Quantitative feeding ecology of the hydromedusan Nemopsis bachei in Chesapeake Bay

Abstract

We determined feeding rates of the hydromedusan Nemopsis bachei L. Agassiz in the mesohaline region of Chesapeake Bay, USA during the spring of 1989 and 1990 from gut contents, digestion rates and abundances of medusae and zooplankton. The medusae consumed primarily copepodites of Acartia tonsa, selecting against naupliar stages. The peak abundance of N. bachei medusae was in April to May, when densities averaged more than 10 m-3. Medusa densities were similar in both years, but were greatest (maximum of 132 medusae m-3) along a southern transect sampled only in 1990. At peak densities, N. bachei medusae consumed 30% d-1 of the copepodite standing stocks, but they consumed <1% d-1 at the lower densities typical of late May or early June. The predation effects were generally greater than those reported for other hydromedusan species. But even at peak predation, N. bachei medusae could not have controlled or reduced A. tonsa copepod populations, which had a production rate of 85% d-1 at that time. Medusa feeding rates were highest at nighttime, and were correlated with prey density in the field, but not in the laboratory.