The Size and Age Structure of California Floater (Anodonta californiensis) and Western Pearlshell (Margaritifera falcata): A New Method of Aging Thin Mussel Shells

Freshwater mussels play vital roles in ecosystems by providing services that improve water quality, sequester harmful substances, cycle nutrients, and serve as food for animals; however, many mussel species are rapidly declining and several species are at risk of extinction. Challenges in finding and assessing mussel populations have hindered collecting basic information, e.g., age structure, size distribution, and habitat preferences, which ultimately reduces effective management strategies. Our study assessed the length-age relationship and calculated growth rates of the Western Pearlshell—Margaritifera falcata (A. Gould, 1850)—and the California Floater—Anodonta californiensis I. Lea, 1852—in western Wyoming, U.S.A. We collected deceased mussel shells along the Bear River and measured shell length, width, age, and distance between annuli. We developed a new method of aging thin-shelled mussels using transmitted light. The oldest California Floater was 12 yr, and plots of length by age exhibited linear growth whereas width of annuli showed logarithmic growth. Both methods displayed logarithmic growth for the Western Pearlshell, and the oldest individual was 60 yr. We calculated the age of live mussels measured in the field based on our relationships; however, growth and size can differ among regions due to differences in habitats, and we suggest creating relationships for each population. Our study contributed basic data about two declining western mussel species providing information to assess populations and inform management strategies to mitigate threats.