Examination of small- and large-scale influences on the diet of an omnivorous polychaete indicates weak effects of upwelling

The relationship between the distributions of predators and their prey is particularly strong for predators with specialised diets. Predicting the diet of omnivores is, however, more complex. We investigated the diet of the omnivorous nereidid polychaete, Pseudonereis variegata with respect to spatial scale, upwelling and small-scale behaviour. To investigate the diet of P. variegata across a range of spatial scales, shores in the Eastern and Western Regions, within the Warm Temperate South Coast (Agulhas) biogeographic province of South Africa were sampled. Two shores, kilometres apart within an area of upwelling and two within a non-upwelling area were sampled within each region. Gut-contents were composed of plants and animals. Neither region nor upwelling was found to affect the overall diet of P. variegata, with significant differences occurring only between shores in the Eastern Region. When the species were separated into plants and animals, in the Eastern Region, significantly more animals were consumed by P. variegata in the upwelling area compared to the non-upwelling area. Carefully designed laboratory experiments for preference showed that P. variegata consumed amphipods (various species of Gammaridae) and macroalgae (Hypnea spicifera), but rarely ate gastropods (Afrolittorina knysnaensis). It was clear that P. variegata rarely chose to eat gastropods, but there was no experimental evidence to support that they preferred amphipods and macroalgae over gastropods. Findings from the experiment based on gut-contents required knowledge of whether P. variegata were making active choices among the range prey that were available in the field. Similarly, the laboratory experiments in which individuals were offered different types of food required background knowledge of the entire suite of prey consumed and also an understanding of the broader-scale environmental factors influencing the diet of P. variegata. This study, therefore, highlights the usefulness of combining large-scale studies with carefully planned laboratory experiments that consider preferences for specific types of food.

► Diet of Pseudonereis variegata from mussel beds was investigated.
► Neither region nor upwelling affected diet, differences were only between shores.
► P. variegata consumed amphipods and macroalgae but rarely ate gastropods.
► There was no evidence that they preferred amphipods and macroalgae over gastropods.
► We highlight the need to combine large scale studies with preference experiments.