Non-size-selective predation on fish larvae by moon jellyfish Aurelia aurita under low oxygen concentrations

Shoji Jun

doi:10.3800/pbr.3.114

The moon jellyfish Aurelia aurita has increased in abundance in coastal waters around Japan during recent decades. Since the moon jellyfish is highly tolerant of low dissolved oxygen concentrations, predation impacts by moon jellyfish on zooplankton can increase during summer hypoxia in coastal waters, which is often caused by anthropogenic effects such as an increase in nutritional loading from the land. Laboratory experiments were conducted in order to test the hypothesis that summer hypoxia leads to qualitative changes in predator-prey interactions between moon jellyfish and larval fish. Larvae of a common coastal fish, red sea bream Pagrus major (2.9, 4.1, 6.2 and 8.6 mm in standard length), were used for the experiments. Predation rates (% of larvae preyed on by a moon jellyfish per 10 min.) were examined at four oxygen concentrations (1, 2, 4 and 5.8 mg L⁻¹) in 10 L tanks (4 replicates). Size-selective predation was observed at the two highest oxygen concentrations (4 and 5.8 mg L⁻¹): about half of the 6.2 and 8.6 mm larvae survived the 10 min. trials while more than 85% of the 2.9 and 4.1 mm larvae were captured. Larval body size did not affect the predation rates at the two lowest oxygen concentrations (1 and 2 mg L⁻¹): more than 90% of larvae in all size classes were caught. These results indicate that trophic flow from ichthyoplankton to moon jellyfish increases during summer hypoxia in coastal waters and a qualitative change in predator-prey interaction, i.e., shift from size-selective to non-size-selective predation occurs at oxygen concentrations <2 mg L⁻¹.

Non-size-selective predation on fish larvae by moon jellyfish Aurelia aurita under low oxygen concentrations

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