Passage III

When Moina micrura (microscopic aquatic crus­taceans) are exposed to unfavorable environmental condi­tions, they produce ephippia (specialized eggs that each contain a dormant embryo). The embryos remain dormant until the ephippia are exposed to favorable environmental conditions that cause the ephippia to hatch. Two experi­ments examined how pH and light intensity affect the hatching of M. micrura ephippia.

Experiment 1

Each of 15 identical beakers received 50 mL of water and 120 freshly laid M. micrura ephippia. The beakers were then equally divided into 5 groups (Groups L-P). For each group of beakers, the water was maintained at 1 of 5 different pH values (see Table 1).

Each beaker was then incubated at 27°C and received 12 hr of light per day at a light intensity of 650 lux. At the end of 7 days, the number of ephippia that had hatched in each beaker was counted, and the average number of ephip­pia hatched in each group was calculated (see Figure I).

Experiment 2

Each of 20 identical beakers received 50 mL of water and 120 freshly laid M. micrura ephippia. The beakers were then equally divided into 5 groups (Groups Q-U). Each beaker was incubated at 25° C, and the water in each beaker was maintained at a pH of 7.0. Group Q was kept in the dark, and each of Groups R-U received 12 hr of light per day at 1 of 4 different light intensities (see Table 2).

At the end of 7 days, the average number of ephippia hatched in each group was determined as in Experiment 1 (see Figure 2).

Figures adapted from N. E. T. Rojas, M. A. Marins, and O. Rocha, "The Effect of Abiotic Factors on the Hatching of Moina micrura Kurz, 1874 (Crustacea: Cladocera) Ephippial Eggs." ©2001 by the International Institute of Ecology.