m mesh to remove clumps. Algal and yeast cell density was estimated using haemocytometer. For each rotifer species we offered food in the following ways: 1. only alga, 2. only yeast, and 3. alga+yeast in equal density.
Several algal species have been used while testing the use of freshwater rotifer species. Since algal cultivation under controlled conditions is laborious, time consuming and expensive, alternative food types such as yeast, wastewater from food industry and livestock have been used for mass rotifer cultures (Klekot and Klimowicz 1981). However, controlled laboratory experiments using these food types for rotifers are necessary to compare the population growth with conventional algal diets. It is also not known whether different Brachionus species show different population growth rates when grown on yeast. At the same time, a comparative information about the growth of different rotifers grown on alga, yeast and their mixture separately has rarely been published (Guevara et al. 1996).
The aim of this study was to test the effect of different concentrations of green alga and yeast and their mixture on the population growth of two brachionid rotifer species commonly found in freshwater systems.
Materials and methods
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Chlorella was cultured using Bold-basal medium. Log phase algae were harvested, centrifuged and resuspended in EPA medium. Commercially available baker's yeast was freshly procured, resuspended in EPA medium and filtered using a 20 m mesh to remove clumps. Algal and yeast cell density was estimated using haemocytometer. For each rotifer species we offered food in the following ways: 1. only alga, 2. only yeast, and 3. alga+yeast in equal density.
Each food type was offered in two densities viz. low (1x106) and high (3x106 ind. ml-1). For each food type and density, we maintained four replicates. Thus for population growth of B. calyciflorus, we maintained a total of 24 plastic jars (3 food types x 2 food concentrations x 4 replicates), each containing 20 ml of EPA medium with appropriate food density. The initial density of rotifers in each test jar was 5 ind. ml-1. For the population growth of B. patulus also the above design was used. Experiments were conducted at 23±2°C.
Following inoculation of B. calyciflorus or B. patulus, at every 24 h interval, we counted the number of female rotifers alive under a stereomicroscope. For this we either counted the whole volume of the test vessel or two aliquot samples each of 1-5 ml depending the density of rotifers per container. After estimating the density, the individuals were transferred to fresh EPA medium containing appropriate food type. The transfer of rotifers to fresh medium was done either individually, when the densities were low, or using a 50 m mesh during later stages of the study. Experiments were terminated after 15 days when most populations began to decline. Thoughout this study males were not encountered.
The rotifer population growth was obtained from a mean of 4-5 values during the exponential phase using the equation r = (ln Nt - ln No)/t, where, No = initial population density, Nt = density of population after time t (days) (Krebs, 1985).
Results
The population growth curves of B. calyciflorus and B. patulus reared under three food types and two densities are shown in Figs. 1 and 2. The maximal population density of B. patulus influenced significantly (p<0.01) by food type, its concentration as well as their interaction but not for B. calyciflorus (p>0.05, ANOVA). In general, at any food type or concentration, B. patulus reached higher population density when compared to B. calyciflorus. In 1x106 ind. ml-1, B. calyciflorus reached 77±12 ind. ml-1; at the same food level B. patulus attained 109±26 ind. ml-1. At 3x106 ind. ml-1 density, B. calyciflorus reached a peak density of 103±8 ind. ml-1. Under comparable conditions, B. patulus reached much higher peak abundance 296±20 ind. ml-1. When yeast was used as exclusive food, the maximal abundance values reached by B. calyciflorus were 62±19 and 57±25 ind. ml-1 under low (1x106 ind. ml-1) low and high (3x106 ind. ml-1) food concentrations, respectively. On the other hand, B. patulus showed peak population abundances of 97±17 and 50±6 ind. ml-1 in low and high concentrations of yeast. B. calyciflorus reached a peak abundance of 54±9 and 86±3 ind. ml-1 for low and high food densities when both these food types offered in equal concentrations. Comparable values of B. patulus were 251±12 and 259±32 ind. ml-1.
The day of maximal population density was not significantly different for the food concentrations used for both B. calyciflorus and B. patulus (p>0.05). However, food type had a significant effect on this variable (p<0.01) for both the species. The highest rate of population growth (r) recorded for B. calyciflorus was 0.63±0.04 and the lowest r value (0.13±0.03) was observed for the same species when grown in high concentration of yeast. Regardless of food type and density the r values of B. patulus ranged from 0.19±0.01 to 0.37±0.01 (Fig. 3). The r values for B. patulus were significantly affected by food type, density and their interaction (p<0.001). For B. calyciflorus only the food type had a significant effect (p<0.01). An inverse relation occurred for both B. calyciflorus and B. patulus when the daily rate of population increase was plotted againt the population density of the same day (Figs. 4 and 5). ]]>
| Species | Experiment | Food type | ]]> Food levelr value | Reference | |
| Anuraeopsis fissa | Population ]]> | Scenedesmus obliquus | 0.5 - 8 x 106 cells ml-1 | 0.45 - 0.86 | Dumont et al. , 1995 | Population growth | Scenedesmus acutus | 0.5 - 40.5 x 106 cells ml-1 | 0.44 - 0.88 | ]]> Sarma et al. , 1996
| Life table | - | - | 0.10 | ]]> Ooms-Wilms, 1997||
| Brachionus angularis | Population growth | Stichococcus bacillaris | ]]> -0.58 | Walz, 1993 | |
| B. calyciflorus | Life table | ]]> -- | 2.20 | Wang and Li, 1997 | |
| Population growth | ]]> Scenedesmus acutus0.5 - 40.5 x 106 cells ml-1 | 0.79 -1.49 | Sarma et al. , 1996. | ||
| Population growth | Various types of algae | - | 0.80 | Rothhaupt, 1990 | B. patulus | Life table | Chlorella | 1-4 x 106 cells ml-1 | 0.14-0.61 | Sarma ]]> |
| Population growth | Chlorella | 1-3 x 106 cells ml-1 | 0.12-0.24 | ]]> Sarma and Rao, 1990||
| B. plicatilis | Population growth | Tetrathelmis tetrathele | ]]> 0.05 x 106 cells ml-10.24 - 0.49 | Okauchi and Fukusho, 1984 | |
| Population ]]> | Chlorella | 1.5 x 106 cells ml-1 | 0.16-0.47 | Okauchi and Fukusho, 1984 | |
| B. ]]> | Population growth | Various types of algae | - | 0.80 | Rothhaupt, 1990 | Population growth | Chlorella | 3 x 106 cells ml-1 | 0.79 | Iyer and Rao, 1993 | ]]>
| B. urceolaris | Life table | - | - | 1.32 | Wang ]]> |
| Keratella cochlearis | Population growth | Cryptomonas erosa | 0.005-0.01 ]]> 6 cells ml-1 | 0.28-0.40 | Smith and Gilbert, 1995 |
| Population growth | - | ]]> -0.13 | Ooms-Wilms, 1997 | ||
| K. crassa | Population growth | ]]> Cryptomonas erosa0.005-0.01 x 106 cells ml-1 | 0.32 | Smith and Gilbert, 1995 | |
| K. ]]> | Life table | - | - | 0.15 -0.39 | Stemberger, 1988 |
