Abstract
Many species of freshwater and marine bivalves are imperiled, and non-lethal approaches to gaining biological information are needed. Freshwater and marine mussels produce visible growth rings in their shells that can be used to estimate individual or population growth rates over daily, seasonal or annual cycles. However, viewing and recovering the information held in those growth rings requires destructive sampling of shells by sectioning and frequently requires staining or dying to visualize structures. Here I demonstrate the ability of Rit Dye, a widely available commercial nontoxic dye to increase the visibility of growth rings in a freshwater unionid mussel species of conservation concern (Margaritifera falcata) and five additional taxa of bivalves (Glycymeris spectralis, Venerupis philipinarum, Meretrix lyrata, Corbicula sp., Dreissena bugensis) compared to untreated samples. Rit Dye treated thin sections of shell were qualitatively compared to thick sections of shell treated with a well-established dye method (Mutvei’s solution). Rit Dye was found to be highly effective in M. falcata and was used to analyze shells from dead mussels (hereafter dead shells) collected by the United States Fish and Wildlife Service (USFWS) and Yakama Nation Fisheries Department in 2019-2020 from nine streams in Idaho and Washington. Dead shell length data was compared to live shell data for El Dorado Creek and Musselshell Creek. Dead and live shell size distributions had similar variances, but dead shells had longer mean length than live shells (El Dorado Creek Mean dead shell length=53.1mm, mean live mussel length=43.8mm, Welch two-sample t-tests p<0.0001; Musselshell Creek mean live shell length=47.7mm, mean dead shell length=60.3mm; Welch two-sample t-test p<0.0001). Across all nine populations, mean dead shell length differed significantly (mean length range; 53.3mm-111.8mm). The ratio of shell length to shell width (SCI) did not differ between streams (F=0.04, p=0.5655). M. falcata shells exhibit substantial erosion of the umbo during growth, generating uncertainty in mussel age estimates and age-at-length relationships. The extent of erosion as a percentage of width for dead shells did not correlate with shell size or location and was similar in all locations with the exception of two streams suggesting erosion is not systematically affected by local environment or age of the mussels at these sites. Crossdated growth chronologies were constructed from dead shells for five streams to estimate individual year of death. The resulting dates suggested two peak years of mortality in 2010 and 2020. Data limitations prevented use of standard models for finding age-at-length data and year of recruitment and so a linear modeling approach was developed to generate preliminary estimates of individual recruitment. Results suggest data on growth patterns within stream are required in order to accurately model recruitment. Verification of age-at-length data using live mussels and refinement of these methods could be used to reconstruct information about patterns of death and recruitment from dead shells and allow investigation of growth patterns, successful recruitment events and high mortality events, without the need to sacrifice mussels and minimizing disturbance of living mussels in M. falcata and other species of concern.