studies have used experimental infections to directly test its influence on shoaling decisions. Our results show that Gyrodactylus spp. can influence shoaling decisions in three-spined sticklebacks and affirm the suitability of the Gyrodactylus-stickleback system for studying the role of parasitic infections on host group dynamics.