Patterns of seasonal and spatial variation in the morphology of two species of Daphnia in a Minnesota lake

Background/Question/Methods

Daphnia employ a variety of behavioral and morphological defenses to avoid or inhibit predation by vertebrate and invertebrate predators. Common morphological defenses that deter invertebrate predators (e.g., Chaoborus, Leptodora) are helmets and tail spines that may develop after exposure to predator exudates. Though these defenses lessen the chance of mortality due to predation, studies have shown that their production comes with the cost of lower fertility relative to those that do not alter their morphology. Given this trade-off, Daphnia in environments with a low risk of invertebrate predation should not develop these morphological defense structures to the same extent as those in predator-rich environments. We evaluated this expectation by comparing head length to body length (HL:BL) and spine length to body length (SL:BL) ratios of two species of Daphnia (D. pulicaria and D. mendotae) collected at different times of year and different depths in the water column from Square Lake (Washington County, Minnesota – USA) in 2010. We expected 1) that the smaller-bodied Daphnia species (D. mendotae), which is more vulnerable to predation by invertebrates, would develop helmets and spines to a greater extent than the larger-bodied D. pulicaria, and 2) that HL:BL and SL:BL ratios would be greater at times of year and at depths in which invertebrate predators were more abundant. 

Results/Conclusions

Both the HL:BL and SL:BL ratios differed significantly between species, over time, and with respect to depth (three factor ANOVAs). Specifically, the smaller-bodied D. mendotae exhibited significantly greater HL:BL and SL:BL ratios than did D. pulicaria. In addition, both species developed larger helmets and tail spines in mid- to late summer, but these changes were more pronounced in D. mendotae. Lastly, Daphnia of both species collected from shallow water had significantly greater morphometric ratios for helmets and spines than did animals collected from deep water. Abundances of pelagic predatory invertebrates (e.g., Leptodora) increased as the summer progressed and were higher in samples collected from shallow depths. The findings of this study support our expectations that morphological defenses should develop to a greater extent in species that are more vulnerable to invertebrate predation (i.e., the smaller-bodied D. mendotae), and that Daphnia should invest more heavily in these defenses when the risk of predation is high.