Endpoint consistency and sensitivity By using the method developed in previous sections, the consistency of the postexposure feeding endpoint was assessed exposing D. a to a range of chemicals with different modes of action, then measuring the postexposure feeding response over a 4-h period. Lethality tests were conducted to investigate endpoint sensitivity by testing the hypothesis that feeding depression median inhibitory concentration(IC50) occurs below the median lethal concentration(LC50). Experiments were carried out as follows. Stock solutions were prepared in ASTM hard water 2 d before the experiment. Experimental dilutions were made from stock solutions by adding the appropriate subsamples to ASTM hard water containing vulgaris at a concentration of 5 x 10s cellsml just before the experiment was started. Postexposure feeding response For each concentration, six 175-ml glass jars were filled with 120 ml of the test substance experimental dilution, and five animals were randomly assigned to five of the jars. The remaining jar, with no animals, was used to establish initial algal concentrations. Controls were set up in the same way with ASTM hard water containing C. wulgaris at a concentration of 5 x 10s cells/ml only. Animals were left to feed for 24 h. They were then transferred with a 3-ml plastic pipette into 60 ml glass jars containing 60 ml of ASTM hard water with C vulgaris at a concentration of 5 x cells/ml and allowed to feed for 4 h, after which they were removed from