Feeding of Aurelia sp. (Scyphozoa) and links to the microbial food web

A large fraction of primary production is recycled through a marine protozoa–bacterioplankton predator– prey chain in most aquatic ecosystems. The size of food resources and the ratio between predators and prey determine the organic matter transfer through the ‘classical’ or the ‘microbial’ pathway (Azam et al. 1983). The effect of grazers on the prey can play an important role in selectivity at the lowest trophic levels and the productivity in pelagic systems. Although the predators are usually between one and three times larger in body mass than the prey, they have the ability to discriminate between alternate prey types depending on food quality (Verity 1991; Strom 2000). The numbers of trophic steps between the producers and the top predator may determine the flux of energy and chemical elements, which is defined as food web efficiency (Rand & Stewart 1998; Berglund et al. 2007). Top-down regulation of plankton assemblages was studied in the marine environment (Olsson et al. 1992) and in mesocosm experiments (Granéli & Turner 2002). Researchers have examined the feeding selectivity of gelatinous planktivores, but there is still controversy regarding the prey and feeding of Aurelia (Sullivan et al. 1994; Graham & Kroutil 2001). Dramatic impacts on the dynamics of zooplankton and a strong decline of the pelagic fisheries globally have been attributed to predation and competition from pelagic coelenterates (Niermann 2004). Selective predation has been found in ctenophores, the scyphomedusae Cyanea capillata and Aurelia aurita and various hydromedusae. The influence of jellyfish on the plankton food web may be significant in environments where jellyfish appear frequently in great densities or in environments where they are abundant all year round. A recent review by Purcell et al. (2007) reports on increasing problems with jellyfish blooms worldwide that may have direct negative impacts on humans. Some jellyfish are severe stingers and interfere with tourism, they may kill farmed fish in offshore mariculture and hamper