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Direct measurements of the digestion time of C . finmarchicus fed with 1, 2, 4 or 8
prey was done on separate medusae in the size range 16–51 mm diameter n 5 6 or 8 medusae per meal size, using the same technique as described above.
3. Results
3.1. Short- and long-term variability in digestion time of a single meal The specific test of the individual variability in digestion time showed that high
variability was an inherent factor Fig. 1. The medusa showed some variability in
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feeding intensity between the experiments, ranging from 0.63 to 1.33 prey h as an
average value for the two periods between experimental days Table 1, average of the two last columns. The average digestion time for the 10 individuals over the 9-day
experiment was less variable, ranging from 2.14 to 2.51 h, and with no significant difference between them t-test, P . 0.05. All medusae showed overlapping 95
confidence intervals, and high variability, both within the same day and between different days Fig. 1, with a range in digestion time from 1.3 to 4.0 h. The total
material showed no significant relationship between digestion time and time of the day two-way ANOVA, P 5 0.870 and average values were 2.42, 2.35 and 2.35 h for
morning, noon and afternoon experiments, respectively. However, there was a significant difference between the different experimental days two-way ANOVA, P 5 0.028 and a
significant interaction between day number and time of the day P 5 0.032. Average value for days 1, 5 and 9 was 2.51, 2.48 and 2.14 h, respectively. These differences were
small and a post-hoc test Sheffe’s test did not reveal significant differences between any of the 3 days, although the difference between days 1 and 9 was close to the
significance level of 0.05.
3.2. Digestion time of different sizes of meals Direct measurements of the digestion time for the four groups of medusae that were
given a single meal of 1, 2, 4 or 8 copepods showed overlapping ranges. With a 1-prey meal the digestion time ranged from 2.2 to 3.5 h mean, 2.8 h, n 5 6, whereas for a
2-prey meal digestion time ranged from 1.8 to 2.3 h mean, 2.0 h, n 5 6. A 4-prey meal was digested in 2.3–3.2 h mean, 2.7 h, n 5 8 and an 8-prey meal needed 2.1–4.2 h
mean, 3.1 h, n 5 6 for complete digestion. A correlation analysis of the average values
2
did not reveal any significant effect of meal size t-test on r , P . 0.05. 3.3. Variability during constant feeding
The number of the copepod prey remaining in the stomach of the predator, Aurelia aurita, during experiments with precisely controlled feeding intensity, indicated that the
digestion rate was not absolutely constant, despite the constant feeding intensity. With an ingestion of one copepod per hour the results showed individual variability, with a
stabilisation of 2 or 3 prey in the stomach, indicating an average digestion time between
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Fig. 1. Aurelia aurita; digestion time of a meal of three stage V copepodids Calanus finmarchicus in the morning, noon and afternoon, respectively, on days 1, 5 and 9. The 10 medusae A–J were kept in individual
tanks with food between the experimental days. Grey areas denote 95 confidence limits. See also Table 1 for information about the medusae.
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2 and 3 h Fig. 2, upper left. With doubled feeding intensity the results indicated even larger individual variability and a stabilisation between 1 and 3 prey in the stomach,
corresponding to digestion times of 0.5–1.5 h Fig. 2, upper right. With a further
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doubling in the feeding intensity 4 copepods h , lower left the stomach stabilised
between 10 and 14 prey, corresponding to digestion times from 2.5 to 3.5 h. With the
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highest feeding intensity 8 copepods h , lower right the stomach content showed
individual variability after the first hour, ranging from 8 to 16 prey, and corresponding to a range in digestion time from 1 to 2 h.
3.4. Effects of changed feeding intensity In the first set of controlled tests with pre-defined momentary changes in feeding
Fig. 2. Aurelia aurita; number of prey stage V copepodids Calanus finmarchicus remaining in the stomach
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over time, with a fixed feeding intensity of 1, 2, 4 and 8 prey h , respectively. Separate results of five
individual medusae are shown, some symbols overlapping. The theoretical numbers of remaining prey are given for a digestion time of 1, 2 and 3 h, respectively, as dotted lines. Note the different scales on the y-axes.
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intensity see Table 2, the switch to a new feeding intensity was defined to 2 h after the start of feeding and the total experimental time was 5 h. The stomach content of these
medusae did not strictly follow a theoretical schedule and individual variability in stomach content was large Fig. 3. With a switch from low to medium ingestion rate
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from 1 to 4 prey h stomach content of the five individuals showed a succession that
could be explained by a variable digestion time within the limit 1 and 2 h Fig. 3, upper left. With the opposite switch, prey number remained higher after the switch than
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theoretically predicted Fig. 3, upper right. With a switch from 2 to 8 prey h , stomach
content showed a high variability, both within and between individuals, with a range during the final hour of the experiment from 3 to 16 prey, corresponding to a digestion
Fig. 3. Aurelia aurita; number of prey c-V Calanus finmarchicus remaining in the stomach over time, with a constant feeding intensity for 2 h and a new constant feeding intensity during the following 3 h. Separate
results of five individual medusae are shown, some symbols overlapping. The theoretical numbers of remaining prey are given for a digestion time of 1, 2 and 3 h, respectively, as dotted lines. Note the different
scales on the y-axes.
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time of 0.5 to roughly 2.0 h Fig. 3, lower left. Finally, with a switch from 8 to 2 prey
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h , the variability was lower, with the stomach content during the final hour of the
experiment ranging from 3 to 10 prey. The succession for some medusae were in fairly good agreement with the theoretically predicted one for a digestion time of 1–2 h Fig.
3, lower right. In the final experimental set, with a switched feeding intensity after 4 h and a total
feeding time of 8 h, the observed stomach content showed very high individual
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variability after the first initial hour Fig. 4. The group switching from 1 to 4 prey h showed a range in stomach content from 1 to 4 prey prior to switching, and from 2 to 10
prey at the end of the experiment. These levels would correspond to theoretical digestion times of 0.5 and 2.5 h, respectively Fig. 4, upper left. The opposite change in feeding
Fig. 4. Aurelia aurita; number of prey stage V copepodids Calanus finmarchicus remaining in the stomach
over time, with a constant feeding intensity for 4 h and a new constant feeding intensity during the following 4 h. Separate results of five individual medusae are shown, some symbols overlapping. The theoretical numbers
of remaining prey are given for a digestion time of 1, 2, and 3 h, respectively, as dotted lines. Note the different scales on the y-axes.
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intensity Fig. 4, upper right gave less individual variability, but none of the five medusae followed the theoretical succession for a fixed digestion time. At the end of the
experiment, individual stomach contents corresponded to a digestion time of 5–7 h. The
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medusae that changed the feeding rate from 2 to 8 prey h , also gradually increased the
stomach content, from 3 to 7 prey at the switching moment to 12–23 prey at the end of the experiment. The theoretical digestion time best describing this succession seemed to
be between 1.5 and 3 h Fig. 4, lower left. The final experiment, where the feeding
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intensity switched from 8 to 2 prey h , gave clear evidences of high individual
variability, and an inability to closely fit any underlying theoretical digestion time for most of the medusae Fig. 4, lower right.
3.5. A simple experimental model to estimate digestion time Fig. 5 gives the theoretical basis for a simple experimental method to estimate the
digestion time of predators. The method assumes that the feeding intensity is constant throughout the experiment and that prey loss is solely an effect of predator consumption.
A given number of predators and prey are kept together for a given time T , where T . T the digestion time. The relative reduction in prey abundance should be low, in
order to not obliterate the assumption of constant feeding intensity. At time T the predators are separated from the prey and the number of prey remaining in the
experimental tank is counted and number of prey in the stomach of the predators is determined by dissecting the animals and summing up for all predators. With a constant
feeding intensity the number of prey consumed will increase linearly to N at the end of
Fig. 5. Graphic illustration of the time sequence of ingestion, egestion and stomach fullness of an animal feeding with constant intensity and with a digestion time of 2 h. T, total experimental time; T , digestion time;
N, total ingestion; and N, amount of prey in the stomach.
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the experiment. The number of prey in the stomach of the predators will increase in the same way until the digestion time T is reached. From this point the ingestion will be
balanced by the egestion, and there will be a steady state with a constant number of prey N present in the stomach of the predators. Thus, if a constant feeding intensity holds
true the digestion time can be easily estimated by using simple geometry:
N T 5 N T →
T 5 N ? T N The digestion rate will then be 1 T . The advantage of the method is that the digestion
estimate is based on predators that are feeding, they can be left completely undisturbed and it is easy to get a good statistical basis for the estimate.
4. Discussion
