The Effects of Food Concentration and Quality on the Feeding Rates of Three Size Classes of the Greenshell<Superscript>TM</Superscript> Mussel, <Emphasis Type="Italic">Perna canaliculus</Emphasis>

In order to calibrate carrying capacity models, investigations were conducted into the effects of food concentration and food quality on the feeding rates of small (25–50 mm), medium (60–85 mm) and large (90–115 mm) Greenshell mussels (Perna canaliculus). Experimental diets varying from 3.3 to 6.0 μg l⁻¹ chlorophyll a concentration and 12–25% organic content were fed to mussels housed in individual flow through chambers. Not surprisingly, this study found that the main factor affecting feeding rates is mussel size. Small mussels were observed to maintain a constant filtration rate of approximately 20 mg h⁻¹ irrespective of food concentration or quality, whereas mussels of greater than 60 mm length had more variable filtration rates between 30 and 80 mg h⁻¹. The filtration rates of these large mussels were also observed to increase positively with organic content, and showed no sign of levelling out, even at the highest organic content tested (25%). Highest rejection rates (50–70 mg h⁻¹) were observed when the organic content of the available seston was low, suggesting that P. canaliculus are able to selectively reject organic material, thereby organically enriching their diet. It appears that the organic content of the seston is the primary determinant of the net efficiency with which food is selected from the available seston by the mussel. The present study shows that P. canaliculus of all sizes are capable of adapting their feeding behaviour to compensate for changes in the food supply, which may occur over relatively short time periods, in the culture environment.     

A model for growth of Artemia franciscana cultures based on food ration-dependent gross growth efficiencies

Laboratory cultures of Artemia franciscana grown under batch regimes at constant temperatures (28 °C) and salinity (35 g l^–1), three initial food concentrations (0.1, 0.4 and 1 M cells ml^–1), various daily food rations (0.1–9M Dunaliella tertiolecta cells Artemia ^–1), and different population densities (1–16 ind ml^–1) were used to develop a model of population growth. Growth rates and gross growth efficiencies (K ₁) were largely independent of population densities and initial food concentrations but determined by age and daily amount of food ingested. While maximum growth rates were found with the highest rations, K _{1 max} peaked at rations of 0.5 million cells d^–1 and decreased at feeding levels above this. A contour plot showing the trend relating K ₁ to Artemia size and ingestion rate in combination and was used to model growth in analogous controlled feeding conditions. Computer simulations using this model paralleled published results of final 15-day average individual sizes of Artemia. Optimal results for near constant food utilization are predicted for high initial population densities (100 Artemia nauplii ml ^–1) and daily culls of enough animals to equilibrate food demand with food availability. This strategy could permit a range of Artemia sizes harvested, maximize final individual sizes and retain high total yields (> 1.2 kg dry wt 1^–1). Effects of different culture strategies are discussed.     

Suspension feeding in Bithynia tentaculata (Prosobranchia,Bithyniidae), as affected by body size,food and temperature

The suspension feeding of Bithynia tentaculata was tested in laboratory experiments. The animals were fed in 1-1 aerated glass beakers, and filtration rates were calculated from changes in cell concentrations during the 6-h experiment. Temperature influenced the filtering rate, with minimum values of 5ml · ind^–1 · h^–1 at 5° C and maxima of 17.2 ml · ind^–1 · h^–1 at 18° C. Three food species of different size, motility and cell surface characteristics (Chlamydomonas reinhardii, Chlorella vulgaris and Chlorogonium elongatum) did not affect filtration rates. Suspension feeding increased with increasing food concentrations up to 12 nl · ml^–1, above which feeding rate was kept constant by lowering the filtering rates. Even the smallest animals tested (<4 mm body length) were found to be feeding on suspended food at a rate of 2.7 ml · ind^–1 · h^–1, and increasing rates up to 8.4 ml were found in the 6–7 mm size class. All size classes of Bithynia showed a circannual fluctuation of their filtration rates. The ecological consequences of Bithynia's ability to switch between two feeding modes, grazing and suspension feeding, are discussed.     

