Shell Strength and Appearance in Cultured Blue Mussels Mytilus edulis, M. trossulus, and M. edulis x M. trossulus Hybrids

We studied shell strength and appearance in six mixed-species cultured populations of blue mussels (Mytilus edulis, M. trossulus, and their hybrids) from Newfoundland, Canada. Shell strength (defined as the force causing shell breakage) and shell thickness both increased with shell length. Strength and thickness varied significantly among sites and among genotypes within sites; M. edulis had stronger and thicker shells than M. trossulus. Shell strength was related to shell thickness and shell width, both of which were greater in M. edulis than in M. trossulus. Hybrids tended to be intermediate. Appearance was studied on the basis of shell color and shape. The frequency of brown-shelled mussels varied considerably among sites and was higher in M. trossulus than in M. edulis. Shell width, depth, width-length ratio, and depth-length ratio all varied significantly among sites. Within sites, M. edulis had a greater shell width, width- length ratio, and width-depth ratio than M. trossulus, whereas hybrids were intermediate. Intrasite variability in shell shape characteristics, expressed as total population variance, was greater in the mixed-species populations from Newfoundland than in a unispecific M. edulis population from Prince Edward Island. In a reciprocal seed transfer experiment, shell width, width-length ratio, and width-depth ratio were greater in M. edulis than in M. trossulus of similar age. Width-length and width-depth ratios decreased with increasing age. Fourteen months after transfer, shell width and depth differed between transferred and nontransferred stocks. Interspecific relationships between M. edulis and M. trossulus in shell width and width-length ratio were similar in transferred and nontransferred stocks, while the other shell dimensions and their ratios differed. We infer that genotype-related differences in shell shape may change after stock transfer owing to site-related effects on shell growth. We discuss these results in terms of their potential impact on future industry development, particularly expansion of the seed supply for further industry growth.