Food thresholds and body size in cladocerans

Abstract

ACCORDING to the size–efficiency hypothesis1, body size is one of the most important determinants of the relative abundance of planktonic animals in nature. First, large-bodied species are more vulnerable to predation by visually feeding planktivorous fishes. Second, they are superior competitors for resources, being able to grow and reproduce at lower food concentrations. But whereas the first assumption has been confirmed by many field and experimental data (for review see refs 2–4), the second suggestion lacks experimental evidence and has been frequently questioned5-12. I now present experimental data from eight filter-feeding species (family Daphnidae) that strongly support the cornerstone assumption of the competitive aspect of the size-efficiency hypothesis. These data show that the threshold food concentration necessary to assure that assimilation equals respiration13, is lower for the large-bodied species than it is for the small-bodied species under steady-state and low-mortality conditions.