Dreissenids’ need for speed: mobility as a driver of the dominance shift between two invasive Ponto-Caspian mussel species

Both the quagga mussel (Dreissena bugensis) and the zebra mussel (Dreissena polymorpha) are notorious for dominating hard substrates in freshwater ecosystems throughout most of the Northern hemisphere. Despite widespread observations of a dominance shift favouring D. bugensis, where both Ponto-Caspian dreissenids co-occur, mechanisms driving this shift are still largely unknown. This study assessed whether movement behaviour differs between these two mussel species. That way we aimed at assessing whether mobility might be a contributing driver to the observed dominance shift. The mobility of dreissenids was assessed in an experimental set-up consisting of polyethene tanks marked with squares and concentric circles facilitating location tracking of the dreissenids by time-lapse photography. Specimens were collected at the Haringvliet and Hollands Diep in the Rhine-Meuse river delta. The experiments mimicked unfavourable habitat conditions by drying, cleaning, tagging and placing mussels in a new environment. After these disturbances, the movement rate, duration, distance, pattern and speed of 299 individuals were monitored. For both species, most individuals moved in more or less circular patterns, causing their actual movement distance to be twice as high as their displacement distance. The average movement duration within 24 hours after the start of each experiment was 65 min, with an average speed of 28 cm/h and an average distance of 29 cm. Hereby no significant differences were found between D. polymorpha and D. bugensis. However, a higher top speed was observed for D. bugensis than for D. polymorpha. The fastest individuals of these two species moved at 90 cm/h and 60 cm/h, respectively. Moreover, about twice as many D. bugensis individuals moved during the experiments in comparison to D. polymorpha individuals. Hereby it was recorded that any point in time close to 10% more D. bugensis specimens were moving around. The results support our hypothesis that D. bugensis could have a competitive benefit over D. polymorpha by having a higher top speed and a significantly higher number of individuals moving after a disturbance of their population. Detachment and mobility of sessile mussel species are supposed to be avoidance mechanisms during unfavourable environmental conditions. Therefore, mobility might be one of the contributing drivers of the observed dominance shift between both species.