Growth Biorhythms of the Freshwater Pearl Mussel Margaritifera margaritifera (Bivalvia, Margaritiferidae), Nemina River Population (Karelia)

Abstract Individual linear growth was studied in 23 specimens of the freshwater pearl mussel Margaritifera margaritifera by measuring successive annual growth rings on the shell surface. It is demonstrated that the growth of each mussel can be described by a single equation, of which a particular case is the von Bertalanffy equation with an average growth deceleration coefficient of a = 0.048 year–1 and a coefficient of the initial conditions of d = 5.3 mm/year. It was observed that growth was accompanied by three regular biorhythms with periods of 12.6, 6.4, and 4.0 years, the first two being decaying biorhythms, while the latter had a constant amplitude. We ascertained that the decrease in the amplitude of the decaying biorhythms can be modeled by power equations. The nature of the biorhythms detected is discussed.