Direct and indirect effects of invertebrate predators on population level responses of the rotifer Brachionus havanaensis (Rotifera)

Rotifers, in nature, are consumed by several vertebrates and invertebrates. In response to predation, they show a certain degree of plasticity in their life history traits. In this regard, there is an ample database of information on the effect of Asplanchna on brachionids; much less is known on the effect of other common freshwater predators. We studied the chemical effects of four invertebrate predators, Stenostomum leucops (Turbellaria), Asplanchna girodi (Rotifera), Eosphora najas (Rotifera) and Mesocyclops pehpeiensis (Copepoda) on the demography of Brachionus havanaensis. Population growth and life table demography experiments were conducted in 50 mL vessels with 25 mL of moderately hard water and 0.5 x 10(6) cells mL(-1) of Chlorella vulgaris as food. Two predators of each phylum were introduced into small meshes with 50 mu m mesh and fed 15 individuals of another rotifer species Plationus patulus. Each experiment was set up with two controls, with and without the prey (P. patulus) in meshes, as in all the other treatments. We started the experiments with 15 individuals of B. havanaensis in the population growth experiments and 15 neonates (<12 h old) in the life table studies. The population growth studies were terminated after 20 days and the life table studies after the last individual of each cohort died. We also tested the feeding rates of A. girodi, S. leucops and M. pehpeiensis on B. havanaensis by observing the prey consumed at different time intervals from 15 to 90 min. The presence of P. patulus alone had an impact on the reproductive but not on the survivorship variables of Brachionus as compared to the controls. We found that among the predators, S. leucops had the least impact on the population dynamics and life history variables. The survivorship ranged from 16 to 18 days with no significant impact of infochemicals from any of the test predators. The population growth rate of B. havanaensis ranged from 0.2 to 0.34 day(-1) and was highest in the presence of M. pehpeiensis and A. girodi. We also observed maximal differences in the life history strategy of B. havanaensis, as compared to the controls, in response to the most voracious predator, A. girodi.