The phenomenal success of the SAR11 clade has been attributed to viral-defense or evolved resource-competition adaptations. The latter considered more likely after the discovery of `pelagiphages`. SCUBA-based sampling techniques, flow cytometry, qPCR, dilution experiments and microspheres tracing, combined with next generation sequencing provided evidence for species-specific and size-independent filtration of marine microbes at the phylotypes level, suggesting `grazing-avoidance` as a third mechanism leading to the success of SAR11. In situ sampling of inhaled and exhaled water by undisturbed benthic suspension feeders (ascidians, bivalves, and sponges) and in situ incubations of planktonic tunicates, demonstrate large variations in filtration efficiencies for different picoplanktonic groups irrespective of cell size. Tunicates (ascidia, appendicularia and salps) efficiently filtered photosynthetic eukaryotes and cyanobacteria as well as some other bacterial phylotypes (e.g., Rhodobacteraceae, Sphingobacteriales, and Flavobacteriaceae) and similar sized beads. Surprisingly, and in clear violation of the hydrosol filtration theory prediction, other bacteria, most notably Pelagibacter ubique and other SAR11 phylotypes, were not retained. NGS based experiments are now underway to measure the mortality rate inflicted on different common bacteria phylotypes by nano and pico planktonic grazers.
