Evidence for constant and highly specific active food selection by benthic ciliates in mixed diatoms assemblages

Observational and experimental studies have shown that phagotrophic ciliates are highly selective predators. However, little is as yet known about the actual mechanisms involved in prey selection. We used direct behavioral observations to study the mechanism of prey selection in benthic algivorous ciliates feeding on mixed assemblages of diatom species. Four ciliate species, viz. three Strombidium species and a Pseudochilodonopsis species, and three diatom species from intertidal sediments in the Schelde estuary were used for the experiments. In each experiment, a single ciliate species was offered a mixture of two diatom species. The feeding preferences of the ciliates were estimated as well as relative encounter rates, attack probabilities, and capture successes for the prey species. The feeding preferences of the ciliates were distinctly predatorߚspecific and highly discerning with respect to the nature of the prey species. They appeared to result mainly from active selection at the time of the encounters and between the encounter and attack of the diatoms. Our observations suggest that selective encounters with the diatoms were caused by noncontact detection of individual prey items, at least for the Strombidium species. Additional experiments confirmed that these ciliates were able to distinguish between diatom species on the basis of soluble chemical cues. Grazing was also constant, i.e., prey switching was not observed. Moreover, neither total prey density nor feeding history influenced the preferences. We hypothesize that these highly specialized trophic interactions may be an important driving force in shaping structure and diversity of benthic diatom communities in intertidal sediments.