Abstract

Grouping behaviours (e.g. schooling, shoaling and swarming) are commonly explicated through adaptive hypotheses such as protection against predation, access to mates or improved foraging. However, the hypothesis that aggregation can result from manipulation by parasites to increase their transmission has never been demonstrated. We tested this hypothesis using natural populations of two crustacean hosts (Artemia franciscana and Artemia parthenogenetica) infected with one cestode and two microsporidian parasites. We found that the cestode parasite modified both the swarming behaviour and the red colour of its host, which likely results in increased parasite transmission to its final avian host. Furthermore, we found that the microsporidian parasites modified both the swarming and the surfacing behaviours of its host. We demonstrated with experimental infections in the laboratory, that these concurrent manipulations result in increased spore transmission to new hosts. Hence, this study suggests that parasites might play a prominent role in host grouping behaviours.