How fast can plankton feed? Maximum ingestion rate scales with digestive surface area

Abstract

Allometric scaling rules have been classically used in plankton ecology to describe how the maximum ingestion rate (Imax) under steady-state conditions changes with the body size of the consumer. Empirical and theoretical concerns, however, motivate a more accurate and mechanistic description of size–ingestion relations. Here, I propose to relate Imax to the digestive surface area, which expresses the capacity in preprocessing and digesting food items. This surface area depends on both the body size and the optimal prey size of the consumer. The allometry in Imax, hence, includes a second major variable which describes different feeding modes within a consumer size class. Species with a small optimal-prey-to-predator-size ratio and, thus, a small “internal” surface-to-volume ratio, as is typical for filter feeders, have large intra-body transport lengths and lower Imax than raptorial-feeding species of the same body size. Digestive surface scaling tries to mechanistically accommodate feeding ecology, physiology and geometry. It does not explicitly resolve further possible factors affecting maximal ingestion such as nutritional quality. Still, digestive surface scaling explains the variability in published data compilations better than classical approaches when applied to the entire plankton size range. This is corroborated in further applications where the theory precisely fits anomalously steep scaling relations reported for heterotrophic nanoflagellates, ctenophores and a scyphomedusa. By introducing feeding mode and related morphological diversity into the size-dependency in ingestion rates, digestive surface scaling can be expected to improve the accuracy of size-based plankton models.