Mussel ages were estimated by counting the growth lines (annuli). Shell preparation for growth line counting was followed to standard method [28-30]. We hypothesized that the growth line of M. laosensis is formed in winter, and yearly individual growth can be estimated from length between consecutive growth lines because the growth lines of other margaritefirids, especially those that also have a southern range (M. laevis, M. auricularia, M. falcata and M. hembeli) are known to form in winter [31-34]. Growth rings of M. laosensis, in comparison to M. margaritifera, for example, are wider and clearer, and therefore easier to count. Estimates weremade for the three smallest and three largest individuals from each river (n = 12)