Behavior of Nemopsis bachei L. Agassiz, 1849 medusae in the presence of physical gradients and biological thin layers

In pelagic systems, thin layers (discontinuities with narrow vertical extents and high concentrations of organisms) create patches of food, and aggregations of gelatinous zooplankton can exploit such resources. The establishment, maintenance, and trophic effects of these functional relationships depend on behavioral responses to thin layers by individuals, which remain largely unexplored. In this study, we used laboratory experiments to test the hypothesis that a common and abundant hydromedusa predator, Nemopsis bachei L. Agassiz, 1849, would respond similarly to salinity gradients with and without thin layers of algae and copepods. Approximately 75% of the hydromedusae remained in both types of discontinuities. These distributions were not created solely by passive responses related to osmoconformation or an inability to swim through salinity gradients because approximately 25% of hydromedusae swam through or away from salinity gradients or biological thin layers. Biological thin layers stimulated feeding. Feeding success was related directly to encounter rates and it was independent of swimming, as expected for an ambush predator. Feeding increased at higher prey densities, and capture, handling time, and ingestion were not saturated even at 150–200 copepods l−1. The proportion of N. bachei that ceased feeding and began swimming increased when encounters with prey decreased to approximately 2 encounters hydromedusa−1 10 min−1. Thus, hydromedusae may seek new patches of prey once encounter rates and subsequent feeding success fall below a threshold. Exposing N. bachei to salinity gradients with and without biological thin layers indicated that these hydromedusae will remain in discontinuities and exert predation pressure that should be considered when assessing trophic webs and estimating carbon flux.