Competition between two congener invaders: Food conditions drive the success of the quagga over zebra mussel in a large shallow lake

Abstract

Dreissena rostriformis bugensis (quagga mussel, QM) has spread into areas occupied by an earlier invader, Dreissena polymorpha (zebra mussel, ZM) in Europe and North America. Usually QM displaces ZM within a few years or both species coexist, although the mechanisms driving these outcomes have not been uncovered clearly. In Lake Balaton (central‐eastern Europe), QM displaced ZM in the oligotrophic (food‐limited) basin, whereas they coexist in the eutrophic (food‐rich) basin. Searching for the drivers of interactions in dreissenid assemblages, we compared survival, growth, allometry, shell hardness, biomacromolecule content and superoxide dismutase (SOD) expression (indicating nutrition stress) of dreissenids collected in both basins in a field survey, and in individuals collected from the food‐rich basin and experimentally transplanted (10 weeks) to the food‐limited or food‐rich (i.e. the same) basin. In the field survey, QM from the food‐rich basin showed the greater height increment per unit length than coexisting ZM and food‐limited conspecifics. ZM had the hardest shells of all the mussel populations. In the food‐rich basin, ZM did not differ from QM in weight, protein, and carbohydrate contents, but had higher lipid content and SOD expression. Food‐limited QM, compared to conspecifics from the food‐rich basin, had weaker shells, but their protein, carbohydrate, and lipid contents showed faster increments per unit size, thus adults made up for the initial advantage of the food‐rich population. QM survived better than ZM after transplantation irrespective of the basin. Shells were harder in ZM versus QM and in the food‐rich versus food‐limited conditions. QM grew at both locations, whereas ZM only in the food‐rich basin. The protein and carbohydrate contents were greater in the food‐rich versus food‐limited basin, with no interspecific differences. Lipid content in QM was higher in the food‐limited versus food‐rich basin, whereas the opposite held for ZM. We demonstrated that the dreissenid species could coexist in food‐rich conditions, despite the higher level of stress in ZM (as shown by weaker survival, higher SOD expression), whereas QM displaced ZM under food‐limiting conditions, probably due to the ability to replace missing storage carbohydrates with accumulated lipids. Nevertheless, QM from the food‐limited basin also showed symptoms of nutritional stress (changes in biomacromolecule content, lower shell hardness). Results suggest that the ability to show a rapid change in metabolism could be an important advantage of QM over ZM in their competition.