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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
The scyphomedusa A . aurita was used as model organism because of its transparent
body and ability to accept prey put directly into the stomach. Because of this, stomach content could be controlled without killing the animals and the ingestion rate could be
precisely controlled through the frequency by which the animals were manually fed. Medusae used to study the long-term days and short-term h individual variability
˚ in digestion time were collected with a hand-held net in Vagsbøpollen, a semi-enclosed
small bay near Raunefjorden, western Norway 608139N, 58169E, in June 1995. Data on the size of the 10 medusae over the experiment are presented in Table 1. Prey
zooplankton were collected with a 180-mm WP2-net in vertical hauls from ca. 30 m to the surface. The material was brought to the laboratory in a 30-l container with seawater
21
and stage V copepodids Calanus finmarchicus average dry weight, 120 mg individual sorted out with a wide-mouthed pipette within 1 h. New, recently collected prey material
was used for each experimental day days 1, 5 and 9, respectively. Medusae and prey were stored at 108C in a constant-temperature room prior to experiment. During each
experimental day the digestion time of the medusae was measured at 3 times with 3 h between the measurements, starting the first experiment around 09.00 and the third one
around 15.00. During these measurements medusae were kept on a cooling table 10628C in 300-ml glass bowls, and three stage V copepodids C
. finmarchicus were put into the manubrium of the medusae with a pipette. The stomach content was checked in
a Wild M5 dissection microscope every 10 min in the beginning and every second minute when stomach content began to disappear, keeping the medusae continuously in
Table 1 Aurelia aurita; bell diameter and average predation rate of 10 individual medusae used in repeated digestion
a
experiments with the copepod Calanus finmarchiucus three individuals of stage V copepodids as prey
21 21
Medusa Bell diameter mm
Pred. rate prey h ind.
Day 1 Day 5
Day 9 Days 2–4
Days 6–8 A
43 39
38 1.06
1.60 B
34 30
31 0.89
1.15 C
42 33
30 0.86
0.68 D
38 32
34 0.63
1.00 E
42 36
37 0.86
0.92 F
41 35
40 0.75
1.18 G
36 36
31 0.54
0.72 H
37 32
32 0.69
0.85 I
41 40
34 1.29
1.14 J
40 40
41 0.88
1.00
a 21
The prey concentration during days 2–4 and 6–8 was 25 prey l .
˚
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. Bamstedt, M.B. Martinussen J. Exp. Mar. Biol. Ecol. 251 2000 1 –15
the bowls. Prey rejection only occurred a few times and then in connection with the mechanical feeding procedure. New prey was then added. We therefore never observed
any lost prey in any of the experiments. The digestion time was defined as the time of complete disintegration of prey with no
solid remains. After the third measurement of an experimental day each individual
21
medusa was transferred to an aerated 10-l aquarium with 25 fresh C . finmarchicus l
as food, and kept in a constant-temperature room at 108C and darkness until the next
experiment. The average predation rate was calculated for the two periods between digestion experiments as the reduction in prey number per unit time see Table 1. All
medusae were healthy and active throughout the experiment and a thorough control after finished experiment indicated perfect condition of all 10 medusae.
Ephyrae originating from a laboratory stock of polyps, were reared on a diet of Artemia nauplii and natural zooplankton to small medusae in the size range 12–70 mm
diameter and used in experiments where the ingestion rate was either constant or changed in a pre-defined manner see Table 2. The medusae were used in the
experiments when attaining an average diameter between 21 and 56 mm Table 2. The medusae were kept individually in 300-ml glass bowls on a cooling table at 108C as
described above. The remaining visible prey in the stomach of the medusae were counted under a stereomicroscope immediately before new prey were added, and without
taking them away from the bowl. Remains in the stomach that could be identified as a prey was recorded as a present prey, even though it could be almost completely digested.
One problem in displaying the results from such records then is that the continuous digestion is not reflected properly, and the plots give an impression of a discontinuous
process. This will be most obvious during a low feeding intensity, when there will be few prey in the stomach, and where individual differences of only 61 prey in the
stomach content may change the calculated digestion time to double or half. However, this has no implication for the precision in the experimental method described in the
paper, where only end-point measurements are used.
Table 2
a
Aurelia aurita; experimental design and biometric data for medusae used in predefined ingestion schedules Exp.
Ingestion schedule Diameter
Wet wt. Dry wt.
AFDW
21
Time h Prey h
range mm mg
mg mg
1 510
110 21–28
0.9960.35 40.5614.1
32.2610.8 2
510 210
37–51 4.0961.14
1856149 123638.5
3 510
410 27–33
1.6360.33 67.0611.6
51.069.3 4
510 810
40–46 4.1360.83
164634.7 126625.1
5 213
114 14–32
1.3061.19 55.1651.1
37.7637.3 6
213 411
12–27 0.9560.50
40.6622.8 27.7615.9
7 213
218 22–39
2.0360.81 81.4640.3
56.0630.4 8
213 812
30–48 2.6061.54
109664.5 79.1647.9
9 414
114 48–50
7.8960.96 333643.2
242629.0 10
414 411
44–61 8.5562.74
3646116 263683.0
11 414
218 47–57
8.7362.08 368695.9
266664.9 12
414 812
45–70 10.7264.34
4426169 3326138
a
Each experiment comprised five individuals.
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. Bamstedt, M.B. Martinussen J. Exp. Mar. Biol. Ecol. 251 2000 1 –15 5
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
