The oldest recorded animal in Finland: ontogenetic age and growth in Margaritifera margaritifera (L. 1758) based on internal shell increments

Three large and robust shells of freshwater pearl mussel (Margaritifera margaritifera) from Lapland were studied. Internal shell-growth increments, showing annual periodicity, provided information about the ontogenetic age and growth of the mussels. Increment widths generally ranged between 0.4 mm and 30 μm and showed exponential growth trends. Moreover, it was shown that the increment series of different specimens could be temporally synchronized by their common growth signal. This enabled crossdating of the increment series, after which they were averaged into one sclerochronology. One of the shells showed past growth disturbance due to breakage of the shell. Subsequent growth increase, not obsereved in the other shells, had occurred over the next 5 to 8 years. The oldest specimen exhibited 162 annual increments. Considering the width of the umbonal corrosion and the widths of the increments next to the corrosion, the loss of 17 young increments could be estimated. Moreover, a number of four years was added to the age estimate due to presumably minimal shell growth during the early post-parasitic years. Our results indicate that, if not anthropogenically disturbed, super-centenarian ages may not be a rarity in the region and the oldest mussels may reach an age of even two centuries.