School of Engineering and Science - Jacobs University

CHAPTER IV
(24, 48, 72 hours of incubation: df = 6, t = 1.66, 0.46, 1.11, p = 0.15, 0.66, 0.31, twotailed
t-tests). This was also reflected in the mean growth rate of 0.77 d -1 over 72 hours
of incubation in both cases. A completely different picture was observed in the smaller
predator G. dominans as it always showed significantly different growth rates at each
sampling day (24, 48, 72 hours of incubation: df = 6, t = 5.32, 5.81, 4.28, p = 0.002,
0.001, 0.005, two-tailed t-tests) in the two predator treatments compared to the single
predator treatments (Figure 2a). Growth rates were twice as high as in the single
predator treatments (mean 0.58 and 0.66 d -1 ) for the first two days. This was surprising
as predation on G. dominans by the larger predator F. ehrenbergii has been shown.
During the last 24 hours of the experiment growth of G. dominans dropped to a mean
value of 0.01 d -1 along with the complete disappearance of the prey S. trochoidea. As
our starving control of G. dominans displayed positive growth rates after the first two
days of starvation (mean value 0.20 d -1 ) we conclude that this drop was due to
pronounced feeding of F. ehrenbergii on G. dominans when S. trochoidea disappeared
as potential prey during day three of the experiment.
Chemical stimulation of G. dominans by F. ehrenbergii
We exposed G. dominans for 24 hours to a filtrate of F. ehrenbergii. Measured growth
rates in treatments with filtrate (mean: 0.04 d -1 ) and in the control (mean: 0.1 d -1 ) were
lower than those observed in the first experiment but were statistically not different
from each other (Figure 4a) (t = 1.85, df = 6, p = 0.11, two-tailed t-test). The same
pattern was found for ingestion rates (t = 0.52, df = 6, p = 0.62, two-tailed t-test). G.
dominans only showed a weak ingestion in treatments that received filtrate (mean 0.06
prey cells predator -1 d -1 ) as well as in the control (mean 0.02 prey cells predator -1 d -1 )
(Figure 4a), which was probably due to differences in predator condition compared to
the first experiment. We additionally investigated swimming patterns and velocity
during this experiment for both, the predator and the prey. There were no visible
differences in the swimming patterns of both species, when looking at the paths of the
cells. Swimming velocity was not different between treatments with and without filtrate
of F. ehrenbergii (G. dominans: t = 0.45, df = 118, p = 0.66, S. trochoidea: t = 0.25, df
= 118, p = 0.80, two-tailed t-tests). It differed significantly between both species (G.
dominans: 177 µm s -1 , S. trochoidea 414 µm s -1 , t = 26.14, df = 238, p < 0.0001, twotailed
t-test).
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