Variation in thermal tolerance among three mississippi river populations of the zebra mussel, dreissena polymorpha

To investigate the occurrence of thermal adaptation in the zebra mussel, Dreissena polymorpha, inhabiting the lower reaches of the Mississippi River, we compared lethal heat-tolerance among three populations (mussels collected at Lake Pepin, MN; Alton, IL; and Baton Rouge, LA). We determined time-to-death at 32°°C for 160 individuals per site, for mussels collected at a water temperature of 15°°C and then maintained in the laboratory for about 8 wk under uniform conditions. Both shell length and condition index significantly affected survival time and were included as covariates in the analysis for interpopulation differences in heat-tolerance. Zebra mussels from our southernmost location had a higher heat-tolerance than those from the two northern locations. This difference in heat-tolerance among sites may indicate adaptation to local temperature regimes. In addition, in a comparison of heat tolerance within populations, we separated mussels into size classes (where larger mussels have been exposed to local conditions longer) and calculated an adjusted mean time-to-death (TTD). We found a different TTD/size relationship depending on sampling location. Minnesota mussels had decreasing heat tolerance as size increased, where Louisiana mussels had the opposite relationship. These patterns of heat-tolerance within populations indicate a selection pressure for increased heat-tolerance at Louisiana. However, even if the selection pressure is strong at the Louisiana site, it has not (at least not yet) resulted in an adaptation, as high heat-tolerance is not ubiquitous within this population. Zebra mussels may have insufficient genetic variation for heat-tolerance or gene flow may have been too strong for genetic adaptation to occur in the short amount of time that zebra mussels have occurred in the lower Mississippi River.