Brood space limitation of reproduction may explain growth after maturity in differently sized Daphnia species

Abstract

Our aim was to determine if a limitation of reproduction by available brood chamber space could explain indeterminate growth in Daphnia, and what conditions might affect this. We evaluated the brood chamber volumes of the larger Daphnia longispina and smaller D. cucullata species during the first three clutches of eggs and compared this with the volume of eggs produced in low and high food conditions. We also examined whether these Daphnia retained significant levels of unused lipids in their bodies after reproduction, and if the volume of eggs deposited in brood chambers of different capacities corresponded to the maximal egg volume specified by a geometrical model. Our results revealed that, at least during the early clutches, both Daphnia species produced as manyeggs as permitted by the available brood chamber volume in both high and low food conditions, thus indicating that “brood space constraint” can explain the simultaneous investment in growth and reproduction. The detailed evaluation of this hypothesis required not only brood chamber volume assessment, but also consideration of the effect of egg size on packing efficiency, especially in low food conditions and for the smaller Daphnia species. The significant lipid reserves detected in the Daphnia body after reproduction also support the limitation of reproduction by brood chamber volume, particularly in larger species.