Studies on the formation and the hatching of fertilized eggs of the rotifer Brachionus plicatilis. I. Combined effects of environmental conditions on the hatching of fertilized eggs of the rotifer Brachionus plicatilis collected from an outdoor pond.

The effects of 70 combinations of temperatures, chlorinities and photoperiods on the hatching of fertilized eggs of the rotifer Brachionus plicatilis were investigated in a laboratory. Egg samples were collected from an outdoor pond. Almost all the tested eggs hatched simultaneously. Both hatching rate and time required for hatching vary depending on external conditions. The best hatching in regard to hatch-ability and incubation period was observed at 25°C or 8‰ Cl'. Though lighting condition also affected the hatching rate of the eggs, it did not have any effect on incubation period. Combined effects of the environmental factors on the hatching were observed more remarkably than those caused by a single factor.Transference into other environmental condition can be effective to bring about hatching of the eggs which were unhatched and still remained dormant within twice the incubation period of the first hatch. It is also suggested that the rousing of dormant egg is affected by physical stress, such as manual handling, because some eggs were observed to hatch when they were transferred into another lot of the same chlorinity. The effect of lighting is considered to be subject to other environmental conditions, that is, temperature and chlorinity.