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

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.

Fine-scale vertical distributions of Mnemiopsis leidyi ctenophores: predation on copepods relative to stratification and hypoxia

“…1). Those locations are known for chronic summertime hypoxia in previous years (Keister et al 2000, Breitburg et al 2003 , was calculated as the difference between adjacent depths in seawater density (σ t ), as calculated from T and S using a seawater density calculator (Tomczak 2000), based on Fofonoff & Millard (1983). Although sampling was necessarily at discrete depths, all variables form continuous gradients in the field.…”

“…Graham et al 2001, Jacobsen & Norrbin 2009), the possible importance of density discontinuities and stratification per se must be considered. Both Arai (1992) and Frost et al (2010) concluded that hydromedusae behaviorally aggregate at salinity discontinuities. Aurelia sp.…”

“…Ctenophores, medusae, and copepods are known to accumulate at density discontinuities in the water column (Arai 1992, Graham et al 2001, Malej et al 2007, Jacobsen & Norrbin 2009, Frost et al 2010. Arai (1992) concluded that Sarsia tubulosa hydromedusae behaviorally aggregate at salinity discontinuities.…”

“…Arai (1992) concluded that Sarsia tubulosa hydromedusae behaviorally aggregate at salinity discontinuities. Similarly, hydromedusae of Nemopsis bachei remain in discontinuities created by salinity gradients with and without thin layers of algae and copepods (Frost et al 2010). Most Aurelia spp.…”

“…Thus, gelatinous predators and their prey actively aggregate at density discontinuities. Although medusae and their feeding increase at clines (Frost et al 2010), we know of no previous studies on ctenophore feeding in situ in relation to stratification.The mesohaline Chesapeake Bay (USA) and its tributaries can have high densities of mesozooplankton, Mnemiopsis leidyi ctenophores, and Chrysaora quinquecirrha scyphomedusae during summer. The eurythermal and euryhaline calanoid copepod Acartia tonsa is the most abundant component of the mesozooplanktonic assemblage, with cyclopoid copepods Oithona colcarva and meroplankton also abundant at times (Brownlee & Jacobs 1987).…”