Feeding selectivity and niche characteristics in Southern Ocean salps

Abstract Salps are a group of pelagic tunicates with widespread distribution and the potential to strongly impact ecosystem dynamics through their rapid reproduction, carbon export and filtration rates. Like most filter feeders, they are considered to be nonselective in their feeding, although recent reports of differences in the proportions of prey types in salp diets compared to their availability have challenged this paradigm. We investigate the potential for selective feeding and its impacts on salp niche breadth and overlap using scanning electron microscopy of the gut contents of 58 salps from the Southwest Pacific east of New Zealand representing seven species: Salpa thompsoni, Thetys vagina, Thalia democratica, Salpa fusiformis, Ihlea magalhanica, Soestia zonaria and Pegea confoederata. We then compare their diet to water column plankton communities via FlowCam and flow cytometry. While most prey types were consumed without preference as expected, dinoflagellates were positively selected across five of the seven salp species regardless of water column prey compositions. Large, spinous diatoms and submicron particles were consistently negatively selected. These selectivities resulted in high niche breadths that still fell short of true generalists, highlighting that selectivity is a spectrum rather than a dichotomy.