Interactive effects of algal level and predator density (Asplanchna sieboldi) on the life-history strategy and morphology of Brachionus calyciflorus.

Predation plays an important role in mediating the coexistence and the community structure of rotifera. In response to predation stresses, prey rotifers develop morphological defenses and change their life-history strategy for the reallocation of energy investment. Yet, how these changes respond to different total energy ingestion remains largely unknown. In the present study, we investigated the life-table demographic, population growth and morphological parameters of Brachionus calyciflorus in response to various densities of Asplanchna sieboldi at two food levels (1.0  ×  10 and 2.0  ×  10 cells per ml of Scenedesmus obliquus). To resist predators, B. calyciflorus developed long postero-lateral spines. The life-table demographic results showed that predation only significantly shortened the generation time of B. calyciflorus at the algal level of 2.0  ×  10 cells per ml, but not affect other demographic parameters. The population growth tests revealed that predator density, algal level and their interactions all significantly influenced the population growth rate of prey rotifers. At 1.0  ×  10 cells per ml of S. obliquus, treatments with two, four, and eight A. sieboldi individuals per 50 ml significantly reduced the population growth rate of B. calyciflorus. In contrast, the population growth rate was not affected in response to two A. sieboldi individuals per 50 ml at 2.0  ×  10 cells per ml of S. obliquus. These results suggested that algal level probably affected the inhibitory effects of predation on the population growth of prey rotifers, and the underlying mechanisms should be further investigated.