Comparative studies on the feeding behavior of two heterotrophic nanoflagellates: the filter-feeding choanoflagellate Monosiga ovata and the raptorial-feeding kinetoplastid Rhynchomonas nasuta

Even though bacterivorous flagellates have been recognized as being able to structure the bacterial community, their behavioral basis for food selecti on is hardly known. We investigated the feeding behavior of the filter-feed ing choanoflagellate Monosiga ovata and the raptorial-feeding kinetoplastid flagellate Rhynchomonas nasuta feeding on a bacterial community dominated by Pseudomonas putida and compared it to that of interception-feeding flage llates. M. ovata handles several food particles simultaneously. In contrast , R. nasuta handles only 1 particle at a time. The handling time for 1 prey particle is in the range of 3.7 s for R. nasuta but more than 300 s for M. ovata. The speed of food particles within the feeding current of M. ovata was only 9.3 +/- 5.7 mum s(-1) but due to the large surface of the collar t he filtered water volume was 6.4 nl h-l and therefore comparable to that of interception-feeding flagellates of a similar size. Bacteria ingested by M . ovata were significantly smaller compared to the bacteria in the medium ( p = 0.001). Size selection of food particles occurs during the processing o f food items. This is in contrast to raptorial-feeding R. nasuta, which sho ws passive food size selection for bigger particles during the contact phas e. Attachment of bacteria proved to be an efficient protection mechanism, d efending them from being grazed by flagellates. There are significant speci es-specific differences in the processing of food particles which explain t he coexistence of various bacterivorous nanoflagellates in the size range o f 3 to 5 mum and indicate the existence of specific predation pressure on d ifferent bacteria.