Rotifer Threshold Food Concentrations and the Size‐Efficiency Hypothesis

Abstract

Using published data, we develop a physiological explanation for the positive relationship between threshold food concentration (the food concentration at which the population growth rate is zero) and body mass in planktonic rotifers. The exponent describing maximum clearance rate as a function of body mass is similar to the exponent expressing energy intake rates at low food concentrations. The former exponent (0,42) is considerably lower than the exponent for respiration (0.66). Consequently, the difference between energy intake and metabolism should decrease with increasing body size at low food concentrations. Rotifer swimming speeds are rather uniform for a wide range of body sizes and species. Consequently, length—specific swimming speeds (body lengths moved per unit time) and, particularly, mass—specific swimming speeds (distance moved per unit time per unit mass) decrease rapidly with increasing body size. We conclude that swimming speed, which is under the energy constraints of ciliary locomotion and which directly determines the rate of food encounters, provides the mechanism for the positive relationship between body size and threshold food concentration.