No Response to Bidirectional Size-Based Selection in the Rotifer Brachionus rotundiformis

Although rotifers have been considered the best feeding option for several species of fishes in aquaculture, they are sometimes larger than appropriate for the early larval stage of some marine fishes. Thus, we aimed to determine whether size-based selection of the parents could affect the average body size of their progeny in two clonal populations of the rotifer Brachionus rotundiformis. From each of the clones, 20 individuals were bi-directionally selected toward both smaller and larger sizes and each individualbased selection was conducted for 10 consecutive generations. The results showed that although there were sometimes differences in mean body size between parents and their progeny, no directional trend was observed in all selected lines of both clones. We demonstrated that artificial selection in a rotifer stock cannot lead to an expected size range although they appear to exhibit a large degree of body size polymorphism.