PS 13-122 - Daphnia morphological variations across an ion gradient

Background/Question/Methods

Both low calcium and elevated salinity conditions have been shown to provide stressful conditions for Daphnia, an important zooplankton herbivore in freshwater lakes and ponds. Both salt toxicity and calcium limitation can impair growth, reproduction, and the ability to produce induced predator defenses (e.g., helmets and tail spines). We explored the question of how Daphnia body size and predator defenses varied among populations from lakes varying in ion availability.



We surveyed daphniid assemblages in 20 Maine lakes along gradients of both calcium and chloride availability. The lakes were chosen from historic data provided by Maine’s Department of Environmental Protection and represent the ion range found in the region. For each lake we identified the Daphnia species present and measured a subsample of individuals representing the two most common species (Daphnia ambigua and Daphnia catawba) to estimate body length, helmet length (for D. ambigua), and tail spine length.

Results/Conclusions

We observed a positive relationship between Daphnia species richness and calcium availability (p=0.004) but no relationship with chloride concentrations (p=0.466). Body size and shape responses to ion availability varied among species. For D. catawba, body length decreased with increasing concentrations of both chloride (p=0.006) and calcium (p=0.006). D. catawba tail spines were larger with increasing chloride levels (p=0.007) and calcium (0.013). There was a marginally significant negative relationship between body length and chloride for D. ambigua (p=0.06). Calcium had no effect on size or defenses for D. ambigua (p=0.282 and 0.106).

These findings support that Daphnia catawba populations may have adapted to elevated chloride levels by decreasing their body size, as observed in an experimental evolution study using closely related Daphnia pulicaria. However, we saw no other evidence that chloride levels harmed Daphnia diversity or altered their morphology. We were surprised to see that calcium levels had a community level impact but a negative or absent effect on size and predator defense. Future research will explore other physiological impacts that calcium limitation may have on Daphnia species in the region.