A method for in situ estimation of prey selectivity and predation rate in large plankton, exemplified with the jellyfish Aurelia aurita (L.)

A method to estimate predation rates of large predatory zooplankton, such as jellyfish and ctenophores, is outlined. Large plankton size allows direct visual tracking of the predator during the process of foraging. The presented method is novel in the sense that it measures predation rate of a specific individual plankton predator in situ.After prey has been evacuated from the gut of an individual predator, the predator is incubated in situ, and observed by SCUBA-divers who recapture the individual after a defined time. Given that this incubation time is shorter than prey digestion time, predation rate can be calculated as increase in gut content over time. Clearance rates for different prey can be calculated from predation rates and prey concentrations in the water, allowing accurate estimates of prey selectivity. Thus, the problem of unknown feeding history and feeding environment, which can otherwise be a problem in prey selectivity studies of in situ-captured predators, is circumvented. Benefits and limitations of the method are discussed.The method was applied to adult medusae of the common jellyfish Aurelia aurita. A large variation in number of captured prey was detected both among individual jellyfish and among the various oral arms and gastric pouches within individuals. Clearance rates varied strongly with prey type. The medusae selected large crustacean prey (cladocerans and copepods/copepodites) over echinoderm larvae and copepod nauplii. Prey distribution within the medusae indicates that both tentacles and oral arms were used as prey capturing sites. Food passage time from prey capturing organs to gastric pouches was estimated.