The green alga, Scenedesmus acutus, grows in culture in unicellular form, but it forms colonies (coenobia) when exposed for 48 h to a chemical released by the grazer Daphnia magna. The colony-forming response can be evoked only in growing cells. The Daphnia factor affects colony size but not algal growth rate. The minimum concentration of Daphnia factor that induces colony formation is equivalent to a Daphnia biomass of 0.5 mg dry wt liter-* in the culture medium. Actively feeding daphniids induce a stronger response than starved ones. Homogenized Scenedesmus, homogenized Daphnia, ammonium, and urea are not effective. The Daphnia factor is a nonvolatile, organic substance of small molecular mass (< 500 Da). It is moderately lipophilic, heat stable, pH-resistant in a range from 1 to 12, and not affected by treatment with Pronase E. The chemical activity is not lost when the substance is dried but disappears during incineration. Colony formation can be interpreted as a grazing defense mechanism. The phenotypic response may have evolved because of the tradeoff between higher sinking rates and grazing resistance of colonial forms.

JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org.Abstract. Mixotrophic flagellates can compete with obligately heterotrophic flagellates for the uptake of food particles, whereas mixotrophs and obligately phototrophic phytoplankton can compete for soluble nutrients. Competition for soluble nutrients is expected when photosynthesis covers a significant portion of the mixotrophs' carbon metabolism. When heterotrophy predominates, however, mixotrophs may release soluble nutrients and facilitate phototrophs. Mechanistic resource competition theory predicts that, due to their ability to utilize substitutable C and P sources, mixotrophs should be able to coexist with their more specialized competitors under certain conditions of resource supply.In laboratory experiments, the mixotrophic flagellate Ochromonas excluded heterotrophic flagellates when only phototrophic growth was possible. However, Ochromonas was excluded by heterotrophic flagellates when only phagotrophic growth was possible. Both coexisted when food bacteria and light were supplied simultaneously. Ochromonas coexisted with a P-limited phytoplankter, Cryptomonas sp., when soluble reactive phosphorus (SRP) and bacterial phosphorus were available as alternative P sources. In nature, the mixotrophic strategy may be successful when resources are limiting. This is supported by published data on the occurrence of mixotrophic chrysophytes.

The food algae Scenedesmus acutus and Cyclotella meneghiniana were grown in chemostats in a modified WC medium at identical growth rates (0.4 d-l) but under different nutrient regimes. Nitrate or phosphate was reduced to produce N-or P-limited algae. Brachionus rubens did not differentiate between nonnutrientlimited and nutrient-limited Scenedesmus and ingested both at comparable rates. The rotifers reached highest maximum growth rates with nonnutricnt-limited Scenedesmus. N-limited Scenedesmus permitted similar growth rates at low food concentrations (below the incipient limiting level, ILL), but maximum growth rates at high food concentrations (above the ILL) were significantly reduced. With regard to the high N content of Brachionus, direct mineral N deficiency of the food appears to be possible. P-limited algae (Scenedesmus and Cyclotella) permitted no positive growth rates of the rotifers at all. P-limited algae that were short-term enriched with phosphate contained similar amounts of P as nonnutrient-limited algae but still were of lower nutritional quality, thus suggesting the importance of essential biochemical constituents.

Size-selective feeding of four Brachionus strains was studied with three experimental setups: selection between polystyrene spheres of different sizes in short-term ( 10 min) fee'ding experiments, selection between pairs of dual-labeled algal taxa in short-term feeding experiments, and selection among three algal taxa in long-term (24 h) feeding experiments. Food size preferences were related to body sizes between strains but not within one strain (Brachionus calyciflorus). Brachionus angularis preferred food items <5-~m equivalent spherical diameter (ESD), Brachionus rubens strain F fed most efficiently on particles -5km ESD, and B. caZycijlorus on particles of -lo-pm ESD. Brachionus rubens strain B ingested particles from 3.5 to 12-pm ESD equally well. Apart from particle size effects, feeding was unselective. Polystyrene spheres of the appropriate sizes were ingested readily.