The Effects of Environmental Cues on Production of Dormant Eggs in an Exotic Zooplankton

Abstract

Daphnia lumholtzi is an invasive zooplankton species that has spread rapidly throughout the United States since its accidental introduction in 1989. Because of its ability to rapidly colonize diverse habitats, Daphnia lumholtzi provides a unique opportunity to investigate the traits that characterize successful aquatic invaders. One trait that may support rapid range expansion is the production of dormant eggs (DE’s) that can be dispersed to new environments by migratory animals and human activities. Some studies have suggested that D. lumholtzi produces more DE’s compared to native Daphnia, contributing to rapid expansion in this species. DE’s are typically produced when environmental cues induce a population of asexually reproducing females to generate males, which then fertilize the DE’s. While it isn’t clear which cues induce DE production by D. lumholtzi in new habitats, some studies suggest that multiple cues may be required, including changes in temperature, population density, and water quality. In this study, we investigated the effects of combined environmental cues on DE production in three distinct clones of D. lumholtzi by varying the initial population density coupled with changes in temperature to simulate cooling seasons (from 22°C to 16°C) or warming seasons (from 23°C to 30°C). Clones were kept in one liter aquaria and monitored for production of males, population growth, and DE production. Preliminary results show that low population density (5 individuals/L) plus warming temperatures stimulated population growth (10 individuals/L/day) by asexual reproduction, while high density (10 individuals/L) populations began producing males at moderate temperatures (17-23°C) and produced DE’s when temperatures were increasing (23-30°C). Experiments are underway to determine if there are differences in responses to these environmental cues among the three D. lumholtzi clones.