Pelagic flatworm predation on daphniids in a subtropical reservoir: different effects on Daphnia galeata and on Ceriodaphnia quadrangula

Abstract

Following a reduction in fish populations in 2004–2005, a new, single annual pulse of pelagic flatworms was observed in early summer during 2006–2009 in Liuxihe, a freshwater reservoir in South China. As soon as these worms appear, Daphnia galeata retreats into dormancy, one month earlier (July) in the presence of flatworms than previously observed (August) with fish, while the population of the related Ceriodaphnia quadrangula tends to increase. We show, through in situ lake sampling, in large enclosures and by laboratory observations, that Ceriodaphnia, although perhaps competitively inferior in its ability to acquire algal food, has a higher tolerance to flatworm toxins. As a result, Ceriodaphnia manages to coexist with and proliferate in the presence of the flatworm. Observations in the laboratory suggest that flatworm population autoregulates by being sensitive to their own toxins and that Ceriodaphnia, even if prey to the worms, likely incurs more benefit than cost from their presence.