REGULATION OF ROTIFER SPECIES BY INVERTEBRATE PREDATORS IN A HYPERTROPHIC LAKE - SELECTIVE PREDATION ON EGG-BEARING FEMALES AND INDUCTION OF MORPHOLOGICAL DEFENSES

Temporal changes in the biomass of rotifer plankton were examined in a shallow hypertrophic lake during 1 month (April/May), when the predat ory rotifer Asplanchna brightwelli develops. The abundance of herbivor ous rotifers was regulated by predation from A.brightwelli and from th e copepod Acanthocyclops robustus. The densities and fecundity rates o f Keratella cochlearis and Keratella quadrata were negatively related with the biomass of predators. Stomach analyses showed that Asplanchna fed selectively on reproductive females of K. cochlearis, reducing th e fecundity of this species. Predators induced longer caudal spines in K. quadrata, which were negatively related to the fecundity of this r otifer, suggesting a reproductive cost associated with spine productio n. In contrast, spine length of K. cochlearis was not related to preda tors, but to temperature. These results showed that predators can redu ce rotifer densities through increasing mortality and through decreasi ng rotifer fecundity rates regardless of phytoplankton biomass. We als o show that morphological defences of K. cochlearis and K. quadrata ar e induced in different ways.