Downing, J.A. 1997. Relationship between habitat characteristics and the extinction of lake mussels in Minnesota. Final report submitted to the Minnesota Dept. of Natural Resources. 10 pp.
The objective of this study was to determine factors driving lacustrine mussels to local extinction by seeking patterns in the physical, chemical, biological and geographical characteristics of lakes from which they have recently disappeared or in which they are experiencing population decline. Although not foreseen in the proposal, molecular genetic methods were also used to assess genetic diversity of some populations. We studied patterns of recruitment, density, and mortality of freshwater mussels in 25 lakes, in a region of north central Minnesota where mass die-offs have been observed. Although 88 % of the lakes contained extant mussel populations, more than 40 % of them showed signs of serious decline. Recruitment rates in Pyganodon grandis grandis appeared lowest in small lakes with low fetch that were closest to the main river course. Recruitment in Lampsilis radiata siliquoidea was also low in small lakes and lakes containing populations with skewed sex ratios, but paradoxically increased with decreased abundance of potential host fish. Sex ratio (males to females) ranged from 1:6 to 4:1 in Lampsilis populations, and appeared to be under environmental control. The sex ratio was strongly negatively correlated with the average pH. Lake pH reduced to around 7.0 apparently led to male-biased sex ratios and recruitment failure. We also developed methods employing random amplified polymorphic DNA (RAPD), heteroduplex, and single-stranded conformational polymorphism (SSCP) analyses using nuclear DNA markers to investigate the influence of population size on heterozygosity in seven different sized populations of the freshwater mussel Lampsilis radiata siliquoidea. Within populations, heterozygosity values ranged from 0.161-0.356 for the RAPD loci and from 0.000-0.300 for the repeat flanking locus that formed heteroduplex molecules. There was only a weak correlation between population size and heterozygosity for the RAPD or repeat flanking locus, but the power of analyses was low due to restricted sample sizes. This result is consistent with the hypothesis that small, isolated populations may experience inbreeding. A combination of subtle environmental changes and genetic degradation may be endangering a large proportion of lake-dwelling mussel populations.