Cost of reproduction in selected species of zooplankton (rotifers and cladocerans)

Abstract

Reproduction is an energetically costly biological process. Among the freshwater zooplankton, rotifers and cladocerans reproduce parthenogenetically and the cost of reproduction can be estimated using the life table data from demographic studies. Reduced probability of future survival or future reproduction as a result of current investment in offspring production (trade-off) is the central theme of the cost hypothesis. Correlations using present reproduction vs. future reproduction (called the reproductive costs) or future survival (called the survival costs) can be used to evaluate the cost hypothesis. In this work sets of correlations were made: (1) between present reproduction (mx) vs. future survival (lx+1, lx+2, lx+3 etc. for the entire lifespan) (survival costs), and (2) present reproduction (mx) vs. future reproduction (mx+1, mx+2, mx+3 etc. for the entire reproductive span) (reproductive costs). These correlations were plotted against the cohort age-classes in order to quantify survival and reproductive costs in rotifers (Asplanchna girodi, Brachionus macracanthus, B. variabilis and Platyias quadricornis) and cladocerans (Ceriodaphnia cornuta – one strain maintained on Chlorella and another strain adapted to Microcystis), Daphnia carinata, D. laevis, Moina macrocopa, Pleuroxus aduncus, Scapholeberis kingi and Simocephalus vetulus). All the tested rotifer species showed negative tendency in correlation coefficients (when the data of current reproduction vs. future reproduction and future survival were plotted) for both reproductive and survival costs. However, from the total survival and reproductive costs derived, 84% of the former and 42% of the latter were statistically significant. In cladocerans about 80% of the costs (correlations between current reproduction vs. future survival or future reproduction) were negative suggesting that present reproduction had negatively affected both the further survival and reproduction of test populations. In terms of statistically significant survival costs, the cladocerans showed a trend slightly lower (72%) but comparable to rotifers. The reproductive costs were significant in 45% cases. In our study, the simple statistical correlations detected the trade-offs between reproduction and survival. Thus, in more than 60% cases of both survival and reproductive costs in zooplankton were negative, and our data supported the cost hypothesis, in the majority of cases where reproduction by zooplankton of a given age class caused reduced survival and reproduction of the next age class.