Optimal development time in pelagic copepods

According to life history theory, development times have evolved to an optimum solution of the trade-offs between rapid development and high fecundity: rapidly developing individuals have a higher chance of surviving to maturity than do slowly developing ones, but they mature at a smaller size and, thus, have lower fecundity. Here we develop a model to predict egg-to-adult times in marine pelagic marine copepods that maximizes fitness, quantified either as lifetime reproductive output or population growth rate. The model is tested against global data sets of development time as a function of temperature. The optimum development time increases when the enhancement of fecundity with delayed maturation increases, and it decreases with increasing juvenile mortality. Our predictions match the patterns in nature for broadcast spawners, for which most data are available, but appear to underpredict development times for sac-spawning copepods. The partial uncoupling of growth and development rates in copepods demonstrated in laboratory experiments, the substantial variation in age at maturity observed in copepod field populations, as well as the fair match of predicted and observed development times together suggest that development rates in marine pelagic copepods are tuned to the prevailing predation mortality. Observations in freshwater zooplankton suggest that life histories may be adapted to local conditions of growth and predation mortality. The striking lack of similar observations in marine zooplankton calls for work to explore the significance of environmental modulation of life histories of marine zooplankton. Possible future research avenues are proposed, including studies of the effects of seasonal environments, variable presence of predators and the effects of sexual selection on life histories of marine planktonic copepods.