Determination of the filter-feeding rates of salps (Tunicata, Thaliacea)

Salps are filter-feeding pelagic tunicates with a world-wide distribution in oceanic and some coastal waters. Feeding salps filter particles by pumping water past a mucous net suspended in the pharyngeal cavity. Muscular pumping moves water through the net, which retains very small particles. Determination of the rates at which salps of different species and sizes remove and ingest particles is a prerequisite to estimating the energetics of salps and their impact on the surrounding ecosystem. The activity and fragility of the animals makes them difficult to maintain or manipulate without generating artifacts, and a variety of field and laboratory methods have been tried to measure salp feeding. Eight methods to quantify grazing are discussed, including particle clearance experiments, gut pigment analysis, fecal production, and indirect calculations from metabolic rate and swimming behavior. Comparison of the assumptions, weaknesses, and strengths of each method suggests that no single approach may be best in all cases, due to differences in behavior and physiology among salp species. However, analysis of gut contents using pigment as a tracer appears to provide results for several species that are close to probable in situ feeding rates.