The marine cladoceran <i>Penilia avirostris</i> and the “microbial loop” of pelagic food webs1

We used quantitative microscopy to examine feeding of Penilia avirostris on natural (&lt;1 µ m) and cultured (0.5–2.0 µ m) bacterioplankton, autotrophic phytoplankton, heterotrophic microflagellates (2–5 µ m), and bacteria‐sized (0.2–1.0 µ m) fluorescent beads. Natural and cultured bacterioplankton were not appreciably ingested, except for extremely high concentrations (&gt;9.0 × 10 6 cells ml −1 ) of clumped cells from bacterial cultures. Bacterivorous microflagellates were ingested. Most species of available natural phytoplankton (chain‐forming or large diatoms) and Pseudoisochrysis paradoxa (5–6 µ m) were not ingested, but the 4–6‐ µ m diatom Thalassiosira pseudonana and the 10–12‐ µ m Thalassiosira weissflogii were. Food choice and feeding rate appeared related to food concentration as well as cell size. Our results contrast with previous reports of bacterivory by P. avirostris and other cladocerans, possibly due to preferential bacterivory on large or aggregated bacteria, elevated bacterial abundance levels in cultures, or failure to recognize heterotrophic microflagellates as an unseen intermediate trophic step in studies with nonmicroscopic techniques. Although P. avirostris does not feed on free‐living bacterioplankton, it may be an important component of the “microbial loop” between bacterioplankton and higher consumers because of its predation on bacterivorous microflagellates.