Behavior of <Emphasis Type="Italic">Nemopsis bachei</Emphasis> L. Agassiz, 1849 medusae in the presence of physical gradients and biological thin layers |
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Authors: | Email author" target="_blank">Jessica?R?FrostEmail author Charles?A?Jacoby Marsh?J?Youngbluth |
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Institution: | (1) Institute for Hydrobiology and Fisheries Science, University of Hamburg, Olbersweg 24, 22767 Hamburg, Germany;(2) Program in Fisheries and Aquatic Sciences, School of Forest Resources and Conservation, University of Florida, 7922 NW 71st Street, Gainesville, FL 32653, USA;(3) Harbor Branch Oceanographic Institute at Florida Atlantic University, 5600 U.S. Highway 1 North, Fort Pierce, FL 34946, USA |
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Abstract: | 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. |
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