The difference in mean length before and after the trial was tested with a paired t-test for each group. The results showed a significant decrease of the mean length for the reed ecotype in the stonewort substrate (Fig 1).

A two-way ANOVA showed significant increase of the mean length of the isopods before and after the trial when there was no substrate. There was also a significant difference between the ecotypes, with the reed ecotype being larger, but no interaction between the two factors (Fig 2).

With reed substrate only the stonewort ecotype showed a significant increase in mean length (Fig 2).