The Allocation of Resources to Reproduction in Daphnia Galeata: Against the Odds?

Variation in offspring is a size common phenomenon in many organism. In cladoceran zooplankton large offspring are known to have a high starvation resistance. One could, therefore, expect offspring to be large at low food levels, whereas at higher food levels the production of more, but smaller, offspring would yield the highest parental fitness. However, in Daphnia galeata I found that individual offspring were smallest at a low food level and largest at intermediate food levels. Moreover, in contrast to the predictions made by several theoretical models, I found that larger mothers produce larger offspring. The first discrepancy between data and theory could be explained by the existence of a maximum offspring size, combined with the difficulty of Daphnia to produce one egg less than was done in reality. The dependence of offspring size on maternal size could be explained by the higher likelihood of intraspecific competition when large (=old) females are present, and hence higher starvation risks for the offspring. It was found that embryonic respiration was lowest at the lower food levels and hence the smaller individual egg size seemed to be compensated by lower carbon losses during development.