Non‐annual external annuli in the freshwater mussels Anodonta grandis grandis and Lampsilis radiata siliquoidea

1. Two species of freshwater mussels, Lampsilis radiata siliquoidea and Anodonta grandis grandis were measured and permanently marked with pointed, plastic tape positioned at the postero‐lateral edge of the shell. Mussels were returned to original conditions at two sites in an oligotrophic lake, retrieved at yearly intervals, re‐measured, and external annuli that had been added since marking were counted. 2. External annuli were formed much less frequently than annually; the overall median number of annuli formed each year was 0.5. In one of the four populations studied, the rate of annulus formation was >1 in small animals and <1 in large ones. Many mussels showed no new external annuli at all, even several years after marking. 3. Ford‐Walford plots of shell annuli did not yield consistent indices of shell growth. Repeated measurement of mussels in successive years showed that estimates of growth based on shell annuli consistently overestimated real shell growth.