Effect of mussel density and size on the morphology of blue mussels (Mytilus edulis) grown in suspended culture in Prince Edward Island, Canada

Mussels usually exhibit morphological variations in response to their environment. We conducted two field experiments in which we assessed the effect of mussel density and size on the morphology of blue mussels (Mytilus edulis) grown in suspended culture using longlines. Morphological measurements were taken over two and a half years on mussels reared at different initial densities (ranging from 100 to 800 mussels per 30 cm of sock) at two sites in Prince Edward Island, Canada. Generally, growing density of mussels did not have an effect on shell width, shell height or tissue-to-shell ratio. However, in some instances, mussels became narrower (reduced shell width-to-length ratio) and had a lower tissue-to-shell ratio at high density. The effect of mussel size (measured as shell length) was more consistent, whereby the shell width-to-length ratio increased and the shell height-to-length ratio decreased as mussels grew. The lack of density-dependent morphological plasticity and variation in tissue-to-shell ratio may be attributed to density-dependent mortality and fall-off observed in mussels reared on longlines.