Interactive effects of water pH and hardness levels on the growth and reproduction of Heterocypris incongruens (Crustacea: Ostracoda)

pH and water hardness are considered to be important factors for population dynamics in crustaceans. In this study, the effects of pH (6, 7, 8, and 9) and water hardness (40 mg CaCO 3 /l and 160 mg CaCO 3 /l) on the growth and reproductive response of the freshwater ostracod Heterocypris incongruens Ramdohr were investigated. Heterocypris incongruens exposed to a pH level of 6 or 7 had a slower growth rate and shorter carapace length than those exposed to high pH levels. The reproductive output of H. incongruens was higher under hard water conditions. However, there were no significant differences in the effects of pH and water hardness on the egg hatching rate. Consequently, higher pH levels and water hardness resulted in increased population growth rates. Collectively, the results indicate that variations in pH levels and water hardness lead to changes in the life history characteristics of H. incongruens . These characteristics provide a mechanistic explanation for habitats in which H. incongruens can persist.