Effective growth cessation in adult Unio crassus Philipsson, 1788 (Bivalvia: Unionidae) from Germany

Information on the growth and age of the endangered freshwater mussel Unio crassus Philipsson, 1788 is crucial to correctly assess the conservation status of a particular population. This long-term study of individually marked specimens from four populations from central and southwest Germany revealed that zero growth and shrinkage of adult mussels from the upper 50% of their population’s length range were common phenomena. Therefore, overall shell length measurements or counting of growth lines on the shells of live adult mussels may grossly underestimate their age and should be regarded as unsuitable for determining the actual age structure of a population. Key wOrDs: growth cessation, age determination, long-term study, Unio crassus, Unionidae