Parasite-induced alterations of host behaviour in a riverine fish: the effects of glochidia on host dispersal

Parasitic species can affect host behaviour in various ways. Freshwater mussels of the superfamily Unionoidea have a glochidia larva that is parasitic on fish. Our aim was to evaluate whether fish exposed to glochidia have distinct behaviour that could affect the upstream dispersal of the parasite. Many freshwater mussels are highly endangered, and understanding the relationships with their hosts is important for their conservation. However, research on the behavioural effects of parasitism on fish host activity and/or the upstream dispersal of mussel larvae in nature has received little attention. Specifically, we examined a fish (the chub, Squalius cephalus) that hosts the larval stage of a freshwater bivalve (Anodonta anatina) and investigated alterations in host behaviour induced by the parasite. One laboratory and two field experiments were conducted using passive integrated transponder systems and radio-telemetry. Infected fish were generally less active in the laboratory and, in the field, dispersed less far upstream. Moreover, radio-telemetry revealed a habitat shift by the infected fish, which were found further from the riverbank. We suggest that behavioural changes in the fish that are induced by glochidia do not facilitate the long-distance dispersal of the mussel but rather cause reductions in fish activity and slight habitat shifts. Possible consequences of such behavioural alterations are discussed.