Cross-habitat differences in crush resistance and growth pattern of zebra mussels ( Dreissena polymorpha ): effects of calcium availability and predator pressure

We examined the growth pattern and shell strength of zebra mussels Dreissena polymorpha in eight European locations characterised by different survival rates, pH levels and calcium availability, testing whether trait variability can be attributed to anti-predator responses and how their expression might depend on mussel size and water chemistry. Differences in chemical conditions were unrelated to the cross-population gradient in survivorship, suggesting other agents such as predators as determinants of mussel survival. Increased population mortality was associated with production of stronger shells by mussels 8, 10 and 12 mm long, and with slower growth of 12- and 14-mm individuals. Shell strengthening was unrelated to growth rate across populations. Mortality correlated positively with Bertalanffy's growth coefficient and negatively with asymptotic size. Chemical parameters were unrelated to growth patterns, but they had interactive effects with mortality on the crush resistance of 8- and 10-mm mussels. Under low-mortality conditions, higher calcium concentrations and lower pH stimulated production of stronger shells; the positive link between shell strength and population mortality was detectable across lower to medium calcium levels and medium to higher pH values. The results suggest adaptive responses to predation through increasing crush resistance and diversion of resources from growth to reproduction. They indicate that water chemistry can mediate induction of anti-predator responses, and that their efficacy changes over the mussel lifetime. We argue that a full consideration of the post-invasive polymorphism of zebra mussels must incorporate the interplay between environmental conditions and adaptive responses to mortality risk.