Trophic role of ambush-foraging hydromedusae in the White Sea



Andrey A. Prudkovsky, Faculty of Biology, Moscow State University, Leninskie Gory 1-12, Moscow 119991, Russia.



The purpose of this study was to determine the trophic role of three ambush-feeding hydromedusae inhabiting the White Sea, both in the natural environment (in the sea) and in an experimental system in the laboratory. Aglantha digitale (up to 35 ind.·m−3), Bougainvillia superciliaris (up to 4 ind.·m−3), and Sarsia tubulosa (up to 50 ind.·m−3) were the most abundant hydromedusae species apart from Obelia spp. There was a variety of zooplankton prey in the stomachs of ambush-foraging medusae in the sea. A prey selection index C (Pearre 1982) was positive and significant for copepod prey and cirriped nauplii (B. superciliaris and S. tubulosa), copepods, cladocerans and chaetognath eggs (A. digitale). In laboratory experiments, A. digitale was poor at capturing cirriped larvae. It may be that a deficiency in desmoneme nematocysts prevents A. digitale from retaining this prey. Diet, ingestion and digestion rates of these three ambush-foraging medusae did not agree completely. It appears that A. digitale has a shorter digestion time compared with the anthomedusae. Individual predation rates in the sea were low for A. digitale in comparison with anthomedusae. Bougainvillia superciliaris cleared 0.018–0.06 l ind.−1 h−1 with Oithona similis nauplii as a food source in an experimental setting. The average predation rate of medusae in the sea was in the range of experimental values. The prey with the latest onset of reproduction, such as eurythermic copepods, were the most affected by medusae predation. Also, anthomedusae feeding substantially increased mortality rates for cirriped larvae due to high selectivity for this prey.