Asplanchna‐induced polymorphism in the rotifer Keratella slacki1

Asplanchna releases into its environment a filterable factor which induces eggs of Keratella slacki to develop into individuals with slightly larger (≈ 15%) bodies, considerably longer (≈ 30%) anterior spines, much longer (≈ 130%) right posterior spines, and sometimes short, left posterior spines not otherwise present. The Asplanchna‐induced morph is much less susceptible to Asplanchna predation than the noninduced morph, its long spines making it about two times less likely to be captured after being attacked and about five times less likely to be ingested after being captured. The pronounced asymmetry of the posterior spines may minimize the cost of spine production without sacrificing protection.