Predator-specific inducible defenses in the rotifer Keratella tropica

1. A Patagonian strain of Keratella tropica has very different induced morphological responses to two predators – the carnivorous rotifer Asplanchna brightwelli and the interference competitor Daphnia pulex. Asplanchna induces the most exuberant morph. Compared to the basic morph, it has a fourfold longer right posterolateral spine (up to c. 115 μm), greatly elongated anterolateral and anterosubmedian spines, and no left posterolateral spine. Transitional morphs have an incompletely developed right posterolateral spine and a reduced left posterolateral spine. Daphnia induces moderate development of both posterior spines but no elongation of any anterior spines. Induction of these morphs by Asplanchna and Daphnia is mediated by kairomones. 2. The Asplanchna‐induced morph is much better defended against large (0.9 mm) Asplanchna than either the basic or Daphnia‐induced morph. The long right posterolateral spine usually prevents capture or ingestion. The Asplanchna‐ and Daphnia‐induced morphs are similarly susceptible to interference from large (3 mm) D. pulex. 3. Life‐table experiments with cohorts of the basic and Asplanchna‐induced morphs at 5 × 10³ and 2 × 10⁴ cells of Cryptomoas erosa per millilitre indicate little or no cost of the induced defense. Lifetime fecundity (13–15 offspring per female) did not differ significantly between morphs. The mean intrinsic rate of natural increase (r_m day^?1) of the induced morph was very slightly but significantly lower than that of the basic morph at the lower food concentration (0.46 versus 0.48) but no different from it at the higher food concentration (0.53 versus 0.54). However, spine development may involve undetermined allocation costs and environmental costs relating to interactions with other organisms. 4. It is not clear why K. tropica has separate induced responses to Asplanchna and Daphnia. Moderate spine development probably reduces damage or ingestion by small (<1.5 mm) daphniids, as in other species of Keratella, but further development may confer no protection against larger ones. Thus, the ratio of benefit to cost with daphniids (and other cladocerans) may be highest for intermediate spine development. In contrast, much greater spine development seems necessary for effective defense against Asplanchna. The more moderate response to Daphnia also may reflect less likely spatial and temporal overlap.