PROBIT analysis predicts hatching rates of brine shrimp Artemia sp. cysts

Thirty replicates of San Francisco Bay brine shrimp Artemia sp. cysts from one lot were weighed to 0·01 mg. The mean mass of a single cyst was 2·597 μg (±0·040 μg, 95% confidence level). Batches of 1 g from the same lot were hatched in the laboratory, and fraction hatched versus time data were fitted to a nonlinear curve using PROBIT analysis. Median time of hatch was 24·8 h with a standard error of ±3·92 h. Cysts and nauplii in 1-ml aliquots were pipetted at regular intervals and counted under a dissecting microscope. The number of cysts pipetted at the beginning of the experiment was 19% less than estimates based on the known number per volume of hatching medium, indicating that hatching data derived from pipetting procedures may contain large sampling errors. PROBIT analysis allows fraction hatched to be predicted when a particular lot of cysts is hatched under controlled conditions.