Abstract
Parasites can fundamentally alter the
cost–benefit ratio of living in a group, e.g. if
infected individuals increase the predation risk of shoal mates. Here, the effect of an infection with a trematode, Uvulifer sp. (Diplostomatidae) on the shoaling behaviour of female western mosquitofish, Gambusia affinis, was investigated. The parasite examined causes a direct phenotypical
change of the host by forming black spots on its
body surface. When given a choice between a
stimulus shoal and no shoal, we found shoaling
tendencies to be significantly reduced in infected
focal fish. In another experiment, we tested for
association preferences relative to the infection
status of the stimulus fish. Given the choice
between an infected and a healthy stimulus fish,
both infected and healthy focal fish preferred to
associate with non-infected stimulus fish. Our
results suggest that (1) the cost–benefit ratio of
shoaling might be different for infected and noninfected individuals. Infected fish may be more
affected by competition for food within a shoal.
(2) Associating with infected conspecifics appears
to be costly for female mosquitofish, maybe due
to increased predation risk.
Tobler, M