Selection of blue mussels, Mytilus edulis, by common eiders, Somateria mollissima, by size in relation to shell content

Abstract

The blue mussel, Mytilus edulis, is the most important food item for the common eider, Somateria mollissima, but the mussels eaten by eiders vary in length between both areas and seasons. The aim of this study was to experimentally test the hypothesis that common eiders try to minimize shell intake when selecting among mussels of different lengths. Two types of experiments were conducted using wild common eiders in captivity. Experiment 1 tested whether eiders could seek out the mussels with least relative shell content and highest flesh content from among mussels of an array of lengths. They selected the shortest mussels (<20 mm), which had the lowest shell content. In experiment 2, the eiders were given mussels of the same length class (26-31 mm), but with different shell morphology and shell mass, from the subtidal and upper intertidal zones. The birds only ate the subtidal-zone mussels with the lowest shell mass. The results of this study strongly indicate that reducing the amount of indigestible shell is an important factor for eiders selecting among mussels of different lengths.