Comparative energetics and life cycle of the opossum shrimp (Mysis relicta) in native and non-native environments

1. Life history, fecundity and energy ingestion were compared for non-nativ e Mysis relicta in Flathead Lake, Montana, and in Waterton Lake, Montana/Al berta, where the mysid is native. 2. Based on estimates from stomach contents, M. relicta in Flathead Lake co nsumed three to four times more energy per hour while foraging than M, reli cta from Waterton Lake. The primary prey of M. relicta in Flathead Lake was Daphnia thorata. Diaptomus sicilis was the primary prey for M. relicta in Waterton Lake. 3. Mysis relicta in Flathead Lake had a 1-year life history. It took 2 year s for M. relicta in Waterton Lake to complete a generation. Mysis relicta f rom Flathead Lake produced significantly (P < 0.05) more eggs per female (1 9.9 +/- 4.7) compared to M. relicta in Waterton Lake (13.7 +/- 3.6). The Li pid content in eggs from M. relicta in Flathead Lake was also significantly (P < 0.05) higher than eggs from hi, relicta in Waterton Lake. 4. Ln Flathead Lake, M. relicta is able effectively to exploit Daphnia thor ata, consuming greater biomass than M. relicta in Waterton Lake. Mysis reli cta efficiently assimilates the greater biomass, which reduces generation t ime, increases fecundity, and increases lipid reserves of offspring. These results provide evidence of energetic mechanisms that enable M. relicta to colonize and affect foodwebs when introduced into lakes.