Age, Asymptotic Size, and Growth Constants of East Texas Freshwater Unionid Mussels

Age and growth information helps researchers better understand how freshwater mussels (Bivalvia: Unionida)—among the most imperiled aquatic fauna worldwide—cope with environmental change. Shell thin sectioning is the primary method for estimating age and growth. Using a low-speed saw, a thin, radial cross-section of shell is taken and then mounted on an unfrosted microscope slide and read using a dissecting microscope. Thin sectioning can be time intensive, and species-specific issues, such as crowded annuli, can complicate efforts to provide accurate estimates. To date, only 69 of the approximately 300 North American species have age and growth information. Texas illustrates this issue perfectly; population-specific growth estimates are available for only 6 of the 52 species known to occur in the state. For the remaining species, information is either unavailable or inferred from closely related congeners or populations outside the state. This is problematic because incorrect inferences about age and growth can lead to erroneous assumptions about a species' life history, which could result in management and conservation actions that, at best, waste resources, and at worst, lead to population declines. We thin-sectioned eight different mussel species, including three species of conservation concern, then estimated growth parameters using von Bertalanffy growth curves. Our work more than doubled the number of Texas species with age and growth information from populations within the state. We found that growth serves as a good proxy for species position along a continuum contrasting higher growth and shorter lifespans versus lower growth and longer lifespans. Our results should be useful for making inferences about how species respond to environmental change.