Divergent phenotypic responses to predators and cyanobacteria in Daphnia lumholtzi

Daphnia lumholtzi is an exotic species of zooplankton that was first introduced in Texas, U.S.A., and has since spread across most of the areas in the country. Research has focussed on the importance of the production of morphological defences in response to predator exposure as a key explanation for the spread of this species. Very little is known about the influence of other common environmental stressors on the fitness of this species or the potential role for genetic variation in plasticity in the spread of this species. We assessed the effects of two common environmental stressors, invertebrate predator cues and increasing concentrations of cyanobacteria, on the life-history traits in D.lumholtzi. This study also included clones from three reservoirs in Texas to measure potential genetic variation in plasticity. We found that the expression of life-history traits depended strongly upon the combined effects of predator exposure and food quality. Increasing concentrations of cyanobacteria resulted in delayed maturation, slower growth, a smaller size at maturation, declines in clutch size and slower rate of population growth. Yet, such declines were much smaller in the presence of predator cues. Divergent responses observed were the size of defence traits; Daphnia reared in the absence of predator cues increased the size of spines as the concentrations of cyanobacteria increased, while the opposite pattern was observed in the presence of predator cues. This study also revealed extensive genetic variation in the magnitude and direction of phenotypic responses to predators and cyanobacteria. These latter results illustrate that significant clonal variation in plasticity may help promote the spread of this species.