Abstract
Jellyfish (cnidarians and ctenophores) affect the marine food web through high feeding rates and feeding efficiency, but in contrast to their great importance in the ecosystem, our knowledge of their dietary requirements is limited. Here we present the results of respiratory and feeding trials of the rhizostome Rhopilema nomadica, the dominant scyphozoan in the waters of the Eastern Mediterranean, which often establishes massive swarms, mainly in the summer months. Through multiple measurements of oxygen demand in R. nomadica at bell diameters of 3–49 mm, we were able to assess its minimum energetic requirements. These, and the results of the feeding trials on individuals of the same bell diameter range, show that R. nomadica is a very efficient predator. When presented with prey concentrations of 100 prey items per liter, a single hourly feeding session provided between 1.15 and 3 times the estimated daily basal carbon requirement. Our findings suggest that R. nomadica is well adapted to its environment, the hyperoligotrophic waters of the eastern Mediterranean, able to efficiently exploit patches of plankton, possibly at rates even higher than what we observed under laboratory conditions.
Highlights
Most scientific inquiries into the predatory impact of gelatinous zooplankton—such as scyphomedusae, hydromedusae and ctenophores—on plankton communities have focused on investigations of jellyfish diets
Of the 54 R. nomadica used in the respiration trials, 24 medusae were measured for bell diameter (BD)
The proportion of C in the dry biomass is often reported in metabolic studies and the range recorded for R. nomadica in the present study, 5–8%, corresponds well with values reported for other scyphomedusae, e.g., Aurelia aurita (4.3%, 3.7%) ([23,24], respectively), Chrysaora fuscescens (7.7%) [25] and Rhizostoma pulmo (5.6%, for juvenile medusa) [14]
Summary
Most scientific inquiries into the predatory impact of gelatinous zooplankton—such as scyphomedusae, hydromedusae and ctenophores—on plankton communities have focused on investigations of jellyfish diets. These studies have generally involved gut content analysis of jellyfish that were sampled in situ and laboratory studies in which digestion times and clearance rates are determined [1,2]. They are informative, these methods rely on assumptions that may lead to inaccurate conclusions (reviewed by Purcell [3]). These enable us to estimate the predator’s minimum energetic demand and, in turn, to estimate the minimum amount of prey required to sustain it
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