School of Engineering and Science - Jacobs University

CHAPTER IV
feeding rates of F. ehrenbergii have been shown to be inhibited by a number of
dissolved free amino acids (Strom et al., 2007b) which could theoretically also be
released by G. dominans. However, in our work growth rates of the tintinnid in the
presence of the smaller predator were the same as when preying on S. trochoidea alone
and thus such a chemical influence could be rejected.
A different picture was observed in the last experiment with a new F. ehrenbergii
culture. Here we detected a mortality rate of around -0.22 d -1 in the presence of F.
ehrenbergii indicating predation on the smaller G. dominans. However, a pronounced
selective predation on G. dominans that would also promote the autotrophic prey S.
trochoidea due to the partial release of grazing pressure (Stoecker & Evans, 1985) was
not observed in our experiments.
Competitive predator relationship with a commensalistic element
Our findings are in contrast to results of another study where no difference in growth
rates was found for a dinoflagellate or its potential ciliate predator competing for the
same prey when compared to the single predator treatments (Jakobsen & Hansen, 1997)
and another study where intraguild predation between the predators favoured the prey
(Stoecker & Evans, 1985). Even if both predators competed for the same prey organism
in our experiments G. dominans was directly supported by the presence, especially by
the feeding, of the other predator leading to a higher efficiency in resource exploitation.
This observed paradox could only be solved when looking at the feeding behaviour of
F. ehrenbergii. The dinoflagellate directly benefited from immobilised but not ingested
prey cells of the tintinnid. Benefits from “pre-conditioned prey” have been reported for
dinoflagellates before, e.g. when feeding on faecal pellets of copepods (Poulsen &
Iversen, 2008).
Although G. dominans can feed on different planktonic prey in the laboratory
(Nakamura et al., 1995a) it is often highly abundant during red tides of mobile
dinoflagellate prey (Nakamura et al., 1995b, Kim & Jeong, 2004). Interestingly, G.
dominans selected strongly for immobilised dinoflagellates in our experiments even if
mobile prey was available in the same concentration. This is most probably related to
the feeding habit of G. dominans. Gyrodinium species display a smooth pre-capture
swimming behaviour around the prey before it is captured and ingested (Hansen, 1992).
Taking this habit and the swimming speed of the prey organism S. trochoidea into
account it is clear that immobile prey cells are easier captured by G. dominans even if
there were higher encounter rates with swimming prey (Gerritsen & Strickler, 1977).
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