Estimating digestion rate and the problem of individual variability, exemplified by a scyphozoan jellyfish

Shea-term (h) and long-term (days) individual variability and the effects o f momentary change in feeding intensity on digestion time were studied in t he scyphomedusa Aurelia aurita as a basis for developing a method to experi mentally measure the digestion rate with a high precision. Ten individual m edusae showed only small, non-significant differences in average digestion time (range, 2.1-2.5 h at 10 degrees C) over a 9-day experiment, whereas va riability within and between days and between individuals at a given occasi on was high. When medusae were manually kept at a constant feeding intensit y, stomach fullness showed high variability both between individuals and wi thin an individual over time. With a feeding intensity of, respectively, 1, 2, 4 and 8 prey h-L over a 5-h experimental period, stomach fullness of mo st individuals corresponded to a theoretical digestion time of 1-3 h, where as single meals of the same size usually gave somewhat higher digestion tim e. Medusae subject to a switching from a low to a high feeding intensity te nded to increase the variability, but most individuals showed a digestion t ime of 1-3 h. An opposite switching tended to increase the digestion time a nd its variability. It is concluded that the digestion time of A. aurita is randomly variable over time within given limits for a given food and envir onmental condition. This variability is non-synchronised in the population, causing high variability between individuals, and changes in the feeding i ntensity cause additional variability. However, the average digestion time of A. aurita in a physically and nutritionally stable environment is robust , and changes in the feeding intensity give predictable effects. The use of field collected data on stomach contents and laboratory determined digesti on times is therefore an attractive method to calculate predation rare, but the inherent high variability in digestion time must be taken into conside ration when designing the digestion experiments. Based on these findings a simple experimental method to determine the digestion time of aquatic anima ls is outlined and evaluated. The digestion time is simply given as the rat io between number of prey in stomach and total number of prey eaten, times the incubation time, assuming that the feeding intensity is constant. (C) 2 000 Elsevier Science B.V. All rights reserved.