Induction of different defences by two enemies in the rotifer Keratella tropica: response priority and sensitivity to enemy density

P>1. Keratella tropica has qualitatively distinct spine-development responses to kairomones released by the predatory rotifer Asplanchna and the cladoceran interference competitor Daphnia. Asplanchna induces a fourfold lengthening of the right posterior spine (to c.100 mu m), a shortening or loss of the left posterior spine and a lengthening of two pairs of anterior spines. Daphnia induces a moderate elongation of both right and left posterior spines. This study tests three hypotheses regarding the sensitivity of these responses to enemy density, and the response priority when both enemies are present. 2. First, since K. tropica and Brachionus calyciflorus have similarly pronounced and effective spine-development responses to Asplanchna, with no appreciable demographic cost, they should be similarly sensitive to Asplanchna density. This was the case. Both showed an exponential response to increasing Asplanchna density, well described by an asymptotic exponential regression model, and exhibited 50% maximal spine development at statistically similar Asplanchna densities-2.5 and 1.7 mu g dry weight L-1 (1.8 and 1.2 individuals L-1), respectively. Strong selection for these Asplanchna-induced responses clearly has led to a coupling of exuberant and effective morphological defence with an unrivalled sensitivity to predator density. 3. Second, since K. tropica's response to Daphnia is much less pronounced and effective than its response to Asplanchna, it should be less sensitive to Daphnia density. This hypothesis was supported. Spine development increased linearly with increasing Daphnia density and was 50% maximal at 454 mu g dry weight Daphnia L-1, a biomass density 180 times greater than that inducing a comparable response to Asplanchna. 4. Third, since K. tropica's response to Daphnia does not reduce Asplanchna predation, K. tropica should respond to Asplanchna when both enemies are present at densities sufficient to induce spine development. This was the case. The presence of Daphnia neither reduced nor increased the length of the right posterior spine; it only limited the extent to which the left spine was reduced or lost.