Use of swimming speed and egg ratio as predictors of the status of rotifer cultures in aquaculture

This study evaluated the use of egg ratio (eggs rotifer^–1) and swimming speed (mm min^–1) as prediction criteria for production and culture quality in mass cultures of the rotifer Brachionus plicatilis. Egg ratio was determined to be a suitable predictor of rotifer growth and production in the cultures. Low egg ratios (i.e., 0–0.17 eggs rotifer^–1) indicate reduced rotifer population over time (i.e., negative net population growth rates). However, at this time egg ratio dynamics are not suitably understood to predict in advance a sudden population collapse.Swimming speed of reproductive, egg-carrying females in the exponential growth phase was 40–45 mm min^–1. During exponential growth swimming speed was independent of the food used. Lower swimming speeds were obtained in late stationary phase (10–25 mm min^–1) when yeast was used as a food source. Both environmental factors (e.g., accumulating metabolites) and changes in nutritional state of the rotifers may have affected the swimming speed, but environmental factors appear to be the most important. We believe that swimming speed has the potential of becoming an accurate predictor of culture quality in mass cultures of rotifers.     

Rotifers as predators on small ciliates

Clearance rates of Synchaeta pectinata, Brachionus calyciflorus and Asplanchna girodi on Tetrahymena pyriformis (46 µm in length) at a density of 10 cells ml^–1, in the presence of algal food, were 2.5 to 6.1 ml rot.^–1 day^–1. Clearance rates of these rotifers were, respectively, about 2, 3, and 13 times lower on Strobilidium gyrans (58 µm in length) than on T. pyriformis, indicating that the saltations of S. gyrans are an effective escape response. Clearance rates of S. pectinata were considerably lower on Colpidium striatum (81 µm) than on S. gyrans, suggesting that S. pectinata may not be able to ingest ciliates of this size. S. pectinata had a clearance rate of 19 ml rot.^–1 day^–1 on S. gyrans at a density of 1.2 cells ml^–1, in the absence of edible algal food. Rotifers may prey extensively on ciliates in natural plankton communities, ingesting 25 to 50 individuals in the 45–60 µm size range day^–1.     

Feeding kinetics of Brachionus plicatilis fed Isochrysis galbana

Clearance and ingestion rates of Brachionus plicatilis were measured using ¹⁴C-labeled Isochrysis galbana Tahiti. Experiments were conducted at 20–22 °C, 20 ppt salinity, and algal concentrations ranging from 0.13–64 mg C 1^–1. Clearance rates were constant and maximal at concentrations <2 mg C 1^–1, with maximum rates ranging from 3.4–6.9 µl ind.^–1 hr^–1. The ingestion rate varied with food concentration, and was described by a rectilinear model. The maximum ingestion rate varied considerably, and was dependent on the growth rate of the rotifers. Depending on the pre-conditions, B. plicatilis ingested about 0.5 to 2 times its body carbon per day at saturating food concentrations.     

The food and growth of pelagic 0+ Alestes nurse (Characidae) from the Blue Nile,near Khartoum

The food, feeding habits and growth of 0+ Alestes nurse (Rüppel) from the Blue Nile near Khartoum, was investigated. 0+ A. nurse were observed to change their food from phytoplankton and small zooplankton during their pre-metamorphosis stages, to larger zooplankton (Daphnia spp., Chydorus sp.) and insects (larvae, pupae and imagines) during their post-metamorphosis stage.Their growth in length and weight was characterized by two distinct stanzas; during the first stanza, the fish increased in total length at a rate of 0.196 mm mm^–1 wk^–1, and in wet weight at a rate of 0.430 g g^–1 wk^–1. These rates were reduced during the second stanza to 0.046 mm mm^–1 and 0.080 g g^–1 wk^–1, respectively. The calculated value of the exponent b in their length-weight relationship suggested that their growth was allometric.     

Feeding and growth of larval herring,Clupea harengus,in relation to density of copepod nauplii

Synopsis Feeding and growth rates of 1–3 wk old herring larvae from four different stocks were compared in laboratory experiments (8°C). For most of the larval groups, feeding rate was saturated at nauplii (Acartia tonsa, nauplii stages 3–5) densities over 301^–1 (5 g d.w. 1^–1). Specific growth rate increased asymptotically with nauplii density, and reached about 6% d^–1 at densities over 120 nauplii 1^–1. The growth rates attained in the laboratory were similar to field measured growth rates of similarly aged herring larvae at comparable food densities. Since food particles were homogenously distributed in the laboratory tanks, patches of dense plankton concentrations are, thus, apparently not necessary for larval growth and survival in the sea. Growth efficiency differed between larval groups, with large sized larvae being the most efficient in transforming ingested matter into growth. The difference probably relates to different sizes rather than to the different geographical origins of the larvae.     

Feeding behaviour and morphology of filtering combs of Daphnia galeata

The population of Daphnia galeata Sars from the fish pond Velký Pálenec (Blatná, Czechoslovakia) living in high food conditions (7 mgC l^–1) was characterized by a small size of the filtering comb on the thoracic limb 3, measured as seta length, length of the base of the comb and number of setae (population 1). One month cultivation of this population in low food conditions (1.5 mgC l^–1) in the laboratory resulted in twofold increase in size of the filtering comb (population 2). Filtering and ingestion rates of both populations were measured at eight concentrations of food (approximately 0.025–3.2 mgC l^–1) using ¹⁴C labeled Scenedesmus acutus. The results show that size of the filtering combs influences considerably feeding behavior of Daphnia. The comparison of animals with the same body length suggests that the population with a large comb feeds at concentration of food below 0.4 mgC l^–1 more intensively and reaches the maximum of the filtering rate at a lower concentration than the population with a small comb. The situation is opposite at concentration above 0.4 mgC l^–1. The higher values of theoretical flow in population with a small projection of filtering area suggest that this population has to compensate disadvantage of a small comb with the higher appendages beat frequency.     

Zooplankton filtration rates in Lake Huron,Georgian Bay and North channel

On three research cruises in 1981, zooplankton community filtration rates were measured at 4 stations: Saginaw Bay, mid-Lake Huron, Georgian Bay and North Channel. For all four stations, the highest rates were observed during the late-September cruise. The maximum observed rate was 137 000 ml d^–1 m^–3, while the lowest rate was 7200 ml d^–1 m^–3. The grazing experiments were performed on three size classes of radioactively labelled algal food (0.45–5 µm, 5–20 µm and 20–64 µm). In 11 of 12 experiments, the smallest size class of food yielded the highest filtration rate. For the late-May cruise we used published data on phytoplankton biomass for the Georgian Bay and North Channel stations to calculate community feeding rates of 0.09 and 0.015 mg C mg C m^–3 d^–1, respectively, and percent cropping rates of 0.74 and 0.35 per day, respectively. A comparison of our feeding rates to literature values for zooplankton biomass suggests that algal food alone may not be sufficient to sustain zooplankton growth at those stations.     

Evaluation of agro-industrial wastes as diets for culture of the fairy shrimpStreptocephalus proboscideus (Frauenfeld, 1873) (Crustacea:Branchiopoda:Anostraca)

Two micronized waste products were evaluated in a closed recirculation system for their suitability to replace a costly die of live algae in the culture of the Sudanese fairy shrimp,Streptocephalus proboscideus. The test population was kept at a density of 50 individuals 1^–1 (sex ratio:1/1). An agricultural waste product (YM20; mixture of pea and corn), and an industrial waste product (POME; Palm Oil Mill Effluent) were fed at two regimes: 0.1 and 0.2mg DW animal^–1 h^–1. The microalgaSelenastrum capricornutum, used as a reference diet at a density of 2.0±0.82 × 10⁵ cells ml^–1, proved adequate in preliminary screening experiments. The effect of the diets and feeding regimes on selected biological variables and water quality were followed by weekly observations and measurements. Results in terms of growth (=increase in length), cyst production, and mortality were more successful when animals were supplied high densities of YM20 than in all other treatments: mean brood size was 155±6 cysts with a maximum of 266. Length after 6 weeks about 2 cm while this ranged between1.4–1.7 cm for the other treatments. Weekly mortality rate was comparable under high food conditions. Mortality rate gradually increased from 5% in the first week to 15% in the last week. Water quality, especially nitriate concentration (measured as NO₂-N), was slightly better in the dry food fed than in the algae fed cultures. Present results are promising for large-scale culturing ofS. proboscideus in a cost-effective way by making use of agro-industrial waste products.     

Population growth of some genera of cladocerans (Cladocera) in relation to algal food (Chlorella vulgaris) levels

We studied the patterns of population growth of 7 cladoceran species (Alona rectangula, Ceriodaphnia dubia, Daphnia laevis, Diaphanosoma brachyurum, Moina macrocopa, Scapholeberis kingi and Simocephalus vetulus) using 6 algal densities, viz. 0.05×10⁶, 0.1×10⁶, 0.2×10⁶, 0.4×10⁶, 0.8×10⁶ and 1.6×10⁶ cells ml^–1, of Chlorella vulgaris for 18 – 30 days. In terms of carbon content these algal concentrations corresponded to 0.29, 0.58, 1.16, 2.33, 4.65 and 9.31 g ml^–1, respectively. Cladocerans in the tested range of algal levels responded similarly, in that increasing the food concentrations resulted in higher numerical abundance and population growth rates (r). The peak population densities were (mean±standard error) 71±5; 17.1±0.4, 3.6±0.3, 12.7±1.1, 18.2±2.7, 15.8±1.0 and 10.9±0.02 ind. ml^–1, respectively for A. rectangula, C. dubia, D. laevis, D. brachyurum, M. macrocopa, S. kingi and S. vetulus. In general, the lowest r values were obtained for D. laevis (0.01±0.001) at 0.05×10⁶ cells ml^–1 food level while the highest was 0.283±0.004 for A. rectangula at 1.6×10⁶ cells ml^–1 of Chlorella. When the data of peak population density for each cladoceran species were plotted against the body length, we found an inverse relation, broadly curvilinear in shape. From regression equations between the food level and rate of population increase, we calculated the theoretical food quantity (the threshold level) required to maintain a zero population growth (r = 0) for each cladoceran species, which varied from 0.107 to 0.289 g ml^–1 d^–1 depending on the body size. When we plotted the cladoceran body size against the corresponding threshold food levels, we obtained a normal distribution curve. From this it became evident that for up to 1300 m body size, the threshold food level increased with increasing body size; however, beyond this, the threshold level decreased supporting earlier observations on rotifers and large cladocerans.     

Growth rate influences reductive biodegradation of the organophosphorus pesticide demeton by Corynebacterium glutamicum

The organophosphorous pesticide, demeton-S-methyl was transformed byCorynebacterium glutamicum in co-metabolism with more readilydegradable substrates. Glucose, acetate and fructose were tested as growth substrates, and the highest demeton-S-methyl biotransformation average rate (0.78 mg l^-1 h^-1) and maximum instantaneous rate (1.4 mg l^-1 h^-1) were achieved on fructose. This higher efficiency seems to be linked to the atypical behavior of C. glutamicum grown on fructose, characterized by a prolonged period of accelerating growth instead of a constant growth rate observed on glucose or acetate. More precisely, for growth rates in the 0.1–0.4 h^-1 range, a direct coupling between the specific demeton-S-methyl consumption rate and the growth rate was demonstrated on fructose during batch –, steady state continuous – or continuous cultures with a controlled transient growth rate (accelerostat technology). The demeton-S-methyl biotransformation was more favoured during an acceleration phase of the growth rate.     

Ecological problems of Lake Ladoga: causes and solutions

We studied the outcome of competition between a large (Brachionus calyciflorus) and a small (Anuraeopsis fissa) rotifer species at five algal (Scenedesmus acutus) concentrations (0.5 × 10⁶ to 40.5 × 10⁶ cells ml^–1) and with varying initial densities in mixed populations (100 to 0% of B. calcyciflorus or A. fissa), the combined initial biomass being 0.2 µg ml^–1 in all test jars. Experiments were conducted at 28 ± 1 °C.Regardless of food concentration, B. calcyciflorus showed a greater increase in biomass than A. fissa, peak densities (mean ± standard error) at the lowest food concentration in the controls being 1.34 ± 0.31 µg dry weight ml^–1 and 0.82 ± 0.08 dry weight ml^–1, respectively. At the lower food concentrations, A. fissa displaced B. calyciflorus and vice versa at the higher food concentrations. At the intermediate food concentrations of 4.5 × 10⁶ cells ml^–1, B. calyciflorus outcompeted A. fissa only if its initial population density was three times higher. The rates of population growth in controls varied from 0.792 ± 0.06 d^–1 to 1.492 ± 0.13 d^–1 for B. calyciflorus and 0.445 ± 0.04 to 0.885 ± 0.01 for A. fissa depending on food level. When both species were introduced together, low food levels favoured higher abundance of A. fissa than B. calyciflorus, suggesting, in nature, it is likely that small Anuraeopsis colonize oligotrophic water bodies more successfully than larger Brachionus. The results also suggest that the outcome of competition depends not only on the size of the competing species and food availability but also on their colonizing density.     

Feeding in Euchlanis dilatata lucksiana Hauer on filamentous cyanobacteria and a prochlorophyte

Ingestion and assimilation rates of Euchlanis dilatata lucksiana Hauer, isolated from Lake Loosdrecht (The Netherlands) and cultured on lake water (seston < 33 µm), were measured in the laboratory using the ¹⁴C-tracer technique. The five taxa used as tracer foods, together with 6–7 mg C l^–1 of lake seston in each case, included four species of filamentous cyanobacteria (Oscillatoria redekei, O. limnetica, Aphanizomenon flos-aquae, Anabaena PCC 7120) and a prochlorophyte (Prochlorothrix hollandica). Except Anabaena, they are all commonly encountered in eutrophic Loosdrecht lakes, including Lake Loosdrecht, and their dimensions ranged between 150 and 250 µm in length and 2 and 3.5 µm in width. The small and large Euchlanis used as experimental animals had mean lengths of 217–223 µm and 276–305 µm, respectively. Euchlanis fed on all the taxa offered as food. Clearance rates ranged from 51 to 99 µl ind^–1 d^–1 for the large animals and from 22 to 41 µl ind^–1 d^–1 for the small animals. The highest ingestion rate observed, 1.7 µg ind^–1 d^–1, was for the large animals feeding on Aphanizomenon. The daily ration for both size classes far exceeded 100% of body weight, reaching up to 690% for the small animals feeding on Aphanizomenon. The small animals generally appeared to assimilate the ingested food more efficiently (assimilation efficiencies: 37–100%) than the large animals (34–77%). Compared with an earlier study in which only lake seston (<33 µm) was used as food, the specific clearance rates of Euchlanis on the cyanobacteria and Prochlorothrix were generally somewhat lower.     

Larval Mirogrex terraesanctae (Cyprinidae) of Lake Kinneret (Israel): growth rate,plankton selectivities,consumption rates and interaction with rotifers

The rotifer Synchaeta pectinata dominated gut content of first feeding Mirogrex larvae (7 mm, 10 days age) and was a selected prey of neuston-caught larvae up to 15 mm TL. A negative L-value (linear index of selection) applied to predation on nauplii and copepodites by 7 and 8 mm larvae; nevertheless, caloric intake was dominated by copepods in 8–10 mm larvae. Neuston-caught larvae 13–20 mm TL fed selectively on Cladocera, especially Bosmina, and on the rotifer Asplanchna spp.Growth, estimated from otolith ring counts and from analysis of size distribution data, ranged from 3 to 7 mm mo^–1, with higher rates for early spawned larvae. When consumption as estimated from gut content, was compared to amounts of food required for growth, it appeared that the smallest larvae were underfed, while 13–16 mm fish obtained rations close to sufficiency.Rotifer standing stock biomass in Lake Kinneret has decreased in recent years, especially in winter, the spawning period of Mirogrex. Postulated causes are predation by an increasingly large population of Mirogrex larvae, and decrease of external supply. Larval distribution appeared to be linked to S. pectinata abundance; highest densities of both organisms occurred in the area of inflow from the Jordan and Golan streams. Larval food enrichment of inflow water by fish pond drainage might have caused observed increases in Mirogrex stock size since 1960.     

Zooplankton as a compound mineralising and synthesizing system: phosphorus excretion

Data on phosphate excretion rates of zooplankton are based on measurements using the pelagic crustacean zooplankton of Lake Vechten and laboratory-cultured Daphnia galeata. In case of Daphnia sp we measured the effects of feeding on P-rich algae and P-poor algae (Scenedesmus) as food on the P-excretion rates at 20°C. The excretion rates of the natural zooplankton community, irrespective of the influence of the factors mentioned, varied by an order of magnitude: 0.025–0.275µg PO₄-Pmg^–1C in zooplankton (C _zp ) h^–1. The temperature accounted for about half the observed variation in excretion rates. The mean excretion rates in the lake, computed for 20°C, varied between 0.141 and 0.260 µg Pmg^–1C _zp h^–1. Based on data of zooplankton biomass in the lake the P-regeneration rates by zooplankton covered between 22 and 239% of the P-demand of phytoplankton during the different months of the study period.In D. galeata, whereas the C/P ratios of the Scenedesmus used as food differed by a factor 5 in the experiments, the excretion rates differed by factor 3 only. Despite the higher P-excretion rates (0.258± 0.022 µg PO₄-P mg^–1 C h^–1) of the daphnids fed with P-rich food than those fed with P-poor food (0.105 ± 0.047 µg PO₄-P mg^–1 C h^p–1), both the categories of the animals were apparently conserving P. A survey of the literature on zooplankton excretion shows that in Daphnia the excretion rates vary by a factor 30, irrespective of the species and size of animals and method of estimation and temperature used.About two-thirds of this variation can be explained by size and temperature. A major problem of comparability of studies on P-regeneration by zooplankton relates to the existing techniques of P determination, which necessitates concentrating the animals several times above the in situ concentration (crowding) and prolonged experimental duration (starving), both of which manifest in marked changes that probably lead to underestimation of the real rates.     

A laboratory study of feeding and assimilation in Euchlanis dilatata lucksiana

Feeding of Euchlanis dilatata lucksiana, a brachionid rotifer isolated from the Loosdrecht Lakes (The Netherlands), was examined in the laboratory using ¹⁴C-labelled food. The gut-filling time at a food concentration of 9.6 µg C ml^–1 was about 15 minutes. Animals which were fed on 3 size fractions of the lake seston (< 7 µm, 7–15 µm, and 15–33 µm) showed a clear preference (Jacobs' selectivity index) for the largest size fraction. This fraction was composed predominantly of filamentous cyanobacteria. For animals weighing 0.37–0.49 µm C ind.^–1 the daily ration (daily food consumption per unit body weight × 100) ranged from 50 to 100% at food levels of 2 mg C l^–1 and below, but increased to 150–250% at food concentrations of 5 mg C l^–1 and above. The assimilation efficiency was 100% up to 5 mg C l^–1 of food, but decreased to about 80% at higher food levels.     

Recovery Patterns of Moina Macrocopa Exposed Previously to Different Concentrations of Cadmium and Methyl Parathion: Life-Table Demography and Population Growth Studies

In most toxicity studies using Cladocera, bioassays are routinely done to determine median lethal concentration (LC₅₀) or the responses to sublethal exposure. However, information on the patterns of recovery of cladocerans exposed to different concentrations of toxicants is scarce. This is important because cladocerans exposed to toxicants for a short duration may later recuperate under favourable conditions. Using the life table demographic and population growth, the present study was conducted to evaluate the recovery patterns of Monia macrocopa exposed to five different concentrations (0, 25, 50, 75 and 100% of 24 h LC₅₀ for CdCl₂ or methyl parathion) and then returned to toxicant-free medium containing alga (Chlorella vulgaris) at low (0.25 × 10⁶cells ml^–1), medium (0.5 × 10⁶cells ml^–1) or high (1 × 10⁶cells ml^–1) levels. We measured selected life history variables such as average lifespan, life expectancy at birth, gross and net reproductive rates, generation time and the rate of population increase. Results indicated that regardless of food concentration, surviving individuals of M. macrocopa exposed to a median lethal concentration did not recover. The effect of food level was significant at 25 and 50% of the median lethal concentration for cadmium or methyl parathion. Age-specific fecundity curves showed that exposure to either toxicant for a duration as short as 24 h at one-fourth of the LC₅₀ showed reduced output of offspring, especially at a lower food level. At and above exposures of 0.037 mgl^–1 of methyl parathion, no reproduction occurred. The highest gross and net reproductive rates (127 and 55 offspring female^–1) were obtained in controls at the high (1 × 10⁶ cells ml^–1) algal food level. The rate of population increase obtained from life table data was around 0.7 per day in controls but decreased when exposed to toxicant concentrations. The rates of population increase per day derived from population growth data varied from 0.22 to 0.33 per day for the controls, depending on the food levels.     

Utilization of cyanobacteria by Brachionus calyciflorus: Anabaena flos-aquae (NRC-44-1) as a sole or complementary food source

The rotifer Brachionus calyciflorus can utilize the cyanobacterium Anabaena flos-aquae as either a sole or supplementary food source in laboratory culture. Positive population growth rates accompany food densities of 10 or 100 µg dry weight ml^–1, but slightly negative rates are found at a lower density (1.0 µg ml^–1). These results are consistent for rotifers feeding on two strains of A. flos-aquae, UTEX-1444 and NRC-44-1, with slightly enhanced survivorship and reproduction with the latter food. A 1:1 mixture (by dry weight) of Euglena gracilis and A. flos-aquae (NRC-44-1) produces survivorship comparable to that of control rotifer cohorts fed E. gracilis alone, but elicits significantly greater fecundity and population growth rates than found with the control food suspension at the same biomass density.