1. Maternal effects have long been known to influence phenotypic plasticity in rotifers. Females in Brachionus calyciflorus and several other species produce long-spined offspring when the predatory rotifer Asplanchna is present; B. calyciflorus also develops short spines when food concentrations are low. These spines protect against predation and decrease food threshold concentrations.
2. Some strains of B. calyciflorus develop long spines even in the absence of Asplanchna and other environmental stimuli. We demonstrate in this study that spine length in such cases is dependent on the age of the mother.
3. In strains from Florida and Georgia, offspring spine length increased significantly with birth order, sometimes to lengths formerly observed only in the presence of Asplanchna. Significant variation in this trait was found among and within clones of a strain. Offspring body size also increased with maternal age. This is the first time maternal age has been shown to affect rotifer morphology.
4. These birth-order effects may have important ecological implications and explain phenotypic plasticity and polymorphism in body size and spine length in populations when predators are absent and food concentrations are high. They may be a bet-hedging mechanism to assure adaptation to rapid changes in predation pressure or food conditions.