School of Engineering and Science - Jacobs University

CHAPTER IV
out in F/2 medium over 72 hours in 73.5 mL tissue culture bottles as batch cultures with
four replicates. Incubation conditions were the same as described for the predator
cultures before. The total volume of each incubation bottle was fixed immediately with
acid Lugol’s solution at a final concentration of 2% for the determination of cell
concentrations. Samples were taken at the start and after 24, 48 and 72 hours of
incubation. To avoid density-dependent differences and allow comparability between
the experiments we maintained start concentrations, incubation conditions and
replication of this first experiment in the following experiments if not explicitly
otherwise stated.
Specific interactions
As we observed that G. dominans responded to the presence of the larger tintinnid
predator F. ehrenbergii we focused on this species interaction with the other predator
when designing the following experiments.
Chemical stimulation and swimming behaviour
To test whether chemical compounds excreted by F. ehrenbergii influenced the growth
of G. dominans, a well fed exponentially growing F. ehrenbergii culture (15 cells mL -1 )
was filtered over 0.2 µm nylon filters (Falcon). 10 mL of filtrate, equalling a final F.
ehrenbergii concentration of 2 cells mL -1 , was added to incubation bottles containing G.
dominans and S. trochoidea. Controls received 10 mL of F/2 filtrate. As the
enhancement of swimming speed can increase predator-prey encounter rates (Gerritsen
& Strickler, 1977) and thus potentially promotes higher grazing rates, we also
investigated the swimming behaviour and velocity of the prey organism S. trochoidea
and the predator G. dominans in the presence and absence of compounds released by F.
ehrenbergii in this experiment. After exposure to the filtrate for 24 hours, S. trochoidea
and G. dominans cells were filmed under a stereo microscope (SZX16, Olympus) at 50-
fold magnification for 10 seconds at a rate of 15 frames per second. Samples for cell
concentrations were fixed as described above at the start and immediately after filming.
Experiments on the pre-condition of the prey
From the detailed observation of a freshly fed F. ehrenbergii culture we found that not
every S. trochoidea cell captured by the predator was actually ingested. A certain
number of cells was egested again – becoming immobile after this “manipulation” by
the predator. This has led to the hypothesis that F. ehrenbergii promotes growth in G.
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