Interspecies differences in calcium content and requirement in four freshwater cladocerans explained by biokinetic parameters

We tested the hypothesis that cladocerans with higher calcium (Ca) content are more susceptible to Ca limitation by conducting life table experiments using four cladoceran species with contrasting Ca contents (0.06-2.24% of dry weight). Populations of Daphnia carinata and Daphnia galeata with high Ca content might collapse when the ambient Ca concentration is, 0.5 mg L-1, whereas Ceriodaphnia dubia, with intermediate Ca content, and Moina macrocopa, with low Ca content, are well adapted to that low Ca level. However, Ca content is not a good proxy of the susceptibilities to Ca limitation within the genus of Daphnia. We propose an index, which considers both the Ca demand (i.e., Ca content under Ca-sufficient conditions) and the ability of cladocerans to extract and retain Ca in low-Ca environments, to explain the differences in cladoceran tolerances to Ca deficiency. We also used physiologically based biokinetic parameters, including the influx rate and efflux rate constant of Ca, to predict the interspecies differences of specific Ca content. Low-Ca species had a lower influx rate and a higher efflux rate constant of Ca than the high-Ca species. A Ca concentration of 2 mg L-1 was sufficient to keep Daphnia spp. from extinction because of Ca limitation.