Physical attachment of the invasive zebra mussel Dreissena polymorpha to the invasive gammarid Dikerogammarus villosus : supplementary path for invasion and expansion?

Abstract

The gammarid amphipod Dikerogammarus villosus is a relatively recent arrival in Europe, and has become well established in Lake Constance, one of the largest oligotrophic lakes in Europe. Lake Constance has previously been colonized by the notoriously invasive zebra mussel Dreissena polymorpha. Here, using field observations, we report an unusual form of active physical attachment of the invasive zebra mussel D. polymorpha to the invasive gammarid D. villosus in Lake Constance, Germany. Infested D. villosus were collected in January, February and March 2017, where they had naturally colonized the support legs of two instruments, installed in November 2016 for routine monitoring of internal wave velocity. A total of 331 D. villosus were collected among which 11.78 % were "infested" with D. polymorpha. About 66 % of D. villosus "infested" with D. polymorpha were male and 28.5 % of D. villosus were attached to more than one (maximum of three) D . polymorpha. To evaluate the significance of this phenomenon, in February, we counted all D. polymorpha and D. villosus attached to the two devices. We found, at one instrument alone, ca. 3 % of D. villosus were "infested" with D . polymorpha. These results are startling, given that the phenomenon is otherwise almost completely unrecorded. We discuss whether this unusual and dynamic physical attachment might serve as a supplementary path for further invasion and expansion by D. polymorpha, both within and between water bodies. We also describe the potential effects of the phenomenon on D. villosus, in particular the implications for essential activities such as feeding, movement and moulting.