Behavioural changes of zebra mussel Dreissena polymorpha (Bivalvia) induced by Ponto-Caspian gammarids

Abstract

Colonies of an invasive Ponto-Caspian zebra mussel constitute suitable habitats for many invertebrate organisms, including Ponto-Caspian gammarids that are now spreading throughout Europe. There is, however, little known of the impact of these gammarids on their mussel hosts which they live amongst. We studied the behaviour of zebra mussels in the presence of three gammarid species that differed in their origin and degree of association with Dreissena habitats. In the presence of the Ponto-Caspian invader Dikerogammarus villosus that was commonly observed in mussel habitats, mussels responded by increasing their attachment strength and reducing upward movement. The responses to the Ponto-Caspian Pontogammarus robustoides, a species that showed no preferences for mussel habitats, were weaker and limited to the increase of attachment strength. There were no behavioural changes in mussels exposed to the native Gammarus fossarum. As mussels only responded to gammarids moving freely among their shells and not to those kept behind a mesh barrier, the most likely factor causing these changes was mechanical irritation of their soft parts by amphipod appendages. This hypothesis was further confirmed in an additional experiment with a South-American catfish Corydoras paleatus used as another potential source of mechanical irritation. The responses of zebra mussels to gammarids were similar to their anti-predator defences shown earlier in the presence of molluscivorous roach. Thus, it appears likely that Ponto Caspian gammarids within mussel colonies have the capacity to compromise the normal functioning of bivalves through inducing their defensive responses.