Rivals, Hunters and Partners in a Jar of Pond Water

Life in a Drop

1Mr. Okafor's seventh-grade class scooped water from a city pond and grew its tiny organisms in clear jars on a classroom shelf. Each day, students placed a 0.1 mL sample on a slide and counted the organisms under a microscope. Two kinds of single-celled paramecia lived in the samples: Paramecium aurelia, the smaller one, and Paramecium caudatum, the larger one. Both feed on bacteria in the water, and neither one eats the other.

2In Experiment 1, the class set up three jars, each with 200 mL of filtered pond water. Jar 1 started with P. aurelia only, Jar 2 with P. caudatum only, and Jar 3 with both. Every jar was kept at 22 °C under the same lamp and got the same small spoonful of food for the bacteria each day, so the bacteria never became plentiful. Students counted each species every two days for 12 days and recorded the average of three samples (Table 1).

3In Experiment 2, the class added a few Didinium to a fourth jar that held P. caudatum. A Didinium is a single-celled organism that catches a paramecium, swallows it whole and digests it. This jar was kept under the same conditions, and the class counted both kinds of organisms every day (Table 2).

4In Experiment 3, the class studied green hydras, tiny freshwater animals about 1 cm long. Many single-celled green algae live inside some of a green hydra's cells. The algae make sugar using light, and some of that sugar passes into the hydra's cells. Inside the hydra, the algae are sheltered from algae-eating animals and can use carbon dioxide and wastes that the hydra gives off. Brown hydras are a related kind with no algae inside. The class kept three groups of 10 hydras in separate dishes for three weeks without feeding them: green hydras in light, green hydras in the dark and brown hydras in light. Then they counted how many hydras in each group were still alive (graph in Section 3).