Intraguild predatory interactions between the jellyfish Cyanea capillata and Aurelia aurita

While qualitative observations of jellyfish intraguild predation abound in the literature, there are only few rate measurements of these interactions. We quantified predation rates among two common jellyfish in northern boreal waters, Cyanea capillata and its prey Aurelia aurita, both of which also feed on crustacean zooplankton and fish larvae. A series of incubation experiments using a wide range of prey concentrations (0.38-3.8 m(-3)) in large containers (2.6 m(3)) was carried out. By replenishing the prey continuously as they were captured we maintained a nearly constant prey concentrations. Ingestion rates increased linearly up to prey concentrations of 1.92 m(-3), yielding maximum clearance rates of similar to 2.37 +/- 0.39 m(3) predator(-1) h(-1) for C. capillata predators 16 +/- 2.3 cm in diameter. Mean ingestion rate at saturated prey concentrations (1.92-3.85 m(-3)) was 4.01 +/- 0.78 prey predator(-1) h(-1). Behavioral observations suggested that predators did not alter their swimming behavior during meals, and thus that feeding rates were generally handling limited rather than encounter limited. Predators captured more prey than needed, and semi-digested prey was often discarded when fresh prey was encountered.