Decreased Calcium Concentration Interferes With Life History Defense Strategies Of Ceriodaphnia Cornuta In Response To Fish Kairomone

Declining calcium in freshwater lakes has become a serious threat to aquatic organisms, which may affect the anti-fish defense of cladocerans with high calcium demand. To evaluate the effects of low calcium on life history strategies of cladocerans for defending against fish predation, we exposed Ceriodaphnia cornuta to eight concentrations of calcium ranging from 0.5 to 50.0 mg L-1 in the absence and presence of fish kairomone, and observed the changes in body surface, recorded the key life-history traits, and calculated the degree of its induced defense response. Results showed that, on average, low calcium (0.5, 1.0, 1.5 mg L-1) significantly reduced survival, maternal body size at maturation, and total offspring per female by 66.7%, 8.0%, and 48.8%, respectively, compared with those exposed to normal calcium level (10.0 mg L-1). At the normal calcium level, fish kairomone shortened the time to maturation, reduced maternal body size at maturation, and increased total offspring by 15.7%, 11.0%, and 29.6%, respectively, compared with those not exposed to fish kairomone. In the presence of fish kairomone, the reproduction of C. cornuta was affected by a narrower range of calcium concentrations. Calcium concentrations and fish kairomone showed a significant interaction on maternal body size at maturation, total offspring, and body size of neonates, i.e., low calcium impaired the life history defenses involved in maternal development, but enhanced the defense traits involved in reproductive performance, which indicated the diversified interference effects of decreased calcium on the life history defense strategies of C. cornuta against fish predators.