The age could be identified by means of dark annual rings observed on the

shells. Difficulties in counting these rings could appear if mussels stopped their growing after disturbances and formed pseudannuli. Those were not easily to distinguish from true annual rings. Additionally, the age determination could become difficult if several rings were very close (older bivalves) at the margin of shells.

See for instance the paper by Weber (2005): Population size and structure of three mussel species (Bivalvia: Unionidae) in a northeastern German river with special regard to influences of environmental factors. Hydrobiologia 537: 169-183 (see attachment)

Thank you for the answer. We will definitely look at the annual rings!

We collected many individuals from different basins so probably it is possible distinguish between the youngest and older ones.

Hello Ivan,

I agree with Dr. Zettler and you can complete this growing lines with the measurements of the adult shells. For me it is useful only for big mussels eg. Unio, Anodonta, Sinanodonta, as for Pisidiidae you can not use.

Thank you Zoltán !

We definitely combine the length measurements with counting of growing lines.

In Angerapp River I found small shell of Unio consists with only umbonal structure pattern (ribs). I don’t know age this shell but the first year of free-living example of Unio may be missed by mistake. We need to have special investigation of rate of growth in aquaria. Then we have to numbering the shell lines of growth and summarize results. Are anybody may be to prove my assumption? Also we have always umbonal corrosion of shell:) it is difficult to use any lines and marks) only.

http://ashipunov.info/shipunov/school/books/zolotarev1989_sklerohron_dvustv.djvu

Золотарев В.Н. Склерохронология двустворчатых моллюсков. К., 1989, DjVu