Effects of temperature on the energy budgets of Hydra pseudoligactis

Summary Dry matter, nitrogen, and energy budgets were determined for laboratory cultures of Hydra pseudoligactis reared at 10, 15, 20, and 25° C, and fed freshly collected zooplankton. Maximum specific feeding rates increased from 0.24 to 0.91 (KJ food/KJ Hydra) with increasing rearing temperature. The corresponding growth rates increased from 0.14 to 0.29 but gross (growth/ingestion), net (growth/(ingestion-egestion)), and assimilation (gross/net) efficiencies decreased with increasing temperatures. The requisite food densities to maintain maximum ingestion rates (when permitted to feed for 3 hrs day^-1) increased from 0.0056 to 0.021 KJ of zooplankton/KJ of Hydra. The combination of increasing energy requirements, decreasing food supply, and efficiencies of food utilization, with increasing water temperatures may contribute to the rapid declines of Hydra populations that are often observed in temperate lakes.Financial support was provided by the Youngstown State University Research Council, Grant Number 287 and a Grand-in-Aid of Research to W.C. from Sigma Xi, The Scientific Research Society of North America. George Mateja and Karen Gates helped with the laboratory and field work. The Mahoning Valley Sanitary District, Mr. John Tucker, Chief Engineer, provided facilities and access to Meander Creek Reservoir. The figures were prepared by Ms. Susan Geer, Youngstown State University Media Center