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A COMPARISON OF GENETIC VARIATION BETWEEN POPULATIONS OF POCKETBOOK AND PIMPLEBACK MUSSELS ABOVE AND BELOW THE ST. CROIX FALLS DAM
Suzy Szumowski, Department of Biology, Macalester College
Mussels play an important role in freshwater habitats. As filter feeders, they are highly sensitive to water quality, and often are the first to suffer the ill effects of degraded water. There are forty-eight species of mussels native to Minnesota, and more than half of them are listed as endangered, threatened, or of special concern. The St. Croix River is a superior habitat for mussels likely due to relatively light human disturbance in the watershed, although since 1903 there has been a hydroelectric dam at St. Croix Falls. In addition to altering water discharge patterns, hydroelectric dams can affect the temperature, oxygen concentration, and suspended sediment content of a river. It is also significant that the St. Croix Falls and the dam have acted as a geographic barrier to upstream fish travel. Since the life cycle of mussels involves a larval growth phase in which the mussel is dependant on a fish host for transportation, this barrier could greatly affect the dispersal of populations. Both Lampsilis cardium and Quadrula pustulosa are known to have multiple fish hosts, primarily catfishes for Q. pustulosa and sunfishes and perches for L. cardium. It has been observed in previous studies that the mussel assemblages above and below the St. Croix Falls differ, and that this variability is not likely to be a result of variation in microhabitat factors, such as substrate composition and water velocity (Hornbach 2001).
To determine the extent to which the dam isolates mussel populations above and below the dam we collected small tissue samples from the mantles of 20 L. cardium and 20 Q. pustulosa at four locations; two above the dam and two below the dam. L. cardium and Q. pustulosa were chosen as representatives of different subfamilies (Lampsilinae and Ambleminae, respectively) found to vary in prevalence above vs. below the dam. Also, as related species found in the St. Croix River, Lampsilis higginsii and Quadrula fragosa are endangered; L. cardium and Q. pustulosa served as proxies. DNA was extracted from the tissue clippings and processed using molecular techniques in order to sequence the NDI gene, which is commonly used in bivalve genetic studies and encodes NADH dehydrogenase 1, an essential enzyme in cellular respiration processes. The ND1 sequences were edited and aligned using Sequencer and MacClade. Preliminary data from one of the below dam sites indicates the percent of polymorphic sites in the ND1 gene to be 1.7% for L. cardium and 6.4% for Q. pustulosa.
Hornbach, D.J. 2001. Macrohabitat factors influencing the distribution and abundance of najads in the St. Croix River, MN and WI, USA, pp. 213-230. In G. Bauer and W. Wachtler [eds] Ecology and Evolutionary Biology of the Freshwater Mussels Unionoidea. Ecological Studies Vol. 145. Springer-Verlag: Berlin.
Watters, G.T. 1999. Freshwater mussels and water quality: A review of the effects of hydrologic and instream habitat alterations. Proceedings of the First Freshwater Mollusk Conservation Society Symposium. 261-174.