Predator-induced defense in rotifers: developmental lags for morph transformations, and effect on population growth

This study examines two aspects of predator-induced, morphological defense in rotifers: developmental time lags for acquisition and loss of the defense, and cost of the defense. When the predator and its inducing kairomone disappear from a community, the extent to which prey population growth is inhibited by the retention of the induced defense will be determined by the reversibility of the defense and the magnitude of any cost associated with the defense. In Brachionus calyciflorus, Asplanchna-induced posterior spines were lost more slowly than acquired. When females of the induced morph were removed from Asplanchna kairomone after oviposition of their first egg, they continued to produce long-spined offspring throughout their life. Even when cultured from birth to death without Asplanchna kairomone, females born with long, induced spines produced daughters with some induced spine development. In contrast, when females of the basic (non-induced) morph were exposed to Asplanchna kairomone after oviposition of their first egg, some of their second and all of their subsequently oviposited eggs developed into daughters with long spines. In B. calyciflorus and also Keratella tropica, the reproductive cost of Asplanchna-induced defense was determined by comparing population growth rates of basic and Asplanchna-induced morphs. Populations were initiated with individuals of either morph, and Asplanchna was present with the induced morph to assure that all individuals born during the culture period had long spines. In each of three separate experiments with each rotifer species, population growth rates of basic and induced morphs were statistically indistinguishable. Thus, possession of long spines in the absence of Asplanchna is unlikely to inhibit the reproduction of these rotifers through any allocation or energetic cost.