| While the invasive zebra mussel, Dreissena polymorpha , has rapidly spread throughout the North American Great Lakes, it is being displaced by the quagga mussel, Dreissena bugensis, in shallow habitats. Quagga mussels of distinctly different shell morphology have also heavily colonized deeper water habitats (>50 m). In this dissertation, I examined byssal thread attachment and shell morphology as two functional traits that might affect performance, with implications for colonization and competition between the two species. In the first set of experiments, I examined byssal thread synthesis rate and dislodgment of both species in response to water velocities of 0, 50, 100, and 180 cm s1. Byssal thread synthesis rate was two times higher in zebra than in quagga mussels at all velocities. Zebra mussels also had greater plasticity of byssal thread synthesis rate and resistance to dislodgment in response to water velocity than quagga mussels. Using a common-garden experiment, I determined the extent to which differences in quagga mussel shell morphology between shallow and deep habitats could be induced by temperature, food quantity, and water motion. Quagga mussels exhibited developmental plasticity of shell morphology in response to temperature, regardless of the level of food quantity and water motion. Finally, I investigated whether shell morphology might affect the locomotion of zebra and quagga mussels, quantified by moment of inertia, and rotational and translational kinetic energy, on hard and soft sedimentary substrates. Deep quagga mussels had higher rotational to translational kinetic energy than zebra and shallow quagga mussels on hard, but not on soft sedimentary substrate. Shell morphology contributed significantly to the difference in kinetic energy for deep quagga mussels on the different substrates. In conclusion, greater plasticity of byssal thread synthesis rate might enable zebra mussels to maintain dominance over quagga mussels in shallow habitats with high water velocities. However, plasticity of shell morphology might confer quagga mussels with a greater ability than zebra mussels to colonize a broader range of habitats, such as the deeper soft sedimentary substrates of the Great Lakes. |
