Food choice and pharyngeal pumping activity of bacterial-feeding nematodes are driven by different functional traits

The feeding behavior of free-living nematodes is shaped by several factors, including food choice and food consumption. The latter is commonly investigated by determining the pumping rate of the pharynx. However, whether the pharyngeal pumping activity also reflects food selection, i.e. whether the pumping rate for preferred food is increased, is largely unknown. The aim of this study was to investigate the relation between pumping rate and food choice. For this, the preference of the nematodes Diploscapter coronatus , Diploscapter pachys , Plectus similis and Plectus sp. were examined in binary choice assays. The bacteria Bacillus subtilis , Kitasatospora sp., Sphingomonas trueperi , Pseudomonas fluorescens and two Legionella pneumophila strains were tested against Escherichia coli OP50. The nematode pumping rates were determined for each bacterial diet. Additionally, the effects of bacterial species and size, and of nematode species and genus, size of buccal cavity and pharynx morphology, were investigated. The nematodes showed a preference for P. fluorescens (except D. pachys ) and avoidance of B. subtilis , Kitasatospora sp. and L. pneumophila . The determinants of food choice were nematode species, bacterial species and size, while the pumping rates varied with genus and pharynx proportion. The food preference determined by the choice assay was not necessarily associated with an upregulation of pharyngeal pumping, e.g. Plectus preferred P. fluorescens over E. coli OP50 when having a choice but pumped significantly faster when feeding on the latter. Although the pumping rate changed with bacterial species, there was no clear pattern that allowed a correlation with food choice across the tested nematode taxa, suggesting that pumping frequency is no indicator for food preference. Relating these results to nematode foraging strategy, food choice was determined by bacterial traits associated with diet quality and search, while food consumption was linked to nematode traits affecting diet handling. • food selection is mainly based on bacterial traits (e.g. species, size). • pharyngeal pumping rates are determined by nematode traits (e.g. taxon, pharynx morphology). • pharyngeal pumping frequency is no suitable approach to assign food selection.