The development of ageing methods for the freshwater pearl mussel Margaritifera margaritifera (Linn.) and the population structure of exploited and unexploited populations
A valid practical ageing method for the freshwater pearl mussel Margaritifera margaritifera (Linn.) was used to compare the structure of populations of the mussel which differed in their degree of exploitation for pearls and so to provide a basis for assessing the status and prospect for the mussel in Scotland. To demonstrate this method, an accurate study of the shell and ligament structure in relation to age was undertaken. The accuracy of the age estimation is affected by erosion. This erosion on M.margaritifera's shell continues to a conchiolin layer which partially limits the erosion. This study demonstrated that erosion is of two forms (Type 1 and 2). Type 1, which occurs near the umbo, consists of deep pits sometimes still covered by periostracum. Type 2, which is formed over the whole shell surface consists of superficial damage to the layers, beginning with destruction of the periostracum. This investigation established which shell layers of M.margaritifera most clearly show the shell lines. They are clearer on the nacreous layer than they are on the prismatic layer, on which in turn they are clearer than on the periostracum layer. A baking method was developed to separate the shell lines on the nacreous layer and to show that these lines are true layers rather than merely superficial marks on the surface. The cut valves are baked in a muffle furnace at 600°C for 1-2 minutes. The nacreous layer then formed flakes of separate shell layers. This study showed that the shell lines are possibly annual using the following techniques.a) A tagging experiment in which young mussels were released in River Dee and recaptured 14 months later, by when a new shell line had formed.b) Line deposition on the shell of young mussels kept under laboratory conditions for a period of 17 months.c) An oxygen isotope method (where different isotopes are laid down at different temperatures) was used for the analysis of the shell increment formed during summer and winter seasons.