Morphological Variation and Its Significance in a Polymorphic Rotifer: Environmental, Endogenous, and Genetic Controls

The planktonic rotifer, Brachionus calyciflorus, displays extensive variation in the length of its anterior and posterior spines. Notably, posterolateral spines may be absent or near body length. Studies of laboratory and natural populations have identified the different factors controlling this variation and have investigated the trade-offs associated with increased spine development. Low temperature and low food availability can induce modest spine elongation that may reduce sinking rate. A kairomone released by the carnivorous rotifer Asplanchna induces pronounced spine elongation, without detectable reproductive cost, that can provide an effective defense against this predator. Endogenous mechanisms also operate: Spine development is inhibited in females hatched from fertilized resting eggs and can be promoted by increasing maternal age. Genetic variation for the length of spines in both noninduced (default) and induced phenotypes occurs among and within populations. Asplanchna in natural communities likely leads to seasonal selection for genotypes that can develop increasingly long spines.