Zooplankton feeding formulations in plankton models have exclusively focused on the relation between food concentration and ingestion, with respiration and excretion being treated separately, despite experimental evidence for strong links among these processes. We present an optimal current-feeding model linking ingestion, respiration, and assimilation efficiency to foraging activity. The Ivlev model is a special case of our optimal current-feeding model, which applies to static feeding behaviour. We validate our model with experimental data for copepods, ciliates, and dinoflagellates. Parameter estimates suggest that phylogenetic grouping is more important than predator size in determining feeding behaviour. Respiratory costs of foraging, e.g. for generating a feeding current, may be much larger than previously thought, are larger in smaller organisms, and might explain the independent development of feeding thresholds in different micro- and mesozooplankton groups. Both preferential feeding on, and lower feeding thresholds for, larger food particles are predicted to derive from greater capture efficiency owing to enhanced detectability of larger particles. The relation between feeding threshold and prey size appears to depend on feeding strategy but not on predator size, as a common relationship seems to apply for current feeders (ciliates and copepods) spanning a vast size range. Our model exhibits an inverse relationship between ingestion and assimilation efficiency, reducing the contribution of copepods to export of organic matter relative to remineralisation at low food concentrations. Export ratio variations previously thought to require strong shifts in community composition can be generated by changes in feeding behaviour predicted by our model.