The combined effect of temperature, food level and the presence of an invertebrate predator on the body size of the rotifer Brachionus havanaensis were tested in this study. B. havanaensis was cultured at 15, 20, and 25°C under three different Chlorella vulgaris levels (0.5 · 10 6 , 1.0 · 10 6 and 2.0 · 10 6 cells ml À1 ) in the presence and in the absence of Asplanchna girodi. For each treatment we maintained three replicates and constant (0.4 ind ml À1 ) population density of B. havanaensis. In treatments containing A. girodi, the predator was separated from the prey by a mesh (pore size 50 mm). On the last day of the experiment, a portion of the B. havanaensis population was sampled for several morphometric measurements (adult lorica length, width, posterior spine length, body volume, and the egg volume). Size measurements were done by drawing the specimens using a calibrated camera lucida. Statistically significant impact of temperature as well as the predator's presence was observed on the lorica length, posterior spine, and egg volume of B. havanaensis. The interactions of food · temperature, or predatoŕs presence · food · temperature were non-significant (P > 0.05) for lorica length, spine length, body volume, and egg volume. Regardless of the type of treatment, there was a direct positive correlation between lorica length and width. Egg volume was linearly related to the adult size. Notably long posterior spines were observed in treatments containing the presence of A. girodi.
We evaluated the combined effects of food (0.5 · 10 6 , 1.0 · 10 6 and 2.0 · 10 6 cells ml )1 of Chlorella vulgaris) and temperature (15, 20 and 25°C) on life history variables of B. havanaensis. Regardless of Chlorella density there was a steep fall in the survivorship of B. havanaensis at 25°C. Both food level and temperature affected the fecundity of B. havanaensis. At any given food level, rotifers cultured at 15°C showed extended but low offspring production. At 25°C, offspring production was elevated, the duration of egg laying reduced and the fecundity was higher during the latter part of the reproductive period. The effect of food level was generally additive, at any given temperature, and higher densities of Chlorella resulted in higher offspring production. Average lifespan, life expectancy at birth and generation time were 2-3 times longer at 15°C than at 25°C. At 20°C, these remained at intermediate levels. The shortest generation time (about 4 days) was observed at 25°C. Gross and net reproductive rates and the rate of population increase (r) increased with increasing temperature and generally, at any given temperature, higher algal food levels contributed to higher values in these variables. The r varied from 0.11 to 0.66. The survival patterns and lower rates of reproduction at 15°C suggest that the winter temperatures (10-15°C) prevailing in many waterbodies in Mexico City allow this species to sustain throughout the year under natural conditions.
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