Experimental effects of temperature and prey assemblage on growth and lipid accumulation by Mysis relicta loven

The opossum shrimp (Mysis relicta) is a holarctic planktivore that undergoe s diel vertical migration (DVM), ascending at night from deeper water or th e lake bottom into the upper water column to feed. In deep lakes that therm ally stratify, M. relicta spends most of the time on or near the lake botto m in cold water where food resources are low; but, M. relicta migrates dail y into warmer layers to feed where food resources are more abundant. Hence, in deep lakes M. relicta traverses steep temperature and prey density grad ients while migrating through the water column to forage. We conducted a la boratory growth experiment of factorial design that simulated the range of temperatures and densities of natural zooplankton forage encountered by M. relicta during DVM. Somatic growth, measured as change in total length and lipid-free biomass, was not significantly different among experimental trea tments. Analysis of variance showed that M. relicta stored energy in the fo rm of lipid instead of using the energy mainly to drive increased somatic g rowth, but only at the colder temperatures. Lipid storage was 24% (+/- 4.3 sd) in 4 degrees C-high forage treatment compared to 6% (+/- 1.1) at 14 deg rees C-high forage treatment. Energy conversion efficiency (ECE) of M. reli cta was highest (21.3% +/- 5.8) at 4 degrees C and lowest (3.3% +/- 0.9) at 14 degrees C, regardless of prey density. The experiment showed that Mysis relicta feeds at high food densities where ECE is low, but spends most of the time in colder water where ECE is high. These results support McLaren's metabolic efficiency hypothesis, which predicts that feeding in the more p roductive surface waters and then migrating to colder waters is reproductiv ely advantageous. Moreover, owing to high metabolic efficiency in the colde st waters and rapid DVM, M. relicta is able to sequester much of the epilim netic zooplankton production near or on the lake bottom, thereby forcing th e lake food web toward a configuration dominated by mysids, benthic or deep water fishes and small, agile zooplankton.