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. Bamstedt, M.B. Martinussen J. Exp. Mar. Biol. Ecol. 251 2000 1 –15 Keywords
: Jellyfish; Medusae; Digestion; Feeding; Nutrition
1. Introduction
Digestion is the physiological process that converts food matter to energy and material for activity, growth and reproduction of an organism. The amount of food in the
digestive tract together with the rate of digestion thus define how much energy and material will be available for the organism per unit of time. In quantitative food-web
studies, much information can therefore be gained by simply measuring stomach contents and digestion rate. The product of these two factors will give the predation rate,
and this highlights the importance of correctly estimating the digestion rate in such studies. The pioneer work by Bajkov 1935 on gut contents of fish, introduced gut
analysis as a quantitative tool to estimate ingestion rate of carnivorous animals. Mackas and Bohrer 1976 were the first to use analysis of gut contents of algal pigments in
copepods as a method to estimate herbivorous ingestion rate. The gut fluorescence method has since been extensively used and is now one of the most common methods in
field works on algal grazing. Similarly, predation by some invertebrate taxa like chaetognaths, siphonophores, ctenophores and several taxa of medusae, is today mostly
estimated from the gut contents e.g., Purcell, 1981a,b,c, 1982, 1984; Purcell and
˚ Kremer, 1983; Øresland, 1987, 1990; Falkenhaug, 1991; Martinussen and Bamstedt,
˚ 1995, Matsakis and Conover, 1991; Bamstedt, 1998.
In the gut-content methods a correct estimate of the turnover rate of gut contents is essential and this is actually the most critical part in the method, since it has to be
determined experimentally. In the gut-fluorescence method this is commonly done by transferring well-fed animals to starved condition and taking sub-samples over a time
series. For predators with macroscopic prey, direct observations is the usual technique, where the time from ingestion to disappearance in the integument is recorded.
In field studies based on gut contents investigators usually do not allocate much of their total effort to properly define the digestion rate, and many workers just use values
˚ from the literature. Martinussen and Bamstedt 1999 showed a consistently high inter-
and intra-specific variability in average digestion time of gelatinous zooplankton, in spite of using a technique where defined prey was placed in the stomach of the predator at a
precise starting time. Such variability may be related to individual differences and or changes over shorter or longer time. Variability might also be related to change in
feeding intensity. It will be essential both for designing digestion experiments and for the evaluation of the results from them to know how robust the digestion rate is in a
constant and variable food environment.
In order to describe the dynamics in the digestion we here present results for the scyphomedusa Aurelia aurita. Its transparency offers unique possibilities to continuous-
ly follow the digestion process and its simple feeding mechanism makes it possible to precisely control the feeding intensity. We here report results on short- and long-term
individual variability under controlled conditions when digesting a single meal. We also present results from experiments with different constant feeding intensity and with
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. Bamstedt, M.B. Martinussen J. Exp. Mar. Biol. Ecol. 251 2000 1 –15 3
different momentary changes to new feeding intensities. The results will give guidelines in how to design and dimension digestion experiments and they also form a basis for a
simple method to experimentally determine the digestion time, which is outlined here.
2. Material and methods
