Sclerochronology of paired valves: A case study of Margaritifera margaritifera (Bivalvia: Margaritiferidae)

Molluscan growth histories can be retrieved from sclerochronological data. The extreme longevity of the endangered freshwater pearl mussel (Margaritifera margaritifera) (Mollusca: Bivalvia) makes this species ideal for sclerochronological studies where long timeseries are needed. However, the species is included in The IUCN Red List of Threatened Species, for which reason the number of potentially reproducing individuals collected from the extant populations should be kept down. In this study, museum specimens of M. margaritifera, collected originally from northern Finland in 2002, were explored. Timeseries of annual shell growth increments were produced from both valves of each individual mussel. The approach of paired valves benefitted cross-dating between the growth records and improved chronology statistics, helping to keep down the number of explored individual mussels. On the other hand, it was also found that the growth of left and right valves may exhibit differentiated patterns. All specimens provided increment data over the 1976–2002 period, that is, the mussels were around 30–40 years old when sacrificed. Midsummer (July) temperature was the only climate variable correlating statistically significantly with the mean shell growth record over the common period (1976–2002). However, this climatic factor did not fully explain a shell growth curtailment around the year 1991. Likely, the negative excursion in shell growth was caused by a high sediment influx due to anthropogenic changes in the catchment. Our findings are based on one site chronology which means the results should not be straightforwardly extended to other streams and habitats. This limitation notwithstanding, the results suggest that sample collection strategies can be used to build sclerochronological datasets for M. margaritifera and other endangered bivalve species.