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Selective feeding by flagellate and ciliate protists is important in shaping microbial communities in pelagic habitats. While less is known of predation on microbial communities in benthic habitats, the abundance and high filtering capacity of sponges suggests that they are key grazers. We studied the feeding preferences of two of the most common glass sponges of North-East Pacific fjords, Rhabdocalyptus dawsoni and Aphrocallistes vastus. Sponges were maintained in large darkened tanks supplied with running seawater from the nearby fjord. The water inhaled and exhaled by the sponges was simultaneously sampled and analyzed using a flow cytometer. Both sponges showed a similar (but not identical) feeding pattern, efficiently removing up to 99 % of the most abundant bacterial cells, whereas clays, silt, and 'debris' particles were expelled into the exhaled water. Filtration efficiencies were maximal for the relatively large and rare eukaryotic algae (3 to 5 μm, 86±9%) and for small non-photosynthetic bacteria (<0.4 pm, 89 ± 10%), while intermediate sized non-photosynthetic bacteria characterized by higher nucleic acid content were efficiently removed in February (92 ± 3%) when overall plankton concentration was low, but not in July (28 ± 16%). The intermediate sized photosynthetic prokaryote Synechococcus (1.1 to 1.5 pm) was also less preferred. Detailed analysis of the ultrastructure of the glass sponge filtration apparatus argues against possible 'by-pass' routes. We suggest that selective filtration involves individual processing, recognition, sorting, and transport of each particle through the sponges' syncytial tissue. Selective grazing by glass sponges, like their pelagic protozoan counterparts, could be an important mechanism shaping microbial communities in the detrital food web of North-East Pacific fjords.

Size independent selective filtration of ultraplankton by hexactinellid glass sponges