Seasonal variations in mussel, Mytilus edulis L. shell thickness and strength and their ecological implications

The shell thickness and strength of the mussel, Mytilus edulis L. were investigated across the season on the Exe estuary, SW England. Collection and measurements of mussels across the season showed that the average ventral thickness of their shells increased from end of September to mid-November and stayed constant till early December, and then significantly decreased and reached a minimum during mid-December. The initial increase is thought to be due to the selective predation of thin-shelled mussels by oystercatchers Haematopus ostralegus L., but there is no evidence of any processes selectively removing the thick-shelled mussels. The subsequent decrease in average thickness must therefore be due to individual mussels becoming thinner. Furthermore, when mussels were smashed using an artificial oystercatcher bill, results indicated that even with the effect of ventral thickness taken into account, the strength was approximately constant until mid-January then increased and became strongest during mid-March. We suggest that the shell thinning and changes is due to reduced estuarine salinity and temperature, which the increases in thickness at the beginning of winter and in strength at the end of winter are due to selective predation by oystercatchers